Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!uw-june!david
From: david@cs.washington.edu (David Callahan)
Newsgroups: comp.arch
Subject: Re: SRAM vs. DRAM, 33MHz 386 UNIX-PC
Message-ID: <9149@june.cs.washington.edu>
Date: 9 Sep 89 15:04:41 GMT
References: <21936@cup.portal.com> <1082@cernvax.UUCP>
Reply-To: david@june.cs.washington.edu.cs.washington.edu (David Callahan)
Organization: Tera Computer Co., Seattle WA
Lines: 21

In article <1082@cernvax.UUCP> hjm@cernvax.UUCP (Hubert Matthews) writes:
>  Why do Crays have fast memories?
>Because [long vector] is exactly the type of problems they are designed for.

Actually the memories on Cray's are commodity parts and hence are not
that fast. The Cray-2 came out with 120ns DRAMS and later with 55ns
SRAMS which is the context of this discussion of 33-50MHz parts don't
seem that fast. Further, the nominal 55ns is only part of latency of
the memory access; the change from 120ns DRAMS to 55ns SRAMS showed
only a 13% improvement in some benchmarks [reported in Supercomputing
88 by Simmons & Wasserman, I think].

Cray's cope with long memory latencies by using vector loads,
effectively pipelining memory. So you were right about these machines
being designed for vector operations but really a stronger statement
could be made: Cray machines require vector operations to acheive
anywhere near peak performance. 

-- 
David Callahan  (david@tera.com, david@june.cs.washington.edu,david@rice.edu)
Tera Computer Co. 	400 North 34th Street  		Seattle WA, 98103