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					SPECIAL REPORT                                                          fore important to the user. Because the computer is owned
                                                                        and supported as an installation, the incremental cost of
                                                                        running it is very low, and no use charge need be levied on
                                                                        users. We need only to control their relative priority of
A Futures Market in                                                        Reserving Computer Time. Users wishing to sign up in
Computer Time                                                           advance for computer time do so by marking the period of
                                                                        time they wish to use and their initials on a scheduling
                                                                        sheet posted in the computer room. With each such bid,
I. E. SUTHERLAiND                                                       the user must indicate the price he is willing to pay to
Harvard University, * Cambridge, Massachusetts                          reserve the block of time in an arbitrary unit of currency
                                                                        called " y e n . " A user may sign up for a period of two hours
                                                                        from 1:00 to 3:00 in the afternoon by drawing a line in
An auction method is described for allocating computer time             that time period, writing his initials, and indicating for
that allows the price of computer time to fluctuate with the            example, 3 yen as his bid. Users may preempt time already
demand and the relative priority of users to be controlled so           bid provided they are wining to pay a higher rate for the
that more important projects get better access. This auction is         time in yen per hour. To preempt a bid, one writes the
free of the periodic fluctuation in computer use often associated       new bid above it on the schedule. At nine o'clock each
with monthly time allocation schemes.                                   morning, the schedule for the next day is frozen. Thus
                                                                        users know at least a day in advance which of their bids
KEY WORDS AND PHRASES: computer scheduling, auction, time allocation,
                                                                        have been successful. A recorded telephone message is
  operating efficiency
                                                                        provided to inform users of successful bids.
                                                                           In actual practice users soon learn the going price to
                                                                        reserve time on the computer and how to bid accordingly.
  Introduction. At Harvard University users bid in a
                                                                        If a user wants to be sure that his bid will not be pre-
continuous auction to reserve time on the PDP-1 com-
                                                                        empted, he merely bids somewhat higher than the typical
puter. The auction permits even the most impoverished
                                                                        rate for that time of day.
user to have access to the computer if no one else wants it.
                                                                           Different amounts of currency (yen) are assigned to
Assignment of different amounts of bidding currency to
                                                                        different users according to the importance of their projects.
different users provides control of their relative priority of
                                                                        A faculty member or a graduate student in the final stages
access. The bidding currency for the auction, unlike real
                                                                        of completing his thesis, for example, will be assigned 10
money, reverts to the bidder when he uses the time for
                                                                        yen. A student with no particular project in mind, who
which he bid. This automatic return of currency elimi-
                                                                        merely wants access to the computer to learn about its
nates the need to issue new time allocations periodically.
                                                                        use, will be assigned i yen. Project employees with modest
   Very High Computer Utilization. We have found that                    but regular needs for computer time will be assigned 5 yen.
under the auction system described here our computer                     Maintenance and special demonstration functions are
utilization is very high. A user's access is limited only by             assigned a large supply of yen.
the real limit of the inelastic cost installation: the de-                  The key to the success of the auction system is that yen
mands of its other users. Users feel free to use the com-                cannot be consumed. Unlike ordinary dollars, yen revert
puter whenever it is available because their bidding cur-                to the bidder as soon as he consumes the computer time
rency will be returned to them. The computer is never                    for which he bid or whenever his bid is preempted. Yen
idle, as often happens under other allocation schemes,                   are merely tokens with which to reserve computer time in
merely because everyone has used up his current monthly                  advance. They are assigned to a user for the duration of
allotment of time. If the computer ever is idle, its price               his project. Because a user's purchasing power does not
automatically becomes attractively low. I believe that this              decrease through use of the computer, the continuous
auction system could be applied with advantage to sched-                 auction system is entirely free of the feast and famine
ule other pieces of equipment to which many users need                   problems associated with periodic allocations of computer
access for varying periods of time.                                      time. The absurd phenomenon of the computer sitting idle
   The PDP-1 computer scheduled by auction is used by                    at the end of the month merely because all users have con-
individual students and faculty members for experiments                  sumed their quota of time is entirely absent. You can't
which run from 15 minutes to several hours in length.                    use up your yen.
Because direct interaction between a user and his program
                                                                           Rules for the Bidding. The rules which implement
is usually essential, users are expected to run the computer
                                                                        the auction system are fairly simple. The major rule
themselves. Reserving time at a convenient hour is there-
                                                                        is that a user's outstanding bids for future computer time
                                                                        must not exceed his allocation of yen. A user may preempt
* Aiken Computation Laboratory                                          any bid, including his own, by bidding at a higher rate in

V o l u m e 11 / N u m b e r 6 / J u n e , 1968                                               Communications     o f t h e ACM     449
                                                       PDP -1    SOHEDULE
                       ! ~) ~ l C
                            ) ~i-IC
                       - r-   IX-It
                       ) C) 0 1 (       (



       .......    J~

      mTfT                     ii
                               I   |


                                                                                                                       YELLOW 4YEN
                                                                                                                       ORANGE 3,YEN
                                                                                                                       RED    2 YEN
                                                                                                                       13ROWN I YEN
                                                                FIG. 1

yen per hour. A user m a y cancel any of his own bids and                   Users are allowed to indicate a preference for certain
use the yen elsewhere, but he m a y neither lower a bid nor              portions of a block of time by bidding the block at several
rebid on time he has canceled.                                           different rates. T h e rule which permits this declares t h a t
   Three additional rules provide for continuity of time                 bids by a single user for consecutive blocks of time are like
and assure users of blocks of reasonable length. First, it is            a single block which can be preempted only at the begin-
always permissible to preempt an entire block of time, but               ning or the end. Thus a user can guard the beginning of a
if a user chooses to preempt some p a r t of a block, he must            three-hour block b y bidding 3 yen for the first hour, 2 yen
take either the beginning or the end. This assures the                   for the second hour, and 1 yen for the final hour. A pro-
previous bidder t h a t his block of time will not be divided.           tective bid can be placed at the beginning and the end of
This first rule does not a p p l y to blocks six hours or more           a block of time. For example, a user m a y sign for several
long. Second, the effective bidding rate for the remaining               hours with a single yen, but protect this low bid by bidding
period of a partly preempted bid is raised because all of the            high for an adjacent half hour at each end. Such a bid will
yen are considered to be concentrated in the smaller re-                 stand longer than if the same number of yen had been
maining section. Third, if a user preempts part of a block               uniformly distributed, b u t when it falls at one end, the
of time which was an hour or more in length, the remaining               center will vanish entirely.
part must be at least an hour long. This rule assures a                     There is an element of gamesmanship associated with
user who needs an extended period of computer time t h a t               making bids. The ability to protect a low bid with ad-
he will get at least an hour if he gets any time at all.                 jacent high bids is the maior tool, but the other rules can
                                                                         be used too. For example, instead of bidding for an hour
   Additional Bidding Rules. There are several rules                     at a flat rate of 2 yen, which can be preempted b y a simple
designed to simplify the accounting for the auction. Users               3-yeIl bid, one should bid 1 yen for 15 minutes and 1 yen
m a y sign for and use time only with their own yen. Yen                 for the remaining 45 minutes. Such a bid can only be
for muRiperson projects are assigned to the individuals                  preempted b y using 4 yen, 2 for each section, because the
rather t h a n the project to avoid issuing undue preemption              entire hour must be taken if any of it is preempted and
power. Bids m u s t be in integer numbers of yen. There is               the new bid m u s t be at a higher rate than the previous bid.
no m i n i m u m rate, but the minimum bid is for 1 yen,
whatever the period. Bids must begin and end at regular                    The Scheduling Board. The computer schedule is
quarter hours.                                                           maintained on a continuous chart posted in the com-

450              Communications        o f t h e ACM                                               V o l u m e 11 / N u m b e r 6 / J u n e , 1968
purer room. (See Figure 1.) This chart consists of a                                  not consume their yen, they feel free to use the computer
long roll of transparent paper over fixed time and day                                whenever it is available. As a result the computer is rarely
rulings. Users mark their bids on the transparent paper                               shut off.
with colored felt-tipped pens. The color indicates the
                                                                                         Controlling Priority. The relative priority of different
number of yen bid. The transparent paper is moved up
                                                                                      users can easily be controlled by issuing them more or less
each day so that the face of the chart always presents the
                                                                                      yen. A new user can simply be issued a number of yen
next two weeks. The chart is ruled so that a single day runs
                                                                                      appropriate to the importance of his project, because the
horizontally in a strip 30 inches long and 2 inches high.
                                                                                      inflation caused by issuing new yen will raise the price of
Fifteen-minute time periods are shown. Horizontal lines
                                                                                      computer time slightly, automatically taking computer
within each day strip provide for six preemptions, which
                                                                                      availability away from the other users. The system is
seems to be adequate. On only one occasion have all six
                                                                                      self-policing because users can easily see when a competitor
preemption spaces been filled. Rarely are there more than
                                                                                      has committed more yen than his allocation.
three bids for a single block of time.
   Experience has shown that about half the yen available                                Acknowledgments. The continuous auction scheme for
for bidding are posted as bids on the schedule at any one                             computer scheduling described here is a combination of an
time. With 167 yen issued among 30 users, the price of                                auction scheme devised by Charles Blair of the Depart-
time varies from about 3 yen per hour during peak day-                                ment of Defense and a method devised by Lawrence G.
time use to less than 1 yen per hour at night. The price of                           Roberts for allocating time on the TX-2 computer at
time is a strong function of the demand for computer                                  M I T ' s Lincoln Laboratory. I have combined Blair's
time. At the end of the semester, when computer projects                              auction technique with Roberts' idea of limiting the sign-
are due, the price of computing time rises noticeably.                                up power of the individual user. I am also indebted to
   This auction scheme for allocating computer time works                             Professor Steven A. Coons of M I T and Mr. Harold
better than any other I have used. Although users com-                                Schwenk (now our most aggressive preemptor) for the
 plain when their bids are preempted, they are generally                              considerable help they provided as the continuous auction
glad to have a choice between short periods of expensive                              scheme was being defined. The modest expenses for sched-
 prime time and long periods of time at less desirable peri-                          uling board, colored pens, and preparation of this paper
 ods of the day. Because users know that their time m a y be                          were supported by a contract from the Bell Telephone
 preempted, they are less prone to complain when their                                Laboratories, Inc. The PDP-1 to which the auction scheme
time is preempted for maintenance. Because users can-                                 is applied is supported by ARPA under contract SD-265.

                                        "COBOL           EXTENSIONS TO HANDLE DATA B A S E S "

                                   Prepared b y the D a t a Base Task Group of the COBOL Committee

                                                  A Special Joint Issue of A C M S I G P L A N Notices
                                                           and A CM S[GBDT Newsletter

                                                   Available from: Association for Computing Machinery,
                                                           211 East 43 Street, New York, N.Y. 10017
                                Price per single copy, $2.00; in quantities of 50 or more (shipped to the same address), $1.00 per copy.

V o l u m e 11 / N u m b e r 6 / J u n e , 1968                                                                     Communications         o f t h e ACM   451