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Title
.AU
Martin Guy
Chris Downey
May 1988
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This document makes some observations about the loads experienced on
the undergraduate teaching machines over the last year, with a view
to possible upgrades to them in critical areas.
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.SH
Requirements for undergraduate teaching
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Here follows some criteria which should be met by any future plans
for the undergraduate teaching machines,
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Number of simultaneous users
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We currently allow up to 32 simultaneous logins on each machine.
With this many users doing non-trivial work, the Mk0 Orions slow down to a
such a degree that students are forced to work late at night, and in the
day, there are upwards of ten processes competing for the processor, with
no idle cycles at all.
When the machine is working under these conditions, it is into the part
of the multitasking against performance graph where total throughput is
falling, and we have seen short peaks in activity cause a backlog of
CPU work of about 20 processes which takes a quarter of a hour to clear.
.NH
Disk size
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The size of the user partition of the first year machine is 70 megabytes,
and that of each of the second and third year machines is 300 megabytes.
With a small amount of encouragement for the users to have a periodic 
clean out, and keeping tabs on very heavy users, these disks do not fill up.
It appears that the maxim
.ft I
disk usage expands to fill the space available
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does not hold, and that we currently have enough by a comfortable margin.
.NH
Primary memory
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Tricky to estimate, but we have experience of 4, 6 and 8 megabyte
Mk0 Orions and an eight megabyte clipper.
For one year of the course, four megabytes was not enough - the machine
used to thrash during the day.
Six megabytes was just enough, and eight is comfortable.
.LP
A fixed amount of the physical memory is reserved for the kernel.
This amount is 1 meg of a 4 meg Mk0 Orion, 1.4 meg on an 8 meg Orion.
(The size of the disk buffer cache expands according to available memory.)
On a clipper, the disk cache is in separate memory from the prescribed 8 meg,
so the kernel only consumes 0.85 megabytes.
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Clipper memory comes in units of eight megabytes.
When Raven (our test clipper) is idling, it has four and a half megabytes of
this on its free lists for user processes.
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For example, let us calculate the amount of memory required by a
first year class of 25 using a visual editor.
A typical visual editor (\fIvi\fP) needs 100K of resident data, and we add
on 50K for their shell (which we don't want to have swapped out).
This makes 3750K.
.NH
CPU speed
.LP
During the day, the processors on the teaching machines are busy for 100
per cent of the time because people are always waiting for it, and a backlog
of work forms.
.LP
Recently, as third year projects were getting into the implementation and
on-line documentation phases, we avoided logging on to the third year
machine at all because the loadings were so high.
A student had to wait for a quarter of an hour for the text editor to read in
a medium size file that would normally take a matter of seconds.
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The teaching machines can currently just cope.
A faster CPU would not only answer the loading problems, but would largely
remove them because the adverse affects which accompany very high CPU loads
would not be present.
.SH
Thoughts on merging two Mk0 Orions into one clipper
.LP