Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!amdahl!nsc!glennw
From: glennw@nsc.nsc.com (Glenn Weinberg)
Newsgroups: comp.arch
Subject: Re: Sw vs. Hw BitBlit. (Really: Purpose of Cache)
Summary: Caches are there to cut bus traffic too
Keywords: cache, bus, bandwidth
Message-ID: <5326@nsc.nsc.com>
Date: 1 Aug 88 17:31:22 GMT
References: <399@ma.diab.se> <1988Jul28.173301.7275@utzoo.uucp>
Reply-To: glennw@nsc.UUCP (Glenn Weinberg)
Organization: National Semiconductor, Sunnyvale
Lines: 28

In article <1988Jul28.173301.7275@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>In article <399@ma.diab.se> pf@ma.UUCP (Per Fogelstr|m) writes:
>>Assuming we have a fast micro (an 68030 or a NS32535) the would at least be
>>supported by their on chip caches. Even if the hitrate in theese caches are
>>as low as 50% an external hardware Blitter could use the other 50% ...
>
>You miss an important point:  those caches are not there to free up external
>memory cycles, they are there to help slow memory keep up with a fast CPU.
>It's not at all inconceivable to get 50% cache hits (which is low for an
>instruction cache but good for a tiny data cache like the 030's) *and*
>complete saturation of the external memory bandwidth, when one of those
>CPUs gets going.

It is not true that the sole purpose of cache is to help slow memory keep
up with fast processors.  In multiprocessors, in particular, one of the
most important functions of caches (and especially copy-back caches) is to
reduce bus traffic and so allow a relatively slow bus to support a number
of fast processors.

Within the next five years, this "relatively slow bus" will need a
bandwidth of multiple hundreds of Megabytes per second (you read that right--
the bus will need a bandwidth of several Gigabits per second) in order to
support a multiprocessor system made up of, say, 8-16 50-MIPS processors.
And the only way that you limit yourself to "only" needing hundreds of
Megabytes per second is by using copy-back caches.

-- 
Glenn Weinberg					Email: glennw@nsc.nsc.com
National Semiconductor Corporation		Phone: (408) 721-8102
(My opinions are strictly my own, but you can borrow them if you want.)