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From: johnl@ima.UUCP (John R. Levine)
Newsgroups: comp.arch
Subject: Re: caches and the kernel
Message-ID: <450@ima.UUCP>
Date: Mon, 19-Jan-87 11:32:09 EST
Article-I.D.: ima.450
Posted: Mon Jan 19 11:32:09 1987
Date-Received: Tue, 20-Jan-87 00:32:23 EST
References: <14362@amdcad.UUCP>
Reply-To: johnl@ima.UUCP (John R. Levine)
Organization: Javelin Software Corporation
Lines: 21

In article <14362@amdcad.UUCP> phil@amdcad.UUCP (Phil Ngai) writes:
>Several people have told me that ... it
>does seem to be the case that the Unix kernel does not interact with
>caches as well as applications such as nroff. This led me to wonder if
>in that case, one might not do well to turn off the cache while the
>processor is in supervisor mode ...

That's probably not such a great idea.  Even kernel code contains loops, and
I'd expect there to be a huge performance loss if kernel code wasn't cached.
Perhaps you'd be better off leaving instruction cacheing on but turning data
cacheing off.  Or maybe have a separate small I cache for the kernel, so that
when you return to the user, in the presumably common case that the user program
will continue doing what it did before, its caches are still set up.

By the way, has anybody looked at the relative cost of reloading the memory
cache vs. reloading the address translation cache across system calls or
process switches?
-- 
John R. Levine, Javelin Software Corp., Cambridge MA +1 617 494 1400
{ ihnp4 | decvax | cbosgd | harvard | yale }!ima!johnl, Levine@YALE.something
Where is Richard Nixon now that we need him?