Cached file read performance with 6.2-PRERELEASE

[Pieter de Goeje]

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