ALFRED
P.
SLOAN SCHOOL OF MANAGEMENT
CSVZY
The Two Sides of Time Sharin/?
Martin Greenberger
Sloan School of Management and Project MAC
Massachusetts Institute of Technology
127-65
MASSACHUSETTS
INSTITUTE OF TECHNOLOGY
50 MEMORIAL DRIVE
CAMBRIDGE, MASSACHUSETTS
02139
.
C8V£Y
The Two Sides of Time Sharing
Martin Creenberger
Sloan School of Management and Project MAC
Massachusetts Institute of Technology
127-65
Based on material
presented to the IFIP Congress 65.
Work reported herein was supported (in part) by Project MAC,
an
M.I.T.
research
program sponsored by the Advanced
under
Research
Department of Defense,
Projects
Agency,
Office of Naval
Research Contract Number Nonr-U 102( 01)
permitted for any
is
Reproduction in whole or
in
part
purpose of the United States Covernment.
Development of the OPS system was a cooperative undertaking
with Anthony Horry , Malcolm Jones, James Morris, David Ness,
Mayer Wantman, and Stephen Whitelaw,
li.l-feS'
PACF
2
Introduction
1.
There
is
more than one way to hlsect the new suhject of
time sharing into contrasting
aspects.
example /
For
could view the rood and the had of it, a dichotomy
guaranteed
provoke
to
lively
discussions.
little
like
Harvard
the
three-year olds.
sweatshirts
and goals, hut come too early in the
sra
on
see
is
these
They
we
They focus attention on
that
Rut
discussions, although healthy, are premature.
we
important
a
some
issues
developmental
process
to he very meaningful,
My hi sect ion is not going to he into the pood
and
had , hut rather into the system and the user , hoth
we consider good for now.
and
Mo
I
user
,T,'s
physical
is
reflected
Project MAT:
tool
or
distinction
The
Mul
t
i
expresses the hopes of the uspr.
present In the phrase on-
1
i
ne
rp?*
1
-1
the
distinction
suhject
of
the
to
ime
Real
.
while
-1
of
an
of
also
is
depicts
Ime
descrihes
on-1 ne
?
The distinction may even
within the scheduling algorithm
which happens to he
acronym
Machine-Aided Cognition
The
the performance of the processor,
the status of the user.
system
-Access Computer refers
whereas
systpm,
whom
of
hetween
douhle-edged
the
in
the
found
he
on-line
operation,
recent
operations
a
research paper (11),
Speaking of operations research, there was
article hy Heorges Brigham puhlished ahout
the Journal
(
2
)
„
It
of the Operations Research
10
a
very
years
Society
of
ago
fine
in
America
won the I.anchester Prize for the hest 0/R paper
of
.
the year.
factory,
of
and
a
congestion problem
contained an interesting
It
counter.
tool
a
dealt with
It
The
analysis
aircraft
In an
queueing
viewed
analysis
service
the
operation at the tool counter mathematically, first from the
side of the tool clerk or server,
mechanic or user.
This
then from the siHe of
approach
gives
duals.
in
mathematical
two
problems that are completely equivalent.
the
We might call
them
Unless we change the statement of service objectives
passing from one side of the counter to the
other,
both
problems yield identical solutions.
The point
is,
of course,
that
service
the
objectives
normally are different on the two sides of the counter.
server and the user look at the operation through
pi
The
different
asses
Behind the tool counter the clerk moves as
possible.
quickly
as
He strives for efficiency and dispatch to prevent
conpestion on the other side.
In
his
he may fail
haste,
notice which customer has been waiting the
to
This
longest.
oversight does not affect average customer wait, so long
no one starts a brawl
show
a
from impatience.
slight preference for customers
v/ho
wish
tools, a bias that actually reduces average
free periods,
the clerk nibbles
which he tries to
get
done
away
without
at
front of the counter,
back to his job.
also
fewest
During
background
neglecting
the mechanic
His foreman may he
the
wait.
important service operation in the foreground
In
may
The clerk
tasks
more
the
,
is
marking
as
eaper to
time,
and
get
a
costly machine may be sitting idle in need of repairs.
Me
is
miffed at havinp to wait lonper than the next fellow.
is
upset hy
having
accept
to
sinple-head wrench, or
a
Stillson
a
from
available
counter.
the
tools
the
all
a
the
If
would
needs
he
without
counter,
one
of
bolt that turns out to be the wronr
size and requires him to return to
mechanic had his way,
instead
He
waiting,
be
error,
replacement, discussion, or red tape.
The sets of objectives
necessarily contradictory.
of
On the
contrary,
objectives usually are fashioned so as
user, but typically the average user
users,
rather than the
?
nd yi dual
?
tonl-counter
phenomenon
is
familiar
in
the
aggrepate
the
and
a
closed shop, while the
of
two-sided
same
the
fiel^.
computer
conventional computer center the operations manaper
for greater equipment efficiency
the
We really did not need
since
example,
server's
the
accommodate
to
or
not
are
user.
Now let's talk about computers.
the
system
and
user
throuph
programmer
response time and closer touch wi th his run.
a
strives
processing
batch
lonps
In
faster
for
This state
of
affairs has lead to conflict, and this conflict has been the
sinple most important motive for
Time sharing is definitely
sharing.
user and
a
development
the
a
concession
recognition of his point of view.
of
to
time
the
PAGF
5
Time-shar ng Supervisors
2.
i
The first order of business in making time sharing work
on a meaningful
scale was to solve
the
technical
occasioned by this new mode of operation.
bui
1
t
problems
Supervisors
were
to:
Accept
input
continuously from the remote
terminals of a number of simultaneous users,
a.
Parcel out successive pieces of the computer's
time to users equitably,
b.
f
Schedule this time so that the computer seemed
immediately responsive to most requests,
c.
damage
Protect the programs of each user from
by an errant neighbor,
d.
e.
Manage input-output and queues of interrupts.
Transfer programs flexibly between primary and
f.
secondary storage,
g.
Maintain data files,
Provide error detection
unauthorized use, and
h.
and
security
I.
Allow batch
processing to continue
background, concurrently with operation
remote terminals in the foreground.
Programming
the
supervisors
to
in
of
these
solve
from
the
the
techniral
problems were major undertakings, partly because the jor had
not been done
before,
and
available at the time were
partly
not
because
designed
to
the
do
computers
the
joh.
PAGF
Emphasis was p]aced on the server side of
6
operation
the
getting the processor to time share.
A
few time sharing supervisors
with
limited
horizons
did cross over to the user's side (15,18), while the
and broader supervisors
means
others
for
do
to
provided
(3,17)
so,
supervisor at Project MAP,
is
e
PTSS,
framework
a
writing
variety of languages,
and
systems
base,
programs
compiling
including FORTRAN, MAD,
applications,
such
hand
be
can
as
tailored
engineering
user
3.
'
s
a
sense
a
design
fulfillment of
this
particular
stress
(16),
in
a
and
From
of
Fach
CTSS
in
ALGOL,
for
analysis (1), and symbol manipulation (21).
systems is in
Users
(U).
There are flexible facilities for editing.
FAP.
sharinr
modular set of
subroutines that encourages further evolution
may add to CTSS by
the
an*-'
time
the
structured as
bip-ger
a
these
special
domai n.
User Systems
The development of time sharing supervisors has been an
approach to man-machine operation from the server's side
the
counter.
from the
In
user's
have
contrast, certain systems
side
These
(5,13,20).
problem context.
Sharing
prerequisite or Initial
time
concern,
sense as an afterthought, and some
among
users
although
of
these
approached
were
systems
designed to facilitate man-machine interaction in
it
of
a
specific
was
not
a
makes
good
systems
have
PAPF
since moved in that
which
one
is
interactive
direction.
of
earliest
the
user
Sketchpad,
systems,
most
and
heinp
is
for
7
example/
vivid
of
implemented
the
an
in
extended 3-dimens ional form on the Project MAC computer.
The full
power of an on-line system is attained when the
user is able to shift flexibly anH
easily
variety
without
of
different
activities
set-up costs or requiring
awkward
between
incurring
adjustments.
computer can do many wondrous things, as we
all
this versatility does not benefit most users
placed at their fingertips.
average
The
have to carry his water to the faucet.
wMe
a
rh
raw
know,
but
it
is
should
user
i
The
unless
system
The
h
not
should
take care of this for him.
Specifically, it should be convenient for
the
on-line
uspt to regulate echo checking, make
calculations,
display
intermediate results, obtain helpful
puidelines,
key variables.
He shoulH have the ability
move
to
view
and
easily
between program modification and execution without the
for
recompi lat on
i
,
He
should
subroutine by simply referring to
he
it
abl^
treat the compound
as
he
features are important
to
him
mathematical problem, building
a
whether
the
he
such
These
solving
is
a
simulation model, designing
man-machine decision procedure, constructing
ng-ponp:i ng among several
subroutines,
parts.
operation, or simply doing some data analysis.
pi
a
symbolically, alter its
treats
a
execute
to
arguments without fuss, compound it with other
and
need
activities,
a
real-time
And if he is
the
value
of
PAHF
mu
1
1
?
pi
i
provided
when
features,
these
hy
single
a
system,
these features (8,9,10).
multi-purpose to
It
is
an on-line
the individual
user,
uses CTSS and is a natural
An
111
that
is
as
CTSS
is
The
system
OPS
extension of it.
ustration
Suppose we have the followinr prohlem to
processor
single
prnv'de
to
system
just
multi-purpose to the community of users.
.
shares
time
round-rohin scheduling
procedure
N
using
terminals,
with
a
Quantum
one
request
has
to wait,
on the average, for values of N ranging from
we also want to know how much hatch processing
in
a
of
we want to know how long a one-second
accomplished
our
solve:
second;
50;
is
ed.
The OPS system at Project MAC was developed
k
8
the background during periods when
1
to
can
be
none
of
the N terminals is requesting service.
For
equal
N
connected, we can
to
number
the
answer
these
of
terminals
questions
time-sharing operation introspective;
that
gather statistics about itself.
we
device
in
general, unless we
operation to
a
are
But
willing
stream of perturbations in
making
by
is,
N,
the
by having it
cannot
to
currently
use
this
subject
The
the
public
relations side of such an approach to the prohlem gives even
the most callous administrator substantial
reason to pause.
Simulation
is
a
more feasible tack, as any pood student
of the art will
hasten
simulation does
not
tell
to
disturb
rhanrinp
us,
clientele,
the
in
N
a
can
be
is
to
Ry virtue
of
and
accomplished with great dispatch,
still more elepant solution
A
develop
a
queueinp model with
parameter.
as
N
problem
the
to
suitable simplifying assumptions, tbp mo^pl heromps
difference equations.
to
A
set of
a
compact algorithm can be propremmp^
solve the equations recursively.
patberinp
The benefits and drawbacks of empirical data
vs.
simulation
documented,
mathematical
vs.
analysis
well
arr>
What we would really likp to be able to do
little of all
three, back and
forth,
a
gradually
our
until
is
desired
increasing comprehension of the problem becomes the
sol ut ion,
This
over
iterative process can expand
several
weeks
or
into
months
even
production-oriented natch setting-
a
in
Consider
sequence required to propram the algorithm;
statistical analyses necessary for estimating
and
reducing data to
combine
all
that
probability
classical
with
rp.ppptpr':
simulation and contemplation of
i
how this process can be very costly in time.
the process
requires
a
flexible
simulation
Simscript, pood statistical subroutines,
multiple repression, debuppinp aids,
traditional
a
the
debugging
add to this
the
reiat ionsbips
distributions;
complications
results;
t'>
series of runs
^nd
In
of
the
you
see
addition,
like
system,
like Thl-square
like traces and
an^'
dumps,
PACF 10
a
desk calculator, graphical displays, and so on.
In
a
t
ime-shar ng environment/ we can hope for
match of
computer
process.
Indeed,
in
with
the
creative
on-line
system
facilities
provides
that
We
hope
can
with
us
motivations
the
for
an
variety
of
need, and allows us to switch hack
v/e
debugging,
between
prohl pn-sol vi ng
or
this has been one of the main
the development of time sharing.
forth
and
modeling,
calculating,
better
a
i
simulating,
modifying, displays, and analysis, with minimal
effort
and
expense.
The OPS system has these features, and brings us closer
to the
day
when
the
total
process
describing might occupy an able researcher
have
been
lonper
than
we
that
no
one interesting afternoon at his terminal.
JL,
The OPS System
The OPS system
is
a
multi-purpose
with on-line facilities for:
calculations;
statistical
analyses
without
need
for
graduated
entails minimal set-up costs, giving
Subroutines
the
user
added
arc
to
adapt
or
great
is
levels
simple
system
with
to
his
of
and
flexibility
subtracted
the
instant
incremental
Switching among facilities
user interaction.
allowing
multiple
as
compilation;
modeling of computer simulations; and
use.
such
vector
and
FORTRAN and MAO type programming with
regressions;
execution
matrix
symbolic
apparatus
modular
of
ease,
own
;
PA^F 11
speci f! cat ions
on
1
I
.
Information about the system
ne.
The OPS
system
has
recently
available command at Project MAO.
and these will
made
been
giving
In
the user can specify operators stored
OPS
available
Is
In
a
publicly
the
command,
file,
his personal
be loaded with the standard operators of
the
precompiled
RSS
Operators
system.
subroutines.
They
can
arc.
be
simply
loaded
load
at
time,
dynamically any time thereafter.
By means of the OPS system,
1.
Pall
the user can:
operators by name from the console;
Compound operators into MAn or FORTRAN
2.
like
type programs that may themselves be called
operators
3.
Fxecute (or test) while compounding;
k.
Compound (or remember) while executing;
compound operators and their
Fdit
5.
parameters either from the console or from a
compound operator;
6.
Refer symbolically to common
storage
by
cell or array name-
Dynamically modify the mapping
of
common storage at execution time;
contents
7.
Add operators without
them Into core as needed;
8.
limit
level
the
Intensify or reduce
9.
interaction by means of switch settings.
and
of
and
bring
user
or
PAPF 12
Standard Operators
6.
Among the standard operators that cone with the
system
are:
1.
SFT, which
like
is
general
the
statement of FORTRAN and MAD, except that
assignment
(=)
symhols
can
its
denote complete matrices and vectors/ as well
elements.
as
Thus, executing the operator
SFT
may cause
20
x
a
=
n
20
A
20
x
+
R
LOn. (C)
*
matrix
A
to he added
product of
to the
20 scalar matrix (whose elements ara all
equal
matrix
need
lop of the constant T)„
stored
explicitly.
scalar
The
SFT
also
provides
rire
referred
to
by
the
be
of
parenthesis
following
the
in
the
matrix
Flements
customarv
Pimensions may he symbolic, as
notation.
not
general
multiplication and transposition of matrices,
matrices
to
a
example:
SFT n(l,J)
2.
IF
and
I
FR
the MAD conditional
=
(pronounced IF-R), which
statement,
upon the truth value of
3.
a
and cause
a
are>
similar
branch
to
depending
Roolean proposition.
REPFAT and GOTO, which allow for looping,
and unconditional
k*
A(I,C)/SQRT.(B(C,J))
indexing,
transfers.
TYPF, which permits symbolic entry or print out
unformated information to or from common storage.
of
e
PAT
5.
SAVFC and LOADT, (pronounced
SAVF-C
which save and retrieve Information hetween
and
13
LOAD-C),
common
storape
and disc,
6.
SAVFS,
LOADS,
ENTERS, FRASFS,
RFSFTS,
PRINTS
and
(prounouncd SAVF-S etc), which save, retrieve, modify,
print the symhol
7.
SAVFK,
tahle.
LOADK,
FO TK
I
PRINTK, FNTFRK, FRASFK, NAMEK,
,
CALLK, RFTRNK, and TAKFP, which save,
delete,
and
initiate,
execute,
rename,
name,
retrieve, edit, print,
terminate
and
compound operators, and distribute their parameters.
which
TFXT,
8.
prints
prespec'fie^
out
textual
information durinp execution.
9.
a
FIT, which performs
dependent variable on
multiple linear repression of
a
set of independent variables.
a
For
exampl
FIT
fits
in
TO XI X2
Y
X3
the observations stored
vectors
XI,
specified, and
X2, and X3.
a
in
the vector Y to those stored
A
vector
of
weights
may
vector containing the residuals of fit
be
may
be created,
10.
RSPHFO,
SCHFO,
OALLRK, and RFTPNA,
operators
for
form
which
on-line
PANTFL,
DFLAY,
a
model inp
LOCAL,
us'np
simulation
of
packapp
en
OALL^K,
apen^a
that
schedules actions and events.
11.
FORMAT,
INPUT,
and
OUTPUT,
which
arbitrarily formated readinp and printinp in
FORTRAN.
make
the
possible
sense
of
.
PAHF 1U
which
VIEW,
12.
pives
snapshot
a
system
key
of
variables for error checkinp.
FRROR, which
13.
diagnostic
provides
information
on
common user mistakes,
CTSS, which allows execution of
lit.
HTSS
any
command
from within the OPS system.
nuiDF, which provides
15.
information
descriptive
on
the OPS system.
DRAW,
16.
exponential,
d
i
which
furnishes
normal,
draw
random
a
thp
probability
rectangular
or
from
str but ion
7.
i
The Fut u re
Future time sharing supprvisors will
havinp
systems
flexibly
modules,
correct
many
a
processors
i
nterconnectpd
computer
on
run
many
.
These
systpms
number of current technical deficiencies
sharing operation.
They
will
for
provide
in
thp
allocation of storape throuph pape turninp, so that
will
not have to specify or
data requirements
when
ComputerLand they will
he
load
bepins
permit
his
all
the
to
assembled
as
pure
identified sepments of memory.
time
Hynamic
a
uspr
en rl
with
the
interact
sharinp
procedures
will
propramminr
by
different
users, not only of processors and storape, but also
packapes
memory
active
and
in
propram
symbolically
.
PARF 15
advances
These
server's
the
on
side
There is still much work to he done
operation.
it will
are
the
there,
and
Progress will also
henefit the user directly.
on the user's side in the form of more natural
much wider range of terminal
of
come
languages,
improvements
equipment/
a
in
graphical and audio input-output facilities, sn^ bigger
an^
better OPS-type systems.
The situation today in computation
after the
turn
of
electricity (12).
ahout what
the
in
it
was
distribution
of
The first electric service
City illuminated the
Fdison
early
bulbs,
light
York
New
in
an^'
did
toaster,
or
There was plenty of skepticism about the benefits
of
little else,
iron.
century
the
is
except run an occasional elevator,
distributing electricity
in
those days-
Fach electric light
installation was required by law to be accompanied by
a
light alongside to cover what was at first considered to
the very substantia^
likelihood of failure.
out, very few of the gas
Much
progress
As
it
gas
be
turned
lights ever had to be used.
been
has
maHe
design
the
in
of
the
system
developments are dwarfed by what has happened on the
user's
television,
hi-fi,
distribution lines and turbines since then, but
s
i
rie=-househol d
appliances,
communications, heating systems,
radio,
air
cond
i
t
i
op nr,
?
...
,
computers
challenge
in
designing better computer system configurations, as well
as
Today
the
there
is
great
intellectual
programming concepts and software required to
make
the
PACF 16
systems work as Intended
(and,
h
field are dedicated to the task.
programming receives
is
s
tor cal
I
Many of the finest minds
than intended).
objective
!
temporarily dominated by
even
y,
and
a
better
computer
the
in
Project
At
priority,
top
1
MAC,
system
twofold
the
MAC
concerted effort
to
foster and implement the utility concept.
The broad system poal of Project MAT may he reparded as
the development and operation of
community utility
a
computer
that is capable of supplying
each
power
to
customer where, when, and in the amount needed, (7)
Work also
is
in
side of the counter,
of examples.
In
progress at Project MAC on
and OPS
is
only one of
a
magnitude
user's
larre
numhpr
facilities
cominp years we may expect
users to expand dramatically in
the
for
importance,
ar\^
just as applications of electricity have expanded during the
past several decades.
More and more
and creative energy will
scientific
brainpower
shift in this direction.
The shift has just hep:un.
underway for some time will
it
Only
after
it
really be meaningful
has
been
to
down and talk about the good and the bad of time sharing.
sit
.
PAGE 17
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