Path: utzoo!attcan!uunet!mcvax!dik
From: dik@cwi.nl (Dik T. Winter)
Newsgroups: comp.arch
Subject: Re: Cray & Amdahl (Really: VM on vector processors) (Was: ...)
Message-ID: <7589@boring.cwi.nl>
Date: 22 Jul 88 19:47:03 GMT
References: <4232@cbmvax.UUCP> <76700035@p.cs.uiuc.edu> <9a0K/cbluk1010IHSPc@amdahl.uts.amdahl.com> <228@sdeggo.UUCP> <5342@june.cs.washington.edu> <7588@boring.cwi.nl> <12174@ames.arc.nasa.gov>
Organization: CWI, Amsterdam
Lines: 39

In article <12174@ames.arc.nasa.gov> lamaster@ames.arc.nasa.gov.UUCP (Hugh LaMaster) writes:
 > In article <7588@boring.cwi.nl> dik@cwi.nl (Dik T. Winter) writes:
 > >the instruction.  That takes a lot of time.  The 205 has two different
 > >page sizes, large pages of 65536 words (8 bytes/word) and small pages
 > >of (site selectable) 512, 2048 or 8192 words.  The number of
 > >associative registers is 16, and these are shared amongst the jobs on
 > >a system.  It appears that the selection of small page size is very
 > >critical.  I have a small (~10 lines) program that will run 2 times
 > 
 > The Cyber 205 does have a "problem" because of its memory to memory
 > vector instruction set, as opposed to Cray's vector registers.  The
 > problem is/was vector startup time was fairly long on the Cyber 205.
 > This problem appears to have been solved on the ETA-10 with better
 > overlap, etc., and short vector performance seems to be much better.
 > (Still the same memory to memory architecture.)  The "problem" with
 > small page sizes is actually an installation option intended to benefit
 > installations with a relatively small amount of main memory.  The
 > solution, if you have more memory, is to use a larger small page size.
 > 
Well, let me also disagree.  The factor of 2 I mentioned was from memory,
and not substantiated by fact; it is more like a factor of 30.  Anyhow,
what we experienced when the 205 was installed next door (1 Mword of memory,
small page size 512 words):  our program which was consuming lots of time
on the 205 (it used in total something like 1 CP-month) was that it ran
twice as fast when small page size was increased to 2048.  The original
runs gave no indication at all that something was wrong; we had typically
something like 250 page faults in a 1 hour run.  The problem was, the
program fitted in memory (with lots of memory to spare), but there were
not enough associative registers to cope with it, so every vector
instruction was interrupted at least 3 times for every 512 elements.
That is quite a lot if you know that vector startup time is 50 to 70 cycles.
The main problem is of course not enough associative registers (you ought
to have enough to address all of real memory).  My estimate, from experience,
is, on a 1-pipe 205 set the small page size to at least 2048, on a 2-pipe
205 use 8192; and what on a 4-pipe 205?
-- 
dik t. winter, cwi, amsterdam, nederland
INTERNET   : dik@cwi.nl
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