Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!oliveb!sun!gorodish!guy
From: guy@gorodish.Sun.COM (Guy Harris)
Newsgroups: comp.arch
Subject: Re: SPARC and multiprocessing
Message-ID: <51321@sun.uucp>
Date: 29 Apr 88 08:53:55 GMT
References: <1521@pt.cs.cmu.edu> <28200135@urbsdc> <4921@bloom-beacon.MIT.EDU> <1671@alliant.Alliant.COM>
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> 	Given that the SPARC must use a virtual cache to get optimal
> 	performance, how does one build a multiprocessor with a SPARC?

Excuse me?  Where is it "given that the SPARC must use a virtual cache to get
optimal performance?"  It is the case that the SPARC requires some form of very
fast memory to get optimal performance, but that's true of a hell of a lot of
machines these days.  The virtual cache permits you to bypass the MMU on a
cache hit, but I don't know that this is *required* for SPARC - or for any of a
number of other microprocessor architectures.

It may be the case that with the current SPARC implementations, with no on-chip
MMU, that it's easier to get high performance with a virtual cache than with a
physical cache, or even that you can't get optimal performance *for those
implementations* with a physical cache (although I suspect the latter is not
true).  This certainly doesn't say that the SPARC *architecture* requires a
virtual cache.

Another way of putting this is that I have no particular reason to believe that
all high-end SPARC machines built by Sun will have virtual caches (it is
already the case that not all SPARC machines built by Sun have virtual caches;
the 4/110 SCRAM memory acts more like a physical cache than a virtual one).