Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ukma!husc6!cmcl2!dasys1!jab
From: jab@dasys1.UUCP (Jeff A Bowles)
Newsgroups: comp.arch
Subject: Re: SRAM vs. DRAM, 33MHz 386 UNIX-PC
Message-ID: <10714@dasys1.UUCP>
Date: 15 Sep 89 17:36:57 GMT
References: <21936@cup.portal.com>
Reply-To: jab@dasys1.UUCP (Jeff A Bowles)
Organization: Datamerica Systems, NYC
Lines: 29

In article <21936@cup.portal.com> cliffhanger@cup.portal.com (Cliff C Heyer) writes:
> 	While multitasking, does flushing the cash waste a 
>measurable amount of run time or is it 
>insignificant compared to swapping, paging, and/or 
>other overhead? In other words, is the cache still 
>beneficial even though it is being flushed? (I 
>assume "yes" since minicomputers such as all VAX 
>models have them.)

First, the old phrase I always heard was "real memory for real
performance."  But for a machine that is(strictly speaking)
CPU-bound, the cache will help you a lot.

Say you change contexts 16 times a second, i.e. you flush the
cache 16 times in a second --- think about how much you can
do in (an average of) 1/16 of a second, on a fast CPU. On a 4 MIPS
machine, for example, that's 256K instructions at a time that run
with benefit of a cache.

All these are numbers pulled out of the air, but the upshot is that
if you are entirely CPU-bound, a cache will help; if you're spending
precious time waiting for a disk, thrashing for memory, or whatever,
the dynamic is different.

	Jeff Bowles
-- 
Jeff A Bowles
Big Electric Cat Public UNIX
..!cmcl2!{ccnysci,cucard,hombre}!dasys1!jab