Date: Wed, 20 Dec 2006 15:36:25 +0100 From: Pieter de Goeje <pieter@degoeje.nl> To: freebsd-stable@freebsd.org Cc: Mark Kirkwood <markir@paradise.net.nz> Subject: Re: Cached file read performance with 6.2-PRERELEASE Message-ID: <200612201536.25497.pieter@degoeje.nl> In-Reply-To: <4589128F.9030404@paradise.net.nz> References: <45888C68.10305@paradise.net.nz> <200612200816.51043.joao@matik.com.br> <4589128F.9030404@paradise.net.nz>
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On Wednesday 20 December 2006 11:38, Mark Kirkwood wrote: > In fact if you note that the PIII HW *can* actually do 700MB/s, it > suggests that your HW is capable of considerably more than 900MB/s - > given that opteron's have excellent cpu to memory bandwidth, and the > speed of your memory! Indeed! 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 a file from cache, because I can only manage to get one fifth (~1GB/sec) of the theoretical speed. (this is with a file that fills more than half of all memory) Note that linux seems to play tricks (zero copy?) when doing dd if=/dev/zero of=/dev/null, because you can reach speeds which are way above the theoretical maximum. (30GB/sec on a P4 1,6Ghz ??? no way) In the context of databases, I think the speeds are limited by the processing done on the data, as long as the read speed stays above a certain limit. It would be more interesting to see how random access to a (cached) file performs in Linux vs FreeBSD, which seems a more logical pattern for a database. Cheers, Pieter de Goeje
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