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From: ferrick@acsu.buffalo.edu (Patrick K. Ferrick)
Newsgroups: comp.unix.bsd
Subject: RAM cache strategies
Message-ID: <Br6ME7.9LM@acsu.buffalo.edu>
Date: 10 Jul 92 16:42:06 GMT
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In the July issue of PC magazine, there was an article discussing, among
other things, RAM cache strategies. I'll mention some of the major points in
the hope that discussion on the net may help those of us about to upgrade our
motherboards to run 386BSD.
Aside from the size of the cache itself, two factors are important:
- organization
- write policy
Organization: direct mapped or set-associative
------------
* direct mapped assigns one line of cache memory (16 bytes) to each block,
and is apparently much easier to design and build
* set-associative assigns multiple lines to each block; typical designs
assign either two or four lines per block (two or four-way associative)
According to the article, set-associative caches work much better with
multi-tasking OSs because multiple processes often reference completely
different blocks of memory. A direct mapped cache suffers from "thrashing",
or repeated cache misses/rewrites, under similar circumstances. Uncool.
Write policy: write through, buffered write, or write back
------------
* write through cache ignores write operations, passing them directly through
to main memory
* buffered write delays writes to memory until the CPU moves to a non-memory
operation. Then buffer contents are written to memory.
* write-back handles both reads and writes
Again, according to the article write through is the simplest (cheapest, most
popular) method. It sounds to me like 386BSD would benefit from a more
sophisticated cache scheme, but which one?
It would seem that a set-associative, write-back cache (optimum?) would only
be found on "top-of-the-line" boards from the big manufacturers. However,
I've read the specs on Micronics motherboards, and find them to utilize a
direct-mapped, write-through RAM cache. So the questions are:
1. Does anyone have an opinion regarding the optimum 386BSD cache?
2. What board makers offer such a cache?
3. Is anyone using such a motherboard for 386BSD?
just wondering,
pat