Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!lll-tis!helios.ee.lbl.gov!nosc!ucsd!ucsdhub!esosun!seismo!uunet!sco!seanf
From: seanf@sco.COM (Sean Fagan)
Newsgroups: comp.arch
Subject: Re: Cray & Amdahl (Really: VM on vector processors) (Was: ...)
Message-ID: <412@scolex>
Date: 23 Jul 88 19:55:35 GMT
References: <4232@cbmvax.UUCP> <76700035@p.cs.uiuc.edu> <9a0K/cbluk1010IHSPc@amdahl.uts.amdahl.com> <228@sdeggo.UUCP> <5342@june.cs.washing
Reply-To: seanf@sco.COM (Sean Fagan)
Organization: The Santa Cruz Operation, Inc.
Lines: 37

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:
>
>It is true that Cray vector instructions are atomic, and those on the
>205 are restartable, but the context is saved quite efficiently on the
>205.  A complete context switch actually takes fewer cycles on the 205
>than it does on the Cray 1/X/Y-MP's, and many fewer than the Cray-2.

My $0.02 worth:  by a complete context switch, I assume you mean something
which will save the vector registers?  The Cray, when it does a context
switch, will save 24 registers (8 address, 8 index/offset, 8 data), plus the
program counter, the starting address of relative word 0, and the limit of
the programs size (incidently, this is, for some unkwon reason, very similar
to the CDC Cyber 170 machines context switch 8-)).  It does not save vectors
(understandable), which the operating system must then do (if it feels the
need; if all it's doing is OS type work, then there is probably no reason to
save them).  Since storing things is so *slow* (relatively speaking), you
try to avoid memory like the plague.  Again, incidently, if Seymour designed
the Cray 1 et al as he did the Cybers, when the machine does a context
switch, the hardware starts storing the exchange package (described above)
and then reading it, at the same time (the cybers had a long wire into which
the signal would go; since there was some travel time, it could safely read
into the registers without worrying about whether or not the values were
done being saved), causing *very* fast context switches (without vector
registers, of course).

As a result, there are two tradeoffs between the two architectures:  Crays
use vector registers, which are a pain to load and store, but very fast for
multiple operations (and very RISCy, of course 8-)), while the 205's (and
ETA's) allow for larger vectors, which somewhat faster memory access.

>  Hugh LaMaster, m/s 233-9,  UUCP ames!lamaster

-- 
Sean Eric Fagan  |  "An Amdahl mainframe is lots faster than a Vax 750."
seanf@sco.UUCP   |      -- Charles Simmons (chuck@amdahl.uts.amdahl.com)
(408) 458-1422   | Any opinions expressed are my own, not my employers'.