Cached file read performance with 6.2-PRERELEASE

Mark Kirkwood markir at
Thu Dec 21 10:20:47 PST 2006

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!



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