Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uwm.edu!cs.utexas.edu!execu!sequoia!helps!wf-aus!gustwick From: gustwick@wf-aus.cactus.org (Bob Gustwick ) Newsgroups: comp.unix.i386 Subject: Re: ESDI caching disk controllers: Reprise Keywords: ESDI cache Message-ID: Date: 6 May 90 17:22:50 GMT References: <4180@infmx.UUCP> Organization: W.F. Associates, Austin, Texas Lines: 17 In article <1736@serene.UUCP> rfarris@serene.UU.NET (Rick Farris) writes: >In article <1424@ssbn.WLK.COM> bill@ssbn.WLK.COM (Bill Kennedy) writes: > >> With a 768K cache memory on one card I routinely get 32% cache hits >> and with 4Mb on the one in ssbn I get 48-52% cache hits. > >That brings up an interesting point. I think I remember learning >that without a cache-hit rate of 90% or better, you're generally >better off without a cache, due to the overhead involved in cache >misses. this is really an implementation question. a cache miss should be a fast search thru some sort of table/tree/etc. a 50/50 hit/miss ratio should still provide a great performance increase. why? a search thru memory is orders of magnitudes cheaper in wall clock time than a disk seek, unless the cache is implemented very poorly.