Path: utzoo!attcan!telly!lethe!torsqnt!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!usc!rutgers!cmcl2!edith!edith.ultra.nyu.edu!freudent From: freudent@eric.nyu.edu (Eric Freudenthal) Newsgroups: comp.arch Subject: Re: Software modularity vs. instruction locality Message-ID: Date: 16 Nov 89 13:33:31 GMT References: <17707@watdragon.waterloo.edu> <23604@cup.portal.com> <6374@dime.cs.umass.edu> <3887@aplcen.apl.jhu.edu> Sender: news@edith.ultra.nyu.edu Distribution: na Organization: New York University, Ultracomputer project Lines: 42 In-reply-to: mef@aplcen.apl.jhu.edu's message of 15 Nov 89 16:10:18 GMT In <1989Nov4.004529.10049@ico.isc.com> Dick Dunn wrote: > Second, I would expect better locality > for code reference than for data reference, hence the I cache ought to do > more good than the D cache. Aren't the pathological cache-busting programs > generally ones which spray data accesses all over the place? No. That is typical of pathological demand-paged-memory-busting techniques. The reason is that typically caches consist of a large set of realatively short entries (typically a few-to-several bytes) as compared to demand-paged-memory which consists of a set of LONG entry-measured in kilobytes. A truly random cache access pattern is pretty good because the lookup scheme is usually a hash function for which it is easy to generate bad access patterns (typically (addr >> m)). Furthermore, the amount of memory moved (therefore the time involved with moving) for a single cache-update is not very great, so if the extra stuff is never used, very little effort was wasted. In comparison a single page-in takes so much time, and moves so much data, that random addressing patterns will cause pathologically bad behavior. Caches, however are only useful if the same data is used repetitively before it is flushed. This seems to be more an issue of algorithm design than programming style. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Eric Freudenthal NYU Ultracompter Lab 715 Broadway, 10th floor New York, NY 10012 Phone:(212) 998-3345 work (718) 789-4486 home Email:freudent@ultra.nyu.edu -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Eric Freudenthal NYU Ultracompter Lab 715 Broadway, 10th floor New York, NY 10012 Phone:(212) 998-3345 work (718) 789-4486 home Email:freudent@ultra.nyu.edu