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Soft Scheduling for Hardware

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					Task Partitioning for
Multi-Core Network
    Processors

 Rob Ennals, Richard Sharp
 Intel Research, Cambridge

 Alan Mycroft
 Programming Languages Research Group,
 University of Cambridge Computer Laboratory
Talk Overview
 Network Processors
    What they are, and why they are interesting
 Architecture Mapping Scripts (AMS)
    How to separate your high level program from low level details
 Task Pipelining
    How it can go wrong, and how to make sure it goes right
Network Processors
 Designed for high speed packet processing
    Up to 40Gb/s
    High performance per watt
    ASIC performance with CPU programmability
 Highly parallel
    Multiple programmable cores
    Specialised co-processors
    Exploit the inherent parallelism of packet processing
 Products available from many manufacturers
    Intel, Broadcom, Hifn, Freescale, EZChip, Xelerated, etc
Lots of Parallelism
 Intel IXP 2800: 16 cores, each with 8 threads
 EZChip NP-1c: 5 different types of cores
 Agere APP: several specialised cores
 FreeScale C-5: 16 cores, 5 co-processors
 Hifn 5NP4G: 16 cores
 Xelerated X10: 200 VLIW packet engines
 BroadCom BCM1480: 4 cores
Pipelined Programming Model
 Used by many NP designs

      Core           Core           Core           Core


 Packets flow between cores
 Why do this?
    Cores may have different functional units
    Cores may maintain state tables locally
    Cores may have limited code space
    Reduce contention for shared resources
    Makes it easier to preserve packet ordering
An Example: IXP2800
 16 microengine cores
    Each with 8 concurrent threads
    Each with local memory and specialised functional units
 Pipelined programming model
    Dedicated datapath between adjacent microengines
 Exposed IO Latency
    Separate operations to schedule IO, and to wait for it to finish
 No cache hierarchy
    Must manually cache data in faster memories
 Very powerful, but hard to program
           72             72         72

                 Stripe/byte align
                                                                                IXP2800

      RDRAM          RDRAM      RDRAM                    MEv2   MEv2   MEv2   MEv2
        1              2          3                       1      2      3      4

                                                                                      Rbuf         S      16b
                                                                                      64 @ 128B
                                                                                                   P
                                                                                                    I
                                                         MEv2   MEv2   MEv2   MEv2                 4
                                                          8      7      6      5                   or
                                           G                                                       C
       PCI                XScale           A                                                       S
64b
                           Core            S                                                        I
       (64b)
                          32K IC           K                                           Tbuf        X      16b
      66 MHz                               E                                          64 @ 128B
                          32K DC           T             MEv2   MEv2   MEv2   MEv2
                                                          9      10     11     12
                                                                                       Hash
                                                                                      64/48/128



                                                                                      Scratch
       QDR            QDR        QDR            QDR      MEv2   MEv2   MEv2   MEv2     16KB
      SRAM           SRAM       SRAM           SRAM
                                                          16     15     14     13     CSRs
        1              2          3              4
                                                                                      -Fast_wr -UART
                                                                                      -Timers   -GPIO
      E/D Q          E/D Q      E/D Q          E/D Q
                                                                                      -BootROM/SlowPort

      18             18         18             18
            18             18         18            18
   IXDP-2400
     Things are even harder in practice…


               IXP2400                 IXP2400


                         CSIX Fabric


Packets from                                     Packets to
  network                                         network

     Systems contain multiple NPs!
What People Do Now
 Design their programs around the architecture
    Explicitly program each microengine thread
    Explicity access low level functional units
    Manually hoist IO operations to be early

 THIS SUCKS!
    High level program gets polluted with low level details
    IO hoisting breaks modularity
    Programs are hard to understand, hard to modify, hard to write, hard
     to maintain, and hard to port to other platforms.
The PacLang Project
 Aiming to make it easier to program Network Processors
 Based around the PacLang language
    C-like syntax and semantics
    Statically allocated threads, linked by queues
    Abstracts away all low level details
 A number of interesting features
    Linear type system
    Architecture Mapping scripts (this talk)
    Various other features in progress
 A prototype implementation is available
Architecture Mapping Scripts
 Our compiler takes two files
    A high level PacLang program
    An architecture mapping script (AMS)
 PacLang program contains no low-level details
    Portable across different architectures
    Very easy to read and debug
 Low level details are all in the AMS
    Specific to a particular architecture
    Can change performance, but not semantics
    Tells the compiler how to transform the program so that it executes
     efficiently
Design Flow with an AMS

  PacLang Program                AMS




                Compiler               Refine AMS




           Analyse Performance




                    Deploy
Advantages of the AMS
Approach
 Improved code readability and portability
    The code isn’t polluted with low-level details
 Easier to get programs correct
    Correctness depends only on the PacLang program
    The AMS can change the performance, but not the semantics
 Easy exploration of optimisation choices
    You only need to modify the AMS
 Performance
    The programmer still has a lot of control over the generated code.
    No need to pass all control over to someone else’s optimiser
AMS + Optimiser = Good
 Writing an optimiser that can do everything perfectly is hard
    Network Processors are much harder to optimise for than CPUs
    More like hardware synthesis than conventional compilation
 Writing a program that applies an AMS is easier
 AMS can fill in gaps left by an optimiser
    Write an optimiser that usually does a reasonable job
    Use an AMS to deal with places where the optimiser does poorly
 Programmers like to have control
    I may know exactly how I want to map my program to hardware
    Optimisers can give unpredictable behaviour
An AMS is an addition, not an
 alternative to an automatic
          optimiser!

 This is a sufficiently important point that it is worth
  making twice
What can an AMS say?
 How to pipeline a task across multiple microengines
 What to store in each kind of memory
 When to move data between different memories
 How to represent data in memory (e.g. pack or not?)
 How to protect shared resources
 How to implement queues
 Which code should be considered the critical path
 Which code should be placed on the XScale core
 Low level details such as loop unrolling and function inlining
 Which of several alternative algorithms to use


                           And whatever else one might think of
AMS-based program pipelining
 High-level program has problem-orientated concurrency
    Division of program into tasks models the problem
    Tasks do not map directly to hardware units
 AMS transforms this to implementation-oriented concurrency
    Original tasks are split and joined to make new tasks
    New tasks map directly to hardware units


          AMS                                Hardware Task Hardware Task
                                             Hardware Task Hardware Task
   User Task                                 Hardware Task Hardware Task
                        Compiler
   User Task                                 Hardware Task Hardware Task
                                             Hardware Task Hardware Task
                                             Hardware Task Hardware Task
Task Pipelining
 Convert one repeating task into several tasks with a
  queue between them

                     A; B; C;


                                Pipeline Transform



      A;               B;                      C;
Pipelining is not always safe

 May change the behaviour of the program:
                    1,2,1,2,...


              q.enq(1); q.enq(2);


                                  Pipeline Transform


                                          Iterations of t1 get ahead of t2
                    1,1,2,2,...

                                                 Elements now written to
        q.enq(1);                 q.enq(2);        queue out of order!
   t1                       t2
Pipelining Safety is tricky (1/3)

 Concurrent tasks interact in complex ways

                 q2.enq(q1.deq);      passes values from q1 to q2



           1,1,...      1,1,2,2,...   values can appear on q2 out of
      q1                q2                        order




      q1.enq(1);        q2.enq(2);


           Pipeline split point
Pipelining Safety is tricky (2/3)

 Concurrent tasks interact in complex ways

           q1.enq(3); q2.enq(4);

      t3                             q1 says: 1,1 written before 3.
                                     q2 says: 4 written before 2.
        1,1,3,...       4,2,2,...    t4 says: 3 written before 4.
       q1              q2            unsplit task says: 2 written before 1,1.

                                     This combination not possible in
                                     the original program.
      q1.enq(1);        q2.enq(2);


           Pipeline split point
Pipelining Safety is tricky (3/3)
                 Unsafe                       Safe

             q2.enq(q1.deq);              q1.enq(q2.deq);



       1,1,...      1,1,2,2,...     1,1,2,2         2,2,...
  q1                q2            q1              q2



 q1.enq(1);         q2.enq(2);    q1.enq(1);      q2.enq(2);


       Pipeline split point          Pipeline split point
Checking Pipeline Safety
 Difficult for programmer to know if pipeline is safe
 Fortunately, our compiler checks safety
    Rejects AMS if pipelining is unsafe
 Applies a safety analysis that checks that pipelining
  cannot change observable program behaviour


 I won’t subject you to the full safety analysis now
    Read the details in the paper
Task Rearrangement in Action
                       ARP


           Classify   IP Options
  Rx                                  Tx
                         IP                  ICMP Err




                      IP Options
                        + ARP
                      +ICMP Err
       Classify
       + IP(1/3)                             Tx
 Rx

                      IP(2/3)      IP(2/3)
The PacLang Language
 High level language, abstracting all low level details
 Not IXP specific – can be targeted to any architecture
    Our toolset can also generate Click modules
 C-like, imperative language
 Static threads, connected by queues
 Advanced type system
    Linearly typed packets – allow better packet implementation
    Packet views – make it easer to work with multiple protocols
Performance
 One of the main aims of PacLang
    No feature is added to the language if it can’t be implemented
     efficiently
 PacLang programs run fast
 We have implemented a high performance IP forwarder
    It achieves 3Gb/s on a RadiSys ENP2611, IXP2400 card
        Worst case, using min-size packets
    Using a standard longest-prefix-match algorithm
    Using only 5 of the 8 available micro-engines (including drivers)
    Competitive with other IP forwarders on the same platform
Availability
 A preview release of the PacLang compiler is available
    Download it from Intel Research Cambridge, or from SourceForge
    Full source-code is available

 A research prototype, not a commercial quality product
    Runs simple demo programs
    But lacks many features that would be needed in a full product
    Not all AMS features are currently working
A Tangent: LockBend
 Abstracted Lock Optimisation for C Programs
    Take an existing C program
    Add some pragmas telling the compiler how to transform the program
     to use a different locking strategy
        Fine grained, ordered, optimistic, two phase, etc
    Compiler verifies that program semantics is preserved



LockBend Pragmas
                             Compiler              Program with Optimised
Legacy C Program                                      Locking Strategy

				
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