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From: MICHELBI@oregon.uoregon.edu (Michel Biedermann)
Newsgroups: comp.sys.ibm.pc
Subject: Re: What is the best way to determine cache size?
Message-ID: <16019@oregon.uoregon.edu>
Date: 11 Feb 90 17:31:40 GMT
References: <2525@leah.Albany.Edu>
Distribution: usa
Organization: University of Oregon
Lines: 35

In article <2525@leah.Albany.Edu>, emb978@leah.Albany.Edu (Eric M. Boehm) writes:
> I currently use a disk cache on my 386/25. I had previously set it to 1
> MB. However, this is ridiculously large (so as to be counterproductive).
> I downsized it to 384 KB. This is probably still much too large. I
> received a suggestion that 64 KB is probably a better size (since that
> is the size of hardware caches on most machines with hardware caches).
> That doesn't seem like a real good reason to me.

If I read you message correctly, I think you are confusing two different types
of caches: memory versus disk cache.

Most 386 computers, in order to perform at 0-wait states without using very
fast RAM, require a small cache, usually around 64K, or super fast SRAM memory
(not DRAM).  The main difference lies in the memory access speed which is
approximately 80-90ns for DRAM versus 15-25ns for SRAM.

In the case of a disk cache, standard DRAM is used to decrease the bottleneck
between the memory and the drive, not the CPU and the memory as seen above.  In
this case, you can indeed have a disk cache as big as you can afford memory.
There is a small penalty mind you.  With very big disk caches (>1024K) if the 
information requested is not in the cache, the computer may actually lose some
time looking for it.  I have found a disk cache of 512K to be a happy medium, 
although I have occasionally used one much bigger (1024K) when I switch
between programs a lot (DeskView or OS/2).  Zenith's latest EISA controller
has a 1MB cache built on the drive controller.  This allows an apparent disk 
access time of *3-6ms* even though the drive itself is only rated at 16ms
which is standard for ESDI drives.

I hope this helps without being too technical...

Please excuse me if I mis-interpretaded you question.

Michel Biedermann	michelbi@oregon.uoregon.edu
U. of Oregon
ZENITH Student Rep.