Messages in this thread | | | From | Nick Piggin <> | Subject | Re: [tbench regression fixes]: digging out smelly deadmen. | Date | Wed, 29 Oct 2008 20:59:37 +1100 |
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On Tuesday 28 October 2008 05:33, Ingo Molnar wrote: > * Alan Cox <alan@lxorguk.ukuu.org.uk> wrote: > > > 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 ;-) > > > [...] 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.
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