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From: LLACROIX@carleton.EDU.UUCP
Newsgroups: comp.os.vms
Subject: re: file system caches
Message-ID: <8707072246.AA11663@ucbvax.Berkeley.EDU>
Date: Mon, 6-Jul-87 23:14:00 EDT
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Posted: Mon Jul  6 23:14:00 1987
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(Gerard K. Newman recently asked why his directory cache performance
decreased when the cache size was _increased_.)

There is probably a good, understandable reason for this, and I'll let
someone else give the "correct" answer :-)  Off the top of my head,
I have two comments which may be relevant.

First, how controled was your experiment when you were measuring the
performance?  If you just monitored normal activity, the activity mix 
may have changed and _that_ could explain the reduced cache performance.

Second, and much more obscure, it is known that certain cache algorithms
(such as FIFO) _can_ have decreased performance for a fixed sequence
of requests when the cache size in increased.  I don't know the details
but I have seen a small example, considering cache sizes of 3 and 4.
Intuitively, I would expect the chances of running into this in the real-life
case of your directory cache to be extremely small.  Intuitively, I wouldn't
have expected the anomaly to exist, either.

- Les LaCroix (csnet: llacroix@carleton.edu)