Date: Thu, 21 Dec 2006 17:22:30 +0100 (CET) From: Oliver Fromme <olli@lurza.secnetix.de> To: freebsd-stable@FreeBSD.ORG, pieter@degoeje.nl Subject: Re: Cached file read performance with 6.2-PRERELEASE Message-ID: <200612211622.kBLGMUt5093373@lurza.secnetix.de> In-Reply-To: <200612211620.03590.pieter@degoeje.nl>
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Pieter de Goeje wrote: > Mark Kirkwood wrote: > > Pieter de Goeje wrote: > > > Copying /dev/zero to /dev/null yields more than 5GB/sec on a simple 2Ghz > > > Athlon64. It imagine there are quite a few extra things done when copying > > On second thought, this is wrong because /dev/zero isn't a real block > of memory It _is_ a real block of memory. To be exact, it's called zbuf[] in src/sys/dev/null/null.c. It's the size of one page (4 KB or 8 KB, depending on architecture). When some program reads from the zero device, that block is copied repeatedly from kernel space to user space. > so these results say nothing about memory I/O speed because > all data is in (cpu) cache. That's true. The test rather benchmarks the CPU, the cache and the overhead involved when copying data between kernel and user space. Best regards Oliver -- Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing Dienstleistungen mit Schwerpunkt FreeBSD: http://www.secnetix.de/bsd Any opinions expressed in this message may be personal to the author and may not necessarily reflect the opinions of secnetix in any way.
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