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Newsgroups: comp.unix.bsd.freebsd.misc Path: euryale.cc.adfa.oz.au!newshost.anu.edu.au!harbinger.cc.monash.edu.au!news.bhp.com.au!mel.dit.csiro.au!munnari.OZ.AU!news.mel.connect.com.au!news.mira.net.au!vic.news.telstra.net!act.news.telstra.net!psgrain!usenet.eel.ufl.edu!newsfeed.internetmci.com!howland.reston.ans.net!psinntp!psinntp!psinntp!usenet2.news.uk.psi.net!uknet!usenet1.news.uk.psi.net!uknet!uknet!newsfeed.ed.ac.uk!edcogsci!richard From: richard@cogsci.ed.ac.uk (Richard Tobin) Subject: Re: FreeBSD memory requirement X-Nntp-Posting-Host: pitcairn Message-ID: <DtAw58.IMp@cogsci.ed.ac.uk> Sender: cnews@cogsci.ed.ac.uk (C News Software) Organization: HCRC, University of Edinburgh References: <Pine.SOL.3.93.960619222235.20516E-100000@bmec.hscbklyn.edu> Date: Thu, 20 Jun 1996 13:29:32 GMT Lines: 48 In article <Pine.SOL.3.93.960619222235.20516E-100000@bmec.hscbklyn.edu> David Zakai <zakaid99@hscbklyn.edu> writes: >Although I understand that FreeBSD will run with less memory, >I am considering adding more (total 12 Meg or 20 Meg RAM). >A recent email message indicated that more memory may not >improve performance greatly because of hardware cache size. >I don't understand this concept. I thought that the >main RAM memory could be used for cache. There are several types of caching at work in your PC. There is the processor's cache, which allows it to access instructions and data faster than if it were in plain RAM. Then there is the operating system's disk cache, which is RAM used to store data from the disk. You can also regard the whole of RAM as a cache for the virtual memory, though if you find that idea confusing just forget it. If you have so little main memory that your programs are continually needing to page, performance will be poor. If you're running X and, say, a few xterms and emacs then you will probably need more than 8 MBytes. 16 MBytes should be fine. More than 20 is likely to be unnecessary unless you're running some big applications. Unix has traditionally allocated a proportion of RAM, decided at boot time, to the disk cache (known as the buffer cache). It scales the amount according to the total RAM available. Several recent unixes, including BSD, don't use a fixed amount for this but in effect adjust it according to what is running. So if you're only running a small program that accesses lots of disk data, it will use most of your RAM as disk cache. As for the processor's memory cache, adding more will improve performance, but if you're short of main memory (ie you're paging) then the effect will be completely swamped by the paging overhead. It's certainly not the case that more memory will be useless because of a shortage of cache. More cache wouldn't hurt, but how much difference it makes will depend a lot on your applications. In summary, yes, get more memory, and don't worry about the cache. [Perhaps you're thinking of a recent thread about someone whose machine went faster when he removed some memory, but that was a very special case only applying to ancient PCs with memory on expansion cards. If your machine takes SIMMs on the motherboard, it's not relevant.] -- Richard -- Don't worry, it's only Usenet.