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

    * Alan Cox <> 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).

    The moment there's real IO it becomes harder to analyze but the same
    basic behavior remains: the more unfair the IO scheduler, the "better"
    dbench results we get.


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      Last update: 2008-10-27 19:35    [W:0.039 / U:35.156 seconds]
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