On Tuesday 28 October 2008 05:33, Ingo Molnar wrote:

* Alan Cox [off-list ref] wrote:

quoted

quoted

The way to get the best possible dbench numbers in CPU-bound dbench
runs, you have to throw away the scheduler completely, and do this
instead:

 - first execute all requests of client 1
 - then execute all requests of client 2
 ....
 - execute all requests of client N

Rubbish. [...]

i've actually implemented that about a decade ago: i've tracked down
what makes dbench tick, i've implemented the kernel heuristics for it
to make dbench scale linearly with the number of clients - just to be
shot down by Linus about my utter rubbish approach ;-)

quoted

[...] If you do that you'll not get enough I/O in parallel to
schedule the disk well (not that most of our I/O schedulers are
doing the job well, and the vm writeback threads then mess it up and
the lack of Arjans ioprio fixes then totally screw you) </rant>

the best dbench results come from systems that have enough RAM to
cache the full working set, and a filesystem intelligent enough to not
insert bogus IO serialization cycles (ext3 is not such a filesystem).

You can get good dbench results come from dbench on tmpfs, which
exercises the vm vfs scheduler etc without IO or filesystems.