*BSD News Article 99075


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From: bde@zeta.org.au (Bruce Evans)
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: Re: dd `benchmark'
Date: 6 Jul 1997 09:51:31 +1000
Organization: Kralizec Internet
Lines: 24
Message-ID: <5pmmm3$8fa$1@godzilla.zeta.org.au>
References: <u7wwn5jrs0.fsf_-_@japonica.csl.sri.com> <5pm7gn$ds6$1@flea.best.net>
NNTP-Posting-Host: godzilla.zeta.org.au
Xref: euryale.cc.adfa.oz.au comp.unix.bsd.freebsd.misc:43997

In article <5pm7gn$ds6$1@flea.best.net>,
Matt Dillon <dillon@flea.best.net> wrote:
>:In article <u7wwn5jrs0.fsf_-_@japonica.csl.sri.com>,
>:Fred Gilham  <gilham@japonica.csl.sri.com> wrote:

>:>dd if=/dev/zero of=/dev/null bs=1m count=1000
>:>
>:>as a memory system benchmark.  I'm very curious if anyone knows
>:>exactly what this measures.  The reason is that I have results I find
>:>curious.

>    Basically all this measures is the size of your cache, which you
>    know anyway:

Nope.   Under FreeBSD, this measures something closely related to the
main memory write bandwidth if bs > size of L2 cache, and something not
so closely related to L2 memory write bandwidth if bs is about half the
size of the L2 cache.  Under other systems, it is meaningless unless
you know the implementation of /dev/zero and /dev/null.

P5's have about twice the effective main memory bandwidth as P6's at
the same (memory) clock speed, because P6's do write allocation.
-- 
Bruce Evans  bde@zeta.org.au