Re: [tbench regression fixes]: digging out smelly deadmen.

Nick Piggin
2008-10-29T10:00:07Z

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.