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Semantic Data Caching and Replacement

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					                            Semantic Data Caching and Replacement
                           Shaul Dar*                      Michael J. Franklin+                   Bj6rn.T. J6nssont
                      DataTechnologies Ltd.                   Universityof Maryland               Universityof Maryland
                          da@dtl.co.il                        franklin@cs.umd.edu                   bthj@cs.umd.edu

                                           Divesh Srivastava                      Michael Tan*
                                               AT&T Research                     Universityof Maryland
                                           divesh@research.att.com                mdtanx@cs.umd.edu

Abstract                                                              the local client memory and/or disk to cache the data that
                                                                      they have receivedfrom the serverfor possible later reuse.
              a
Wepropose semantic        modelfor client-side          and
                                                caching replace-
                                                                          Data-shipping architectures were popularized by the
mentin aclient-server    database         and          this
                                   system compare approach
                                                                      early generations of Object-Oriented Database Manage-
                    and                          Our
to pagecaching tuplecachingstrategies. cachingmodel
                                                                      ment Systems(OODBMS). These systemswere aimed, in
is basedon, and derivesits advantages        from, threekey ideas.
                                                                      large part, at providing very efficient support for nuviga-
First, the client maintainsa semantic                 of
                                          description the datain
                                                                      tional accessto data (i.e., pointer chasing), as found in
its cache,which    allowsfor a compact               as
                                        specification, a remainder
                                                                      object-orientedprogramming languages. Data-shipping is
query, of thetriples          to        a
                       needed answer querythatarenotavailable
                                                                      well suited to navigational access,as it brings data close
in thecache.Second,      usage              for
                                information replacement      policies
                                                                      to the application, allowing for very lightweight interaction
                 in
is maintained an adaptive        fashionfor semantic regions, which
                                                                      betweenthe application and the database     system.
                  with
are associated collectionsof tuples. This avoidsthe high
                                                                          When cachingis incorporatedinto a data-shippingarchi-
             of
overheads tuplecachingand,unlike pagecaching,is insensi-
                                                                      tecture, serversare usedprimarily to service cachemisses,
tive to badclustering.Third, maintaininga semantic        description
                                                                      and thus, client-server interaction is typically fault-dtiven.
           data          the
of cached enables useof sophisticated functions  value           that
                                                                      That is, clients request specijic data items from the server
incorporate   semantic   notionsof locality,not just LRU or MRU,
                                                                      when such items cannot be located in the local cache. The
                            We
for cachereplacement. validatetheseideaswith a detailed
                                                                      relationship between the client and server in this case is
                study
performance that includestraditionalworkloads well as      as
                                                                      similar to that between a databasebuffer manager and a
a workload               by
              motivated a mobilenavigation       application.
                                                                      disk manager in a centralized databasesystem. Not sur-
                                                                      prisingly, the techniques used to manageclient cachesin
1 Introduction                                                        existing data-shipping systemsare closely related to those
1.1 I&a-shipping           Architectures                              developed for databasebuffer managementin traditional
                                                                      systems. That is, a client cache is managedas a pool of
A key to achieving high performance and scalability in
                                                                      individual items, typically pagesor tuples. An individual
client-server databasesystemsis to effectively utilize the
                                                                      item can be located in the cache by performing a lookup
computationaland storageresourcesof the client machines.
                                                                      using its identifier, or by scanningthe contentsof the cache.
For this reason, many such systems are based on data-
                                                                          As with traditional buffer managers,one of the key re-
shipping. In a data-shippingarchitecture, query processing
                                                                      sponsibilities of a client cache manager is to determine
is performed largely at the clients, and copies of data are
                                                                      which dataitems should be retainedin the cache,given lim-
brought on-demand from servers to be processedat the
                                                                      ited cache space. Such decisions are made using a cache
clients. In order to minimize latency and the needfor future
                                                                      replacement policy; each of the items is assigneda value
interaction with the server,most data-shippingsystemsuse
                                                                      and when spacemust be made available in the cache, the
    *Theworkof Shaul and Michael Tan was performed when they
                         Dar                                          item or items with the leastvalue are chosenasreplacement
were at AT&T Bell Laboratories,Murray Hill, NJ, USA.                  victims. The value function for cache items is typically
    tsupported in part by NSF Grant IRI-9409575, an IBM SUR award,    based on accesshistory, such as a Least Recently Used
and a grant from Bellcore.
                                                                      (LRU) or a Most Recently Used (MRU) policy.
Permission to copy without fee all or part of this material is granted
providedthat the copies are not made or distributedfor direct commercial
advantage, the VLDB copyright notice and the title of the publication and     1.2 Incorporating    Associative Access
its date appear, and notice is given that copying is by permission of the
Very Large Data Base Endowment. To copy otherwise, or to republish,           In recent years, it has become apparentthat large classes
requires a fee at&or special permission from the Endowment.                   of applications are not well-served by purely navigational
Proceedings of the 22nd VLDB Conference                                             to
                                                                              access data. Such applicationsrequire associative access
MumbaQBombay), India, 1996                                                    to data, e.g., as provided by relational query languages.


                                                                        330
Associative accessimposes different demandson a cache            tions that incorporate semantic notions of locality can be
managerthan navigational access.For example,using asso-          devisedfor traditional query-basedapplications as well as
ciative access, data items are not specifieddirectly, but are    for emerging applications such asmobile databases.
selectedand groupeddynamically basedon their data val-              We validate the advantagesof semantic caching with
ues. Becauseof the differencesbetween navigational and           a detailed performance study that is focused initially on
associativeaccess,                               that
                     many client-server systems focus on         traditional workloads, and is then extended to workloads
associativeaccessforego the data-shipping architecture in        motivatedby a mobile navigation application.
favor of a query-shipping approach,whererequestsare sent
from clients to serversusing a higher-level query specifi-       2 Architectures for Cache Management
cation. The traditional query-shipping approach,however,
as supportedby most commercial relational databasesys-           In order to evaluate the performance impact of semantic
tems,doesnot supportclient caching. Thus, query-shipping         caching, we compare it to two traditional cache manage-
architecturesareless ableto exploit client resources per-
                                                      for        ment architectures:pagecaching and tuple caching. In this
formanceor scalability enhancement.                              section, we first outline the primary dimensions for com-
   In this paper, we propose a semantic model for data           paring the three architecturesin the context of associative
caching and replacement. Semanticcaching is a technique          query processing.We then describethe approaches lightin
that integrates support for associative accessinto an ar-        of thesedimensions. We focus on the particular instantia-
chitecture basedon data-shipping. Thus, semanticcaching          tions of the architecturesthat arestudiedin this paper,rather
providesthe ability to exploit client resources,while alsoex-    than on an analysis of all possible design choices. More
ploiting the semanticknowledge of data that arisesthrough        detailed discussionsof the traditional architecturescan be
theuseof associativequery specifications.In this approach,       found in, among other places, [DFMV90, KK94, Fra96].
serverscan processsimple predicates(i.e., constraint for-
mulas) on the database,sending back to the client those          2.1 Overview of the Architectures
tuples that satisfy the predicate. The results of thesepredi-    In this paper, we assumea client-server architecture in
catescanthen be cachedat the client. A novel aspectof this       which client machineshave significant processingand stor-
approach,however,is that rather than managing the cache          ageresources,and arecapableof executing queries. We fo-
on the basis of individual items we exploit the semantic         cuson systems   with a single server,but all of the approaches
information that is implicit in the query predicatesin order     studiedhere can be easily extendedto a multiple serveror
to more effectively managethe client cache.                      even a peer-to-peerarchitecture, such as SHORE [C+94].
                                                                 The databaseis stored on disk at the server, and is orga-
1.3 Semantic Caching                                             nized in terms of pages. Pagesare physical units - they
                                                                 are fixed length. The databasecontains index as well as
Our semantic caching model is basedon, and derives its
                                                                                          that
                                                                 datapages.We assume tuples are fixed-length and that
advantages    from, three key ideas.
                                                                 pagescontain multiple tuples. Pagesalso contain header
    First, the client maintains a semanticdescription of the
                                                                 information that enablesthe free spacewithin a pageto be
data in its cache, instead of maintaining a list of physical
                                                                 managedindependently of spaceon any other page.
pagesor tuple identifiers. Query processingmakesuse of
                                                                    In this study, there are three main factors that impact
the semantic descriptions to determine what data are lo-
                                                                 the relative performanceof the architectures: (1) datagran-
cally available in the cache,and what data are neededfrom
                                                                 ularity, (2) remainder queries vs. faulting, and (3) cache
the server. The data neededfrom the server are compactly
                                                                 replacementpolicy. We addressthesefactorsbriefly below.
specified as a reyainder query. Remainder queries pro-
vide reduced communication requirements and additional
parallelism comparedto faulting-basedapproaches.                 2.1.1 Data Granularity
    Second,the information used by the cachereplacement          In anysystemthat usesdata-shipping,the granularity of data
policy is maintained in an adaptivefashion for semantic re-      managementis a key performanceconcern. As described
gions, which are associated    with setsof tuples. These sets    in [CF’Z94,Fra96], the granularity decisions that must be
are defined and adjusteddynamically basedon the queries          made include: (1) client-server transfer, (2) consistency
that are posed at the client. The use of semanticregions         maintenance, and (3) cache management. In this study
avoids the high storageoverheadsof the tuple caching ap-         (in contrast to [DFMV90]), all architectures ship data in
proach of maintaining replacementinformation on a per-           page-sizedunits. Also, we examine the architectures in
tuple basis and, unlike the page caching approach,is also        the context of read-only queries. Thus, the main impact of
insensitive to bad clustering of tuples on pages.                granularity in this study is on cache management. Tuple
   Third, maintaining a semanticdescription of the data in       caching is based on individual tuples, page caching uses
the cache encouragesthe use of sophisticatedvalue func-          statically definedgroupsof tuples (i.e., pages)and semantic
tions, in determining replacementinformation. Valuefunc-         caching usesdynamically defined groups of triples.


                                                           331
   Given that tuples are fixed-length, the main differences       likely to be referenced;pagecaching tries to exploit spatial
between these three approachesto granularity are in the           locality under the assumption that clustering of tuples to
relative spaceoverhead they incur for cache management            pagesis effective. As demonstratedin’section 3, semantic
(buffer control blocks, hash table entries, etc.), and in the     caching enables the use of a dynamically defined version
flexibility of grouping tuples. Tuple caching incurs over-        of spatial locality, that we refer to as semantic locality.
head that is proportional to the number of tuples that can        Semanticlocality differs from spatial locality in that it is not
be cached. In contrast, both page and semantic caching            dependenton the static clustering of tuples to pages;rather
reduce overhead by aggregatinginformation about groups            it dynamically adaptsto the pattern of query accesses.
of tuples. In terms of grouping tuples, semantic caching
provides complete flexibility, allowing the grouping to be        2.2 Page Caching Architecture
adjustedto the needsof the current queries. In contrast,the
                                                                  In page caching architectures (also referred to as page-
static grouping usedby page caching is tied to a particular
                                                                  server systems @XMV90, CFZ94]), the unit of transfer
clustering of tuples that is determinedQpriori, independent
of the current query accesspatterns.                              betweenserversand clients is a page. Queriesare posedat
                                                                  clients, and processedlocally down to the level of requests
                                                                  for individual pages. If a requestedpage is not present in
2.1.2 Remainder Queries vs. Faulting
                                                                  the local cache, a requestfor the page is sent to the server.
Another important way in which the architecturesdiffer is         In responseto such a.request, the. server will obtain the
in the way they requestmissing data from the server. Page         page from disk (if necessary)and send the page back to
caching is faulting-based. It attempts to accessall pages         the client. On the client side, page caching is supported
from the local &he, and sendsa request to the server for          through a mechanismthat is nearly identical to that of a tra-
a specific page when a cache miss occurs. ‘l%ple caching          ditional page-based   database buffer manager. A client can
is similar to page caching in this regard, but takes care to      perform partial scanson indexed attributes by first access-
combine requests for missing tuples so that they can be           ing the index (faulting in any missing index pages)and then
transferred from the server in page-sizedgroups. As de-           accessingqualifying data pages.If no index is presentthen
scribed in Section 2.3, when there is no index available at       a page caching approachwill scan an entire relation, again
the client, then the query predicateand someadditional in-        faulting in any missing pages. As with a buffer manager,a
formation are sent to the server to avoid having to retrieve      pagecacheis managedusing simple replacementstrategies
an entire relation. This is an extension to tuple caching that    basedon the usageof the dataitems, such asLRU or MRU.
we implementedin order to make a fairer comparisonwith
semanticcaching. Semanticcaching describesthe exact set           2.3 ‘Ibple Caching Architecture
of tuples that it requiresfrom the serverusing a query died
                                                                  Tuple caching is in many ways analogousto page caching,
the reminder query. Sending queries to the server rather
                                                                  the primary difference being that with tuple caching, the
than faulting items in can provide severalperformanceben-
                                                                  client cacheis maintained in terms of individual tuples (or
efits, such asparallelism between the client and the server,
                                                                  objects)rather than entire pages.Caching at the granularity
andcommunicationssavingsdue to the compactrepresenta-
                                                                  of a single item allows maximal flexibility in the tuning of
tion of the requestfor missing items. An additional benefit
                                                                  cachecontents to the accesslocality properties of applica-
of the approach is that in caseswhere all neededdata is
                                                                  tions [DFMV90]. As described in [DFMV90], however,
present at the client, a null remainder query is generated,
                                                                  the faulting in of individual tnples (assumingthat tuples are
meaningthat contact with the server is not necessary.
                                                                  substantially smaller than pages) can lead to performance
                                                                  problems due to the expenseof sending large numbers of
2.13   Cache Replacement Policy                                   small messages.In order to mitigate this problem, a tuple
A final issuethat impactsthe perform&nce the alternative
                                            of                    caching systemmust group client requestsfor multiple tu-
architectures is the cache replacement policy. A cache            ples into a single messageand must also group the tuples
replacementpolicy dictateshow victims for replacementare          to be sent from serversto clients into blocks.
chosenwhen additional spaceis required in the cache. Such           , Scansof indexed attributes can be answeredin a manner
policies apply a value function to eachof the cacheditems,        similar to pagecaching. For scansof non-indexed attributes
andchooseasvictims, thoseitems with the lowestvalues. In i        however,there are two options. One option is for the client
traditional systems,value functions typically are basedon         to first perform the scan locally, and then send a list of all
temporal locality and/or spatial locality. Temporal locality      qualifying tuples that it hasin its cache,along with the scan
is the property that items that havebeenreferencedrecently        constraint to the server. The server can then processthe
are likely to be referenced again in the near future; the         scan,sendingback to the client only thosequalifying tuples
LRU policy is basedon the assumptionof temporal locality.         that are not in the client’s cache. An alternative is for the
Spatial locality is the property that if an item has been         client to simply ignore its cachecontentswhen performing
referenced,other items that arephysically closeto it arealso      a scan on a non-indexed attribute. In this case, the scan


                                                            332
constraint is sent to the server,and all qualifying mpies are
return&, duplicate tuples can be discardedat the client.
   Finally, the tuple cache, like a page cache, is managed
using an access-based     replacement policy such as LRU.
Unlike the pagecache,however,thereis no notion af spatial
locality for tuples, so only temporal locality is exploited.

2.4 Semantic Caching Architecture
                                                                                        1
 Semanticcaching managesthe client cache as a collection
                                                                                        c--.-“--.-,,.
 of semanticregions; that is, access information is managed,
                                                                                               2528     30   Age
 and cachereplacementis performed,at the unit of semantic
regions. Semanticregions, like pages,provide a meansfor                             Figure 1: SemanticSpaces
the cachemanagerto aggregateinformation about multiple
tuples. Unlike pages,however, the size and shape(in the             client cache,and anotherthat requires tuples to be shipped
 semanticspace)of regions can changedynamically.                    from the server.In semanticcaching, thepotions of a probe
     Eachsemanticregion hasa constraint formula describing          query and a remainder query correspondto thesetwo por-
 its contents, a count of tuples that satisfy the constraint, a     tions of the query. More formally, given a query on relation
 pointer to a linked list of the actual tuples in the cache,        R with constraint formula Q, if V denotesthe constraint
 and additional information that is used by the replacement         formula describingthe setof tuples of R presentin the client
policy to rank the regions. The formula that describesa             cache,then the probe query, denoted by ‘P(Q, V), can be
region specifiesthe region’s location in the semanticspace.         definedby the constraint formula & A V on R. Further, the
Unlike the replacementvalue functions used by the page              remainder query, denotedby R(Q, V), can be defined by
 andtuple caching architectures,the value functions usedby          the constraint formula Q A (7 V) on R.
 semanticcaching may take information about the semantic                For example, consider a query to find all employees
locality of regions into account.                                   whose salary exceeds 50,000, and who are at most 30
     When a query is posedat a client, it is split into two dis-    years old. This query can be describedby the constraint
joint pieces: (1) a probe query, which retrieves the portion        formula Qi = (Salary > 50,000 A Age 5 30) on the re-
of the result available in the local cache,and (2) a remuin-        lation employee(Name, Salary, Age). Assume that the
der query, which retrieves any missing tuples in the answer         client cache contains all employees whose salary is less
from the server, If the remainder query is not null (i.e., the      than 100,000 as well as all employeeswho arebetween25
query coverspartsof the semanticspacethat arenot cached)            and 28 years old. ‘Ihis can be described by the formula
then the remainderquery is sentto the serverandprocessed            VI = (Salary < 100,000 V (Age > 25 A Age 5 28)).
there. ‘Similar to tuple caching, the result of the remainder           The probe query P( Qt , VI) into the client cache is de-
query is packed into pagesand sent to the client. Unlike            scribed by the constraint formula ((Salary > 50,000 A
tuple c&ching,however,the mechanismfor obtaining tuples             Salary < 100,000 A Age 5 30) V (Salary > 50,000 A
from the serveris independentof the presenceof indexes.             Age 3 25 A Age < 28)). This constraint describesthose
                                                                    tuples in the cache that are answers to the query. The
3 Model of Semantic Caching                                         remainder query ‘R(Qi , VI) is describedby the constraint
                                                                    formula ((Salary 1 100,000 A Age < 25) V (Salary 2
3.1 Basic ‘kmiuology                                                100,000 A Age > 28 A Age < 30)). This constraint de-
                                                                    scribesthosetuples that needto be fetched from the server.
Semanticcachingexploits the semanticinformation present
in associativequery specificationsto organize and manage                When the constraint formulas are arithmetic constraints
the client cache. In this study,we considerselectionqueries         over attributes AI, . . . , A,, they have a natural visualiza-
on single relations, where the selection condition is an ar-        tion as sub-spacesof the n-dimensional semantic space
                                                                    v1 x v2 x -3 * x D,, where ‘Di is the domain of attribute
bitrary constraintformula (that is, a disjunction of conjunc-
                                                                    Ai. Figure 1 depicts the projection onto the Sahry and
tions of built-in predicates); dealing with more complex
                                                                    Age attributes of the semantic spacesassociatedwith the
querieswithin the framework of semanticcaching is an im-
                                                                    employee relation, query Q 1,cachecontents VI, the probe
portant direction of future research. In semantic caching,
the portion of a single relation presentin the client cacheis       query P(Q1, Vi) and the remainder query R(Q1, Vi).
also describedby a constraint formula; the entire contents
of the client cacheare describedby a set of suchconstraint          3.2 Semantic Regions
formulas, one for eachdatabase    relation.                         Client cachesize is limited, andexisting tuples in the cache
    A query can be split into two disjoint portions: one that       may need to be discarded to accommodatethe ples re-
can be completely answeredusing the tuples presentin the            quired to answer subsequentqueries. Semantic caching


                                                              333
            the
 manages client cacheasa collection of semanticregions
 that group together semantically related tuples; each tuple
 in the client cache is associatedwith exactly one semantic
 region. These semantic regions are defined dynamically
 basedon the queriesthat are posedat the client.
     Each semanticregion has a constraint formula that de-
 scribesthe tuples grouped together within the region, and           . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...’   :                                                          j      ,...........................................;



 has a single replacement value (used to make cache re-              (a) Regions after Ql                                                                        @) Regions after 42                                                (c) Regionsafm 43
 placementdecisions) associatedwith it; all tuples within a
 semanticregion have the replacementvalue of that region.                                                             Figure 2: SemanticRegions: Recencyof Usage
     When a query intersectsa semanticregion in the cache,       ,   .............................................                                              ....   ...............   ......................
                                                                                                                                                                                                            *
                                                                                                                                                                                                                  .,
                                                                                                                                                                                                                       j       j_________.            .................           M”“.   ...




 that region gets split into two smaller disjoint semantic
 regions, one of which is the intersection of the semantic                                                           Ql

region and the query, and the other is the difference of the
 semantic region with respect to the query. Data brought
into the cacheas the result of a remainderquery also forms
a new semanticregion. Thus, the execution of a query that
overlaps n semanticregions in the cache can result in the            (a) Regions after Ql                                                                       (b) Regions after Q2                                             (c) Regionsafter 43
formation of 2n + 1 regions; of these regions n + 1 are
part of the query. The question then ariseswhether or not                                                            Figure 3: SemanticRegions: Manhattan Distance
to coalescesomeor all of these regions into one or more
larger regions.                                                   ple or page,correspondingto the latest time the item in the
    A straightforward approach is to always coalescetwo           cachewasaccessed.     Maintaining replacementvaluesbased
regionsthat havethe samecachereplacementvalue, result-            on recency of usagein the semantic caching approachas-
ing in only one region corresponding to the query. With           sociatessuch a value with each semanticregion, basedon
small (relative to cachesize) queries, this strategycan lead      the sequenceof queries issuedat the client. Figure 2 illus-
to good performance.When the answerto eachquery takes             tratesthe semanticregions and their associated replacement
up a large fraction of the cache,however,this strategycan         values, basedon mcency of usage,for a sequenceof thtee
result in semanticregionsthat areexcessivelylarge. The re-       range queries on a single binary relation. The solid lines
placementof a large region can empty a significant portion        show the semanticregions createdwhen full coalescing is
of the cache,resulting in poor cacheutilization.                 performed, the dotted lines depict the additional semantic
                                                                 regions that would result if no coalescingwere performed.
    Another option is to never coalesce. For small queries
that tend to intersect, this can lead to excessiveoverhead,          The constraint formula Ql cotresponding to the tirst
but for larger queries,it alleviates the gram&rity problem.      query is the only semanticregion (with value 1) after Ql
    In our approach,therefore,we use an adaptiveheuristic.       is issued (seeFigure 2(a)). The secondquery Q2 overlaps
Regions with the same cache replacementvalue may be              with the semantic region with value 1, and the constraint
coalescedif either one of them is smaller than 1% of the         formula Q2 is the semantic region with value 2. Since
cachesize. As shown in Section 5.1, this heuristic strikes a     semanticregionshave to be mutually disjoint, the semantic
good balancebetweenthe two extremes.                             region with value 1 ‘Winks”, after Q2 is issued, to the
                                                                 portion that is disjoint with Q2 (seeFigure 2(b)). Similar
                                                                 shrinking occurs when the third query is issued, note that
3.3 Replacement Issues
                                                                 the semanticregion with value 1 is no longer convex, and
When there is insufficient spacein the cache,the semantic        its constraint formula is not conjunctive. In fact, semantic
region with the lowest value and all tupleswithin that region    regions may not be connectedin the semanticspace.
are discardedfrom the cache. Semanticregions are, thus,              An alternativeto using recencyinformation for determin-
the unit of cachereplacement.The value functions usedby          ing replacementvaluesis to usesemanticdistance. Figure 3
semantic caching can be basedon temporal locality (e.g.,         showsthe result of using Manhattan distance in the ptevi-
LRU, MRU), or on semantic locality of regions. Below,            ous example. In this case,each semanticregion is assigned
we describe two caching/replacementpolicies, one where           a replacementvalue that is the negative of the Manhattan
the replacementvalue is based on recency of usage, and           distance betweenthe “center of gravity” of that region and
anotherwhere it is basedon a distancefunction.                   the “center of gravity*’ of the most recent query. With this
   Maintaining replacementvalues basedon recencyof us-           distance function, semanticregions that are “close” to the
age allows for the implementation of replacementpolicies         most recent query have a small negativevalue, irrespective
such as LRU or MRU. Conceptually, tuple caching and              of when they were created,and are hence less likely to be
page caching associatea replacementvalue with each tu-           discardedwhen free spaceis required.


                                                           334
3.4 An Operational Model
Wenow describean operationalmodel of semanticcaching.
In this model the client processes a stream of queries
Ql,...,Qm on relation R. Let Vi-1 denote the cache
contentsfor relation R, and Si- i denote the set of seman-
tic regions of relation R, when query Qi is issued. Vo is
the constraint formula false, and Se is empty. Processing
query Qi, involves the following steps:
  1. Computethe probequery P( Qi, VL 1)andthe remain-
     der query ‘R(Qi,K-1) from Qi and 6-i.              Partly
     answer query Qi from the set of tuples that satisfy                                              and
                                                                              Table 1: Model Parameters Default Settings
     ‘J’(QitV-1).                                                        answeringsuchqueries. Disks aremodeledusing a detailed
  2. Repartition Si-1 into Si and update the replace-
                                                                         characterizationadaptedfrom the ZetaSim model lBro92].
     ment values associated with the semantic regions
                                                                         The disk model includes an elevator scheduling policy, a
     in Si based an P(Qi,K-I),           R(Qi,I$-I), and the
     caching/replacement      policy used.                               controller cache, and read-aheadprefetching. There are
  3. Fetchthe tuplesof R that satisfy the constraint formula             many parametersto the disk model (not shown) including:
     ‘R(Qi, K-1) from the server.                                        rotational speed,seekfactor, settle time, track and cylinder
  4. If the cachedoes not have enough free space,discard                 sixes, controller cache size, etc. In addition to the time
     semantic regions Si, . . . , Sk with low dues among                                                 the
                                                                         spentwaiting for and accessing disk, a CPU overheadof
     the setof semanticregions ,!$, anddiscardtuples in the              Disklnst instructions is chargedfor every disk I/O request.
     cache that SatiSfy the constraint formulas Si, . . . , Sk               The database,the server buffer pool, and the client’s
     until enough spaceis free.                                          disk cache are organized in pagesof size PageSize.Pages
  5. Answer the rest of query Qi by taking the setof tuples              are the unit of disk I/O and data transfer between sites.
     that satisfy ‘R(Qi, K-1).                                           The network is modeled as a FIFO queue with a specified
  6. Compute Vi by taking the disjunction of K-1 and                     bandwidth (NetBw); the details of a particular technology
     ‘R(Qi, K-i), and then taking the difference with re-                (e.g, Ethernet, ATM) are not modeled. The cost of sending
     specttos~,..., Sk; Determine the semantic regions                   a message   involves the time-on-the-wire (basedon the size
     Si in the cache and update their replacementvalues                  of the message),a fixed CPU cost per message       (MsgZnst),
     basedon Si),R(Qi, K-i), the discardedsemanticre-                    and a size-dependent   CPU cost (PerSizeMZ).
     gions Si , . . . , Sk, and the caching/replacement
                                                      policy.                When scarminga relation at the server, there is a ded-
                                                                         icated processwhich attempts to keep the scan one page
4 Simulation Environment                                                 aheadof the consumerat the client. This leads to overlap
                                                                         between disk reads and network messages,      which is most
4.1 Resourcesand Model Parameters                                        apparentwhen the result size is small relative to the amount
Our simulator is an extension of the one used in [FJK96],                of data scanned.In the extremecase,network communica-
written in C++ using CSIM. It models a heterogeneous,                    tion can be done completely parallel to the disk reads. This
peer-to-peerdatabasesystemsuch as SHORE [C+94], and                      overlap doesnot arise when data is faulted in to the client,
provides a detailed model of query processingcostsin such                as there is no dedicatedprocessat the serverin this case.
a system. For this study, the simulator was configured to                   In addition to the CPU costsfor systemsfunctions such
model a systemwith a single client and a single server.                                and
                                                                         as messages I/OS, there are also costs associatedwith
    Table 1 showsthe main parametersof the model. Every                  the functions performed by query operators. The costs
site has a CPU whose speedis specified by the Mips pa-                   that are modeled are those of displaying, comparing, and
rameter,NumDisks disks, and a main-memory buffer pool.                   moving tuples in memory.
At the client, the size of the buffer pool is ClientCache.’
The details of buffer management   overheadfor the different             4.2 Buffer Management at the Client
client caching strategiesare describedin Section 4.2.                  In order to maintain fairness to the different caching ar-
    The CPU is modeled as a FIFO queue. The client has an              chitectures, the ClientCache parameter includes both the
optional disk-resident cache,which also usesthe parameter              space needed for buffer managementoverhead, and the
ClientCache;the memory cacheis not usedin this case.The                spaceavailable for storing data Since we do not consider
disk cache is used for queries on non-indexed attributes,              updatesin this study,we do not model the overheadneeded
and the whole disk cache is scanned in sequencewhen                    to facilitate updates. We also do not model the CPU cost of
   1As eachpageis referencedonly onceper query, and serverbuffersare                        at
                                                                       cachemanagement the client.
clearedbetweenqueries, the buffer size at the serverdoesnot matter.        To estimatethe overheadof pagebuffer management,   we


                                                                   335
                                       relation(tuples)
                   10000 Sizeof database                                      databasesmall and have sized the cache proportionally, in
                                                                              order to makethe running of a large numberof experiments
 ~                l-108                           by
                              % of relationselected eachquery                 feasible. As with all caching studies, what determinesthe
                                                                                                                                    and
                                                                              performanceis the relative sizesof the cache,databases,
                     10%      Size   of thehot region(% of relation)          accessregions, rather than their absolute sizes3 The rela-
     Table 2: Workload Parameters Default Settings
                                and                                           tion has three candidate keys, which we adoptedfrom the
                                                                              Wisconsin benchmark: Unique2 is indexed and perfectly
used the Buffer Control Block of [GR93]. After removing                       clustered; Unique1 is indexed but completely unclustered;
all attributespertaining to updatesand concurrencycontrol,                    Unique3 is both unindexed and unclustered.
 we were left with 28 bytes per page. To model the storage
cost of indexes,we assumethat the primary index takesup                       5 Experiments and Results
negligible space,as also the upper levels of the secondary
index. The leaf level of the secondaryindex, however,has                      In this section we examine the performance of the three
8 bytes per tuple. This addsup to 188bytes of overheadfor                     caching architecturesusing a workload consisting of selec-
a pageof 20 tuples. In a cacheof size 250Kb, we can then                      tion queries on a Wisconsin-style databaseusing various
fit &$ff& a 60 pages.                                                         indexed and non-indexed attributes. As shown in Table 2,
    For tuple shipping the samedata structure can be used                     the access  pattern is skewedso that 90% of the querieshave
for cache management,with two exceptions. luple size                          a centerpointthat lies within thehot region consisting of the
needsto be kept, and tuple identifiers are typically larger                   middle 10% of the relation. In all the experiments in this
than page identifiers. However, since we used fixed size                      section, the client cacheis set to 250Kb, which is sufficient
tuples, and do not have a specific implementation of tuple                    to store the entire hot region, including overhead, for all
identifiers, we chose to use 28 bytes per tuple. With the 8                   three approaches.
bytes for indexes, that addsup to 36 bytes per tuple. In a                       The primary metric usedis responsetime. Whereneces-
cacheof size 250Kb, we can then fit m6 M 1085tuples.                          sary,other metrics suchascachehit rates,message    volumes,
    For semanticcaching, the buffer managementinforma-                        etc. are used. The numberswere obtainedby averagingthe
tion is kept on a semantic region basis. The replacement                      results of three runs of queries. Each run consisted of 50
information needed is similar to page and tuple caching;                      queries to warm up the cache followed by 500 query exe-
however, the page identifier, the frame index and the hash                    cutions during which the measurements     were taken. The
overflow pointer are not needed. Instead, we need addi-                       results presentedhere are a small, but representativeset of
tional pointers to the list of factors in the constraint formula              the experimentswe haverun. In particular we ran numerous
describing the region, and to the list of tuples in the re-                   sensitivity experimentsvarying cachesize, hot region size,
gion. This is a total of 24 bytes. For each factor in the                     tuple size, skew,etc.
constraint formula we need the endpoints of the range of
each attribute (8 bytes per attribute), and a pointer to the                  5.1 Indexed Selections
next factor (4 bytes). For each tuple we need a pointer to                  We first study the performance of the three caching ar-
the next tuple (4 bytes). Note, that we do not needto model                 chitectures when performing single- and double-attribute
a storageoverheadfor indexesat the client, as the semantic                  selections on indexed attributes. Figure 4 shows the re-
cacheusessemanticinformation to organizethe data. Since                     sponsetime for the three caching architectures when the
the overheadis variable, our implementation simply makes                    selection is performed on the Unique2 attribute, which has
sure that the size of the overhead data structures and the                  a clusteredindex. The t-axis of the figure showsthe query
actual data is never more than the size of the cache.                       result size expressedas a percentageof the relation size. In
                                                                            this case,it can be seenthat all three architecturesprovide
4.3 Workload Specification                                                  similar performanceacrossthe range of query sizes. As the
We use a benchmark consisting of simple selections. The                     .query size is increased (while the cache size is held con-
size of the result QuerySize is varied in the experiments,                  stant), the responsetime for all of the architecturesworsens
but is always smaller than the cache. A fixed portion of                    due to lower client cachehit rates. luple caching has the
the queries (Skao) has the semantic centerpoint within a                    worst performancein this experiment and page and seman-
hot region of size HotSpot? The remaining queries are                       tic cachingperform roughly equally. l7.tplecaching’s worse
uniformly distributed over the cold area                                    performancein this caseis due to its relatively high space
   As shownin Table2, we usea single relation with 10,000                   overhead.As describedin Section 4.2, tuple caching incurs
tuples of 200 bytes each. We have intentionally kept the                    an overhead of 36 bytes per .every 200 byte tuple in the
                                                                            indexed case.In contrast,pagecaching incurs an overhead
   2Sincethe only require ent for a hot query is that the centerpoint be
within the hot spot, a sizabl raction of the query may lie outsidethe hot       3We also conductedexperiments where the database,cache, and the
spot The semanticareaadj. ent to the hot spot will thereforealso have a     queries, were all scaled up by a factor of 10. The results (in terms of
significant number of I&.                                                   relative performance)in this casewere nearly identical.


                                                                        336
     1   234567                      6    0   10

             Quety Size I% of Relation]                    Query   Size [% of   Relatlonl                Query Size pl6 of Relation]

  Figure 4: Resp.Time, Unique2                     Figure 5: Resp.Time, Unique1              Figure 6: Overhead,UniquellUniquQ
   Mem. Cache,Varying Query Size                   Mem. Cache,Varyjng Query Size                 Mem. Cache,Varying Query Size

of less than 10 bytes per tuple, and becauseUnique2 is a                and three variants of semanticcaching.
clustered attribute, nearly all of the tuples in an accessed               The storageoverheadfor tuple caching andpagecaching
page satisfy the query. Thus, page caching has approxi-                 is proportional to the number of items that fit in the cache,
mately 10%more datain the cachethan tuple cachinghere.                  so it is independentof the query size. Pagecaching has an
Semanticcachinghasevenlower spaceoverheadthan page                      overheadof 6.5% (including the cost of unusedspaceon the
caching in this experiment; however, this slight advantage              pages)while the overheadof tuple caching is 15.2%for all
 is mitigated by an equally slight degradationin cacheuti-              query sizesin Figure 6. Despite its advantagein overhead,
lization as the query size increases. With larger regions,              however,pagecaching still performsmuch worsethan tuple
 the replacementgranularity of semanticcaching increases.               caching in this experimentbecauseof the lack of clustering
Replacinglarge regionstemporarily opensup largeholes in                 with respectto the Uniquel attribute.
the cache,which is detrimental to overall cacheutilization.                 In contrastto pageand tuple caching, the spaceoverhead
     Figure 5 showsthe responsetimes for the architectures              of semanticcaching is dependenton both the query size and
when the selectionis on Uniquel , the non-clusteredindexed              the coalescingstrategy.The three lines shown for semantic
 attribute. In this figure, the performanceof page caching              caching in Figure 6 show the overheadfor three different
 is shown for two different cache value functions: LRU                               to
                                                                        approaches coalescingregions. The highest spaceover-
 and MRU. In this experiment, the page caching approach                 head is observed when coalescing is turned off (“Never
performsfar worsethanboth the tuple andsemanticcaching                  Coalesce”). Recall that a query that touchesn regions can
approaches.Pagecaching‘s poor performancehere is to be                  result.in the creation of up to tr + 1 new regions. If these
expected; since Unique2 is unclustered, the hot region of               new regions are not coalesced,the overheadincurred can
the relation is not able.to fit entirely in me cache. MRU               be significant. As can be seenin the figure, the overheadis
helps page caching slightly in this case,becausethe non-                significantly worse for smaller queriesthan for larger ones.
clustered index scan processesthe pages of the relation                 For 1% queries, there are 55 regions and nearly 275 fac-
 sequentially. Of course, random clustering is the worst                tors. In contrast,when coalescingis performedaggressively
casefor pagecaching, which is basedon the assumptionof                  (“Always Coalesce”) overhead is decreasedsubstantially
 spatiallocality. Nevertheless, comparingthis graphwith the             (e.g., by 85% for the smallestquery). As statedpreviously,
                              the
previous one demonstrates sensitivity of page caching                   however, aggressivecoalescing can also negatively affect
to clustering. Also the two experiments demonstratethat                 cacheutilization by increasing the granularity of cachere-
the spaceoverheadof semanticcachingis the sameor better                 placement. In this experiment, aggressivecoalescing has
than pagecaching,but that unlike pagecaching, a semantic                as much as 10% lower cacheutilization comparedto never
cacheis not susceptibleto poor static clustering.                       coalescing. Finally, the regular “semantic” line, showsthe
     The first two experiments examined single-attribute                               of
                                                                        effectiveness the default coalescingheuristic describedin
queries. We also studiedqueriesthat are multi-attribute se-             Section3.2. In this case,the overheadis only slightly higher
lections on the combination of Unique1 and Unique2. The                 than that of always coalescing, while the cacheutilization
results in this case(not shown) are similar to those of the             (not shown) is nearly the sameas that of never coalescing.
non-clustered selection of the previous experiment: page                Thus, these results demonstratethat the simple coalescing
caching suffers due to poor clustering; tuple and semantic              heuristic usedby semanticcaching is highly effective.
cachingprovide similar, andmuchbetter performance.The                      Finally, it should also be noted that the spaceoverhead
important aspectof this experiment, however, can be seen                of semantic caching is impacted by the dimensionality of
in Figure 6, which showsthe total spaceoverhead(as a per-               the semanticspace.In this case,sincethe semanticspaceis
cent of the cachesize) incurred by page and tuple caching               two-dimensional, semanticcaching incurs somewhat      higher
    .

                                                                   3‘37           i                                          I
     01    ’    ’      ’       ’    ’    ’        ’   ’     1      --                                                         01     ’    ’      ’       ’    ’     ’       ’   ’     J
       1   2   3       4      5     6    7    6       910           1   2   3       4      5     6     7       6   910          12       3       4      5     6    7        6   910

               Quefy       Size p6 Of Relauc4l]                             Query       Size pb of Relation!                             Query       Size p6 of Relation]

   Figure 7: Resp.Time, Unique3                                 Figure 8: Network Volume, Unique3                           Figure 9: Resp.Time, Unique1
    Disk Cache,VaryingQuerySize                                    Disk Cache,Varying Query Size                            Mem. Cache,Varying Query Size
                                                                                             L-                                                   .
overheaddue an increasein the numberof semanticregions                                         at the server. The result of the sequentialprocessingin this
and the complexity of the constraint formulas that describe                                    experiment is that tuple caching has worse responsetime
them. For small queries,the overheadof the nevercoalesce                                       even than a tuple-basedapproachthat completely ignores
case is over four times higher than in a single-attribute                                      the cache. The main reasonfor this non-intuitive behavior
semanticspace.The default coalescingheuristic, however,                                        is that becausethe selection is applied to a non-indexed
doesnot suffer from this overheadexplosion: its overhead                                       attribute, any data requestsent to the serverresults in a full
evenfor the smallestqueriesis only about one third higher                                      scan of the relation (from disk) at the server. The cost of
than in the single attribute case.                                                             this scandominatesall other activities in this case,andsince
                                                                                               the server is able to overlap communication with I/O, the
5.2 NonIndexed Selections                                                                      communication costs do not factor into the total response
                                                                                               time. Thus, in this experiment, tuple caching performsex-
As describedin Section 2, the z&lability (or lack) of in-
                                                                                               tra work prior to contacting the server,but seesno benefit
dexesat clients dictates the ma;n”e’ in which the page and
                                                                                               in responsetime resulting from this work. Such a benefit,
tuple caching architecturesprocessqueries. In this section
                                                                                               however,is evident in Figure 8 which showsthe numberof
we examine the performanceof the tuple caching and se-
                                                                                               bytes sentacrossthe network per query. In this case,the use
mantic cachingarchitectureswhen performing selectionson
                                                                                               of the client cacheresults in a significant reduction in mes-
an unindexed attribute (Unique3).4 For mple caching, we
                                                                                               sagevolume. In a network constrainedenvironment (e.g.,
explore two approachesto processingselections on unin-
                                                                                               a wireless mobile network), such communication savings
dexed attributes. One approachexploits the client cache
                                                                                               may be the dominant factor. Finally, it should be noted that
by first applying the selectionpredicateto all of the cached
                                                                                               when a memory cacheis used rather than a disk cache,the
tuplesof the given relation andsendingthe list of qualifying
                                                                                               performanceof tuple caching is roughly equal to that of the
tuples, along with the selection predicateto the server.The
                                                                                               “tuple ignore” policy in this experiment.
serverthen appliesthe predicateto the entire relation (recall
that there is no index) and sendsany qualifying tuples that                                        Turning to the performanceof semanticcaching in Fig-
are missing from the cache. The secondapproachsimply                                           ure 7, it can be seenthat semanticcaching provides signif-
ignores the cacheand sendsthe predicate to the server. In                                      icant performancebenefits for small queries. This result
this caseall qualifying tuples are sentto the client.5                                         is unexpected, becauseas described above, any data re-
                                                                                               quest sent to the serverincurs a full relation scan,resulting
    Figure 7 shows the responsetime of semantic caching
                                                                                               in performancesimilar to that of “tuple ignore”. This re-
and the two tuple-basedarchitectureswhen the client uses
                                                                                               sult illustrates another fundamental advantageof semantic
its local disk as a cache,rather than its memory. We use a
                                                                                               caching, namely that by maintaining semanticinformation
disk cachehere, in order to demonstratea fundamentalad-
                                                                                               about cachecontents,a semanticcaching systemcan iden-
vantageof semanticcaching over tuple (or page) caching;
                                                                                               tify caseswhen it can answer a query without contacting
namely, that the use of remainder queries for requesting
                                                                                               the server. In this experiment, over 60% of the small (1%)
missing tuples from the server enables the client and the
                                                                                               queries are answeredcompletely from the client’s cache,
serverto processtheir (disjoint) portions of thequery inpar-
allel. In contrast,for a client to exploit a tuple cachein this                                thus avoiding the disk scanat the server.6In contrast,tuple
                                                                                               caching, which also often had an entire answer in cache,
case,it must scanthe local cacheprior to initiating the scan
                                                                                               was still required to perform a disk scanat the server,only
   4Psgecachingperformssigniticautly worse thanthe othershere due to
the lack of clustering, and is therefon not shown.                                                 %Vhen the query size is so large that no queries ate answeredcom-
   5Note that these approachesassumethat the server has the ability to                         pletely in cache,then the perfoiice of semanticcachingbecomesequal
processselectionpredicates,as is also required for semanticcaching.                            to that of “tuple ignore” in this experiment,


                                                                                        338
to find that no extra tuples were needed, Finally, it should                           QlO   Qll
be noted that in environmentswhere communication chan-
nels are scarce, such as cellular networks, the ability to
operateindependentlyof the servercan result in significant
monetary savingsin addition to performancegains.
                                                                                                   Q19   ‘--I
53    Semantic Value Function
                                                                                                         f      I
The previousexperimentsbrought out severalintrinsic ben-                             i -- i” ---B-B             1
efits of maintaining cache contents using semantic infor-                                    1
mation, including low spaceoverhead,insensitivity to page                              m--d I
                                                                                       !
clustering, client-server parallelism, and the ability to an-
 swersomequerieswithout contacting the server.In this sec-                     Figure 10: Random Query Path
tion we demonstrate                     of
                      anotheradvantage semanticcaching:
the ability to incorporatesemantic locality in cachereplace-         Weuseabenchmarkof simple selectionsof tuples, which
ment value functions. As an examplewe usethe Manhattan                                                   in
                                                                 is characteristicof map data accesses a navigation appli-
distancedescribedin Section 3.3.                                 cation. Each query is in the form of a rectangle of sire
    Figure 9 shows the responsetime for selection queries        8 x 16, oriented along one of the two axesin the semantic
on the non-clustered, indexed attribute Uniquel. As can          spaceof the two spatial attributes of the relation; thus, each
be seenin the figure, the Manhattan distanceprovides bet-        query answerhas 128 tuples. The location and orientation
ter performancefor all query result sizesin this experiment.     of the query rectangle dependson the user’s current loca-
The Manhattandistanceis moreeffective than LRU at keep-          tion and direction of motion. A query path correspondsto
ing the hot region in memory,resulting in a better cachehit      navigating through the 2dimensional spacein a Manhattan
rate. The reason that LRU loses in this workload is that         fashion. Figure 10 gives an exampleof such a query path.
there are a significant number of queries (10%) that land in        We simulated a variety of query profiles: random,
the cold region of the relation. Such cold data is not likely    squares,and Manhattan “lollipops”. The random profile
to be accessed the near future, but it stays in the cache
                 in                                              has a fixed probability of moving in one of the four direc-
until it agesout of the LRU chain. In contrast, using the        tions. In each step, moving left, right or backward is by
Manhattandistancefunction, sucha cold rangewould loose           4 units, moving forward is by 8 units; the difference es-
its value when the next “‘hot range” query is submitted.         sentially models different speedsof motion. The square
                                                                 profile involves the query path repeatedlytraversing a fixed
                                                                 sire squarein the 2dimension space. The Manhattan Zol-
6 Mobile Navigation Application                                  Zipop profile is a squarebalancedon top of a “stick”. Each
In the previous section, we showed that semanticlocality         query path goes up the stick, traversesaround the square
can improve performanceeven in a randomizedworkload.             multiple times, goes down the stick, and then repeatsthe
In this section, we further examine the benefitsof semantic      cycle.
locality by exploring a workload that has more semantic
content than the selection-basedworkloads studied so far.        6.2 Semantic Value Function
The workload models mobile clients accessingremotely-
stored map data through a low-bandwidth wireless com-            Consider the query path in Figure 10. Using a replace-
munication network (see, e.g., [D+96]). Each tuple in the        ment policy like LRU is not very appropriatefor suchquery
databaserepresentsa road segmentin the map, and each             profiles. Assumethat when Q19 is issued, somemap data
page is a collection of such tuples. The application must        must be discardedfrom the client cache. If an LRU policy
update the map data displayed to the user at regular inter-      is used, the map data associatedwith 93 is likely to be
vals, dependingon the user’scurrent location, direction and                                                  for
                                                                 discarded,since it hasnot beenaccessed a long time. A
speedof motion.                                                  semanticcaching policy can recognize the semanticprox-
                                                                 imity of Q3 and & 19, and discard the data associatedwith
                                                                 Q9, Q 10, Q 11in preferenceto the data associated   with Q3,
6.1 Workload Speciiication
                                                                 resulting in better cacheutilization. We now describea se-
The databaseis one relation, two of whose attributes take        mantic value function, the directional Manhattan distance
values between0 and 8 191. This pair of attributes forms a       function, that maintains a single number with each seman-
densekey of the relation; there is a tuple for every possible    tic region basedon its Manhattan distance from the user’s
pair of values. These two attributes can be viewed as the        current location and direction of motion.
X and Y co-ordinatesin a 2dimensional space,The rela-                Assumethat the user’s direction of motion is the positive
tion is clusteredusing the Z-ordering [Jag901on thesetwo         X axis (for other directions of motion, the distancefunction
attributes. Each tuple is 200 bytes long.                        is defined similarly), and let p, , pr , pr and pb denote the


                                                           339
weights that model the relative importance of retaining in
the cache semantic regions that are ahead of, to the left           Size/Path          1 Dir. Manhattan1 LRU / MRU
of, to the right of, and behind the current region. Let                                        Random
(t”, y,,) be the user’s current location, and (z, y) be the          .25/.25/.25/.25    1.OO(29.4 ms)    1.06   2.24
                                                                     .33/.33/.33/.00    1.OO(42.5 ms)    1.05   1.52
centerof a semanticregion S in the cache. The replacement
                                                                     .50/.20/.20/.10    1.OO(44.6 ms)    1.03   1.38
information associatedwith S is computed as -(dpar +                                    1.00 (56.1 ms)   1.01   1.04
                                                                     .80/.10/.10/.00
dperp),where the values d,,,. (parallel distance) and dpe,.,,                                   Square
(perpendiculardistance) are defined as follows:                     32x32               2.29             9.57   1.oO(7.23ms)
       dPar    =    ifa:>t,then(l-p,)*(z-2,)                         160x 160           1.22           1.22     1.00(51.9ms)
                                                                                                 Lollipop
                                                                                         Manhattan
                    else (1 - pa) * (2” - z)                         160/32x32/1        1.86           2.02     1.00 (47.1 ms)
      dperp    =    ify>~,then(l-pl)*(~-y~)                          160/32x32/5        1DO (62.6 ms)    1.22   1.11
                                                                     160/32x32/10       1.00 (49.2 ms)   1.38   1.60
                    else (1 - zh) * (yu - Y)
                                                                     160/32x32/50       1.00 (34.9 ms)   1.69   2.54
6.3 Performance Results                                                             Table 3: Mobile Query Paths
We presenta performancecomparisonof LRU, MRU and                  times larger than the cache size. LRU and the directional
the directional Manhattan distance function for semantic          Manhattandistancefunction essentiallykeep the samedata
cachingfor various query profiles. The metric usedis aver-        in the cache,and hence they perform similarly.
ageresponsetime to answerqueriesover a sequence 500     of            For the Manhattanlollipop query path, the squaresize is
queries. We also studiedthe LRU andMRU value functions            32 x 32, andthe stick length is 160;we considereddifferent
for tuple caching; since they always do slightly worse than                                                          in
                                                                  valuesfor the numberof timesthe squareis traversed each
their semanticcounterparts,we do not discussthem further.         cycle: 1,5,10 and 50 (in this casethe query path doesnot
    A key characteristic of the query profiles we study is        completea full cycle). When the squareis traversedoncein
the possibility of loops in a query path, i.e., the user can      each cycle, the path is very regular and MRU outperforms
visit or be close to a previously visited location. When          the other approaches.When the squareis traverseda large
the query path is random and the loops are small, LRU is          number of times in each cycle, the regularity breaksdown
expectedto perform well since recent data will be retained        and MRU begins to lose. The break-evenpoint between
in the cache. When the query path is regular and the loops        MRU and the directional Manhattan distancefunction is 4
are larger, MRU is expectedto perform well, since older           rounds, and the break-evenpoint between MRU and LRU
                    to
data(guaranteed be touchedagain) will be retainedin the           is between 6 and 7 rounds. The directional Manhattan
cache. We demonstratethat, in contrastto LRU and MRU,             distance function is always better than LRU, and hence is
a value function basedon semanticdistance,performs ro-            the clear winner when the squareis traversedmany times.
bustly, acrossa wide range of loop sixes.
    We study random query paths,for four different choices
of probability values. The directional Manhattan dis-             7 Related Work
tance function is the winner, though LRU is a close sec-          Data-shipping systemshave been studied primarily in the
ond. An interesting point to note is that the directional         context of object-oriented databasesystems, and are dis-
Manhattan distance function performs substantially bet-           cussedin detail in vra96]. The tradeoffs between page
ter than MRU when the query path is totally random                caching (called page servers)and tuple caching (called ob-
(.25/.25/.25/.25). When the query path approachesa                ject servers) were initially studied in [DFMV90]. That
straight line (.80/.10/.10/.00), all approachesperform                                 the
                                                                  work demonstrated sensitivity of pagecaching to static
comparably - there is not much scope for improvement              clustering, and also the message overheadthat results from
in this ~ase.~Our results are summarizedin table 3.               sendingtuples from the serverone-at-a-time. In our imple-
    Each stepfor the squareand the Manhattan lollipop pro-        mentation of tuple caching, we took care to group tuples
files is 8 units long. The squaresixes studiedwere 32 x 32        into pagesbefore transferring them from the server.
and 160 x 160. This query profile - predictable and cyclic           Alternative approachesto making page caching less
- is ideal for MRU, which is the clear winner. The query          sensitive to static clustering have been proposed [KK94,
results for the 32 x 32 squarearejust slightly larger than the    OTS94]. These schemes,known as Dual Buffering and
cachesize. A semanticdistance function can be expected            Hybrid Caching respectively, keep a mixture of pagesand
to be useful in this case,and the directional Manhattan dis-      objects in ibe cache based on heuristics. A page is kept
tance function considerably outperforms LRU. The query            whole in the’cacheif enough of its objects are referenced,
results for the 160 x 160 square are approximately five           otherwise individual objects are extracted and placed in a
   ‘In the absenceof loops, when data is touched at most once,
                                i.e.,                             separateobject cache. These approachesaim to balance
caching is not useful. and no value function will perform well.   the tradeoff between overhead and sensitivity to cluster-


                                                              340
ing. Semanticcaching takesthe different approachof using        to alleviate this problem. In this study,we focusedon query-
predicatesto dynamically group tuples.                          intensive environments;exploring the impact of updatesis
    The caching of results basedon projections (rather than     necessaryto make these techniques applicable to a larger
selections)was studied in [CKSV86]. However, the work           classof applications. We studiedthe utility of conventional
most closely related to ours is the predicate caching ap-       value functions (e.g., LRU and MRU), as well as of some
proach of Keller and Basu [KB96], which usesa collection        semanticvalue functions (e.g., Manhattan distance and its
of possibly overlapping constraint formulas, derived from       directional variant) in traditional workloads as well as a
queries,to describeclient cachecontents. Our work differs       mobile navigation workload. Our plans for future work
from [KB96] in three significant respects.First, in [KB96]      include the further developmentof semanticvalue functions
there is no concept analogousto a semantic region. Recall       for this and other applications as well.
that maintaining semanticregions allows, in particular, the
useof sophisticatedvalue functions incorporating semantic       References
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