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From: gkn@SDS.SDSC.EDU
Newsgroups: comp.os.vms
Subject: RE: File cache hit rates (2nd try)
Message-ID: <870722202753.003@M5.Sdsc.Edu>
Date: Wed, 22-Jul-87 16:27:52 EDT
Article-I.D.: M5.870722202753.003
Posted: Wed Jul 22 16:27:52 1987
Date-Received: Sat, 25-Jul-87 05:01:37 EDT
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	From:	 
	Subject: File cache hit rates (2nd try)
	Date:	 Tue, 21 Jul 87 14:15 CDT
 
	    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.
 
I used AUTOGEN, but it did not see fit to increase the amount of paged
pool that I had, so I told it to.  My file system caches have started
working.

Swimming in pool,

gkn
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	San Diego Supercomputer Center
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