Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!oliveb!sun!ember!dre
From: dre%ember@Sun.COM (David Emberson)
Newsgroups: comp.arch
Subject: Re: Cray & Amdahl (Was: missing Dhrystone 2.1)
Summary: VM, Cray
Message-ID: <60952@sun.uucp>
Date: 22 Jul 88 01:43:06 GMT
References: <4232@cbmvax.UUCP> <76700035@p.cs.uiuc.edu> <5342@june.cs.washington.edu>
Sender: news@sun.uucp
Lines: 16

> 
> Relevant (really?) question: Does it make more sense to buy a little
> bit of very fast memory and slow it down with virtual memory, or to
> buy a whole bunch of fast physical memory and slow it down by putting
> it farther away?  (Assume: $ is no problem).

If money is no object, it makes sense to buy a ton of memory AND have VM.
This may be apocryphal, but I have been told that Seymour Cray, replying to
the question of why the Cray 1 did not have virtual memory, replied, "Because
I don't understand it."

With virtual caches, VM does not cause a performance penalty worth mentioning.
Even on some machines with physical caches, address translation can take place
in parallel with the cache tag access--thus no penalty.

			Dave Emberson (dre@sun.com)