On Wednesday 20 December 2006 22:20, Mark Kirkwood wrote: > 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
On second thought, this is wrong because /dev/zero isn't a real block of memory so these results say nothing about memory I/O speed because all data is in (cpu) cache. > > 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! I'll check 'em out :) Cheers, Pieter _______________________________________________ freebsd-stable@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-stable To unsubscribe, send any mail to "[EMAIL PROTECTED]"
