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    SubjectRe: SMP performance degradation with sysbench
    On 2/27/07, Paulo Marques <> wrote:
    > Rik van Riel wrote:
    > > J.A. Magallón wrote:
    > >>[...]
    > >> Its the same to answer 4+4 queries than 8 at half the speed, isn't it ?
    > >
    > > That still doesn't fix the potential Linux problem that this
    > > benchmark identified.
    > >
    > > To clarify: I don't care as much about MySQL performance as
    > > I care about identifying and fixing this potential bug in
    > > Linux.
    > IIRC a long time ago there was a change in the scheduler to prevent a
    > low prio task running on a sibling of a hyperthreaded processor to slow
    > down a higher prio task on another sibling of the same processor.
    > Basically the scheduler would put the low prio task to sleep during an
    > adequate task slice to allow the other sibling to run at full speed for
    > a while.
    > I don't know the scheduler code well enough, but comments like this one
    > make me think that the change is still in place:


    > If that is the case, turning off CONFIG_SCHED_SMT would solve the problem.

    To chime in here, I was attempting to reproduce this on an 8-way Xeon
    box (4 dual-core). SCHED_SMT and SCHED_MC on led to scaling issues
    when above 4 threads (4 threads was the peak). To the point, where I
    couldn't break 1000 transactions per second. Turning both off (with gives much better performance through 16 threads. I am now
    running for the cases from 17 to 32 to see if I can reproduce the
    problem at hand. I'll regenerate my data and post numbers soon.

    I don't know if anyone else has those on in their kernel .config, but
    I'd suggest turning them off, as Paulo said.

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      Last update: 2007-02-27 21:45    [W:0.038 / U:1.424 seconds]
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