Date: Thu, 21 Dec 2006 10:20:33 +1300 From: Mark Kirkwood <markir@paradise.net.nz> To: Pieter de Goeje <pieter@degoeje.nl> Cc: freebsd-stable@freebsd.org Subject: Re: Cached file read performance with 6.2-PRERELEASE Message-ID: <4589A921.90002@paradise.net.nz> In-Reply-To: <200612201536.25497.pieter@degoeje.nl> References: <45888C68.10305@paradise.net.nz> <200612200816.51043.joao@matik.com.br> <4589128F.9030404@paradise.net.nz> <200612201536.25497.pieter@degoeje.nl>
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Pieter de Goeje wrote: > 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. > Yeah - typically it is creating tuples out of the blocks/pages just read, so for a big memory scan CPU appears to be the limiting factor! > 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. > Agreed, and good point, I'll knock up a simple program to do random and/or sequential access of a file and see what we get! Cheers Mark
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