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From: darrell@sdcsvax.UUCP (Darrell Long)
Newsgroups: net.arch
Subject: Re: not missing anything
Message-ID: <1537@sdcsvax.UUCP>
Date: Tue, 11-Mar-86 23:07:41 EST
Article-I.D.: sdcsvax.1537
Posted: Tue Mar 11 23:07:41 1986
Date-Received: Fri, 14-Mar-86 04:41:59 EST
References: <145@pyramid.UUCP>
Reply-To: darrell@sdcsvax.UUCP (Darrell Long)
Organization: EECS Dept. U.C. San Diego
Lines: 34

In article <145@pyramid.UUCP> dan@.UUCP (Dan Sobottka) writes:
>
>First of all, the problem of multiprogramming effects on caches are common
>to ANY cached machine (RISC or CISC or whatever).  Secondly, (as far as
>'defending' caches) the problem can be reduced greatly: (1) the on-chip
>cache is not likely to be that large.  Even if we flushed the entire on-chip
>cache after every context switch, it wouldn't be any big deal (how often
>are users switched anyway? Couldn't be more than a few hundred times a
>second).  (2) in slightly more elaborate schemes (commonly used for large
>off-chip caches) each cache line's TAG could contain the entire process
>ID and other info, so that a context switch would not require flushing
>of the cache.  Of course, dilution still occurs because of the room the
>old (say, 'context A') cache lines occupied, but in many cases, after
>switching back to context A (from wherever) many of those lines will still
>be there!! Indeed, the more frequent the context switches , the more likely
>it is that those lines will be there, somewhat negating the other degradations
>due to switches.
>Please please not let me be missing something!!!!!!!!!!!!!!!!!! :-)
>
>       sun!pyramid!dan

I must be missing something!  If you are caching PHYSICAL addresses,
I do not see any reason to flush the cache.  In a virtual memory
system then the TLB must be flushed since it contains virtual addresses,
but the cache should not have to be flushed (in a base & bound register
environment the physical address could be compared to the bound register
to prevent illegal access).
-- 
Darrell Long
Department of Electrical Engineering and Computer Science
University of California, San Diego

UUCP: sdcsvax!darrell
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