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From: KIERSTEI@UMKCVAX1.BITNET
Newsgroups: comp.os.vms
Subject: File cache hit rates (2nd try)
Message-ID: <8707222332.AA10394@ucbvax.Berkeley.EDU>
Date: Tue, 21-Jul-87 15:15:00 EDT
Article-I.D.: ucbvax.8707222332.AA10394
Posted: Tue Jul 21 15:15:00 1987
Date-Received: Fri, 24-Jul-87 07:00:51 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
Lines: 32


    I've been watching my file system caches lately, and have noticed that the
    hit rate on all of them was quite low.  So, I increased the sizes of the
    caches and things worked well for the directory index (ACP_DINDXCACHE),
    file extent (ACP_EXTCACHE), file ID (ACP_FIDCACHE), file header
    (ACP_HDRCACHE), and bitmap (ACP_MAPCACHE) caches.

    The hit rates on the directory FCB (ACP_SYSACC), and the directory data
    (ACP_DIRCACHE) caches were still low, so I increased the sizes on those
    again.

    When I did this, the hit rate went *down*;  I mean _through the floor_.
    So, I've shrunken the caches back a bit, and I'm back to where I was
    before.

    Am I misunderstanding something here?  In general, the larger a cache
    is the better the chances that you'll find something in it.  This doesn't
    seem to be the case with at least some of the XQP caches.

I had the same problem when I raised the header cache.  At the time, I
remembered a note on Info-vax that mentioned something about some cache they
were working with was actually set to a lower default value after they raised
it due to not enough pool space to accomodate the increase.  I then did the
parameter change in autogen, which promptly raised my paged pool.  The
parameter change then increased the hit rate, rather than lowering it.  One
note:  If your paged pool increases in size, increase sysmwcnt to keep the
system paging at the previous level.

If anyone has a more detailed explanation, let us know.

                                  Barry Kierstein
                                  Kierstei@Umkcvax1.Bitnet