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    SubjectRe: [patch 1/3] slub: add per-cache slab thrash ratio
    On Mon, Mar 30, 2009 at 10:11:31AM +0300, Pekka Enberg wrote:
    > On Sun, 2009-03-29 at 22:43 -0700, David Rientjes wrote:
    > > The slab_thrash_ratio for each cache do not have non-zero defaults
    > > (yet?).
    > If we're going to merge this code, I think it would be better to put a
    > non-zero default there; otherwise we won't be able to hit potential
    > performance regressions or bugs. Furthermore, the optimization is not
    > very useful on large scale if people need to enable it themselves.
    > Maybe stick 20 there and run tbench, sysbench, et al to see if it makes
    > a difference? I'm cc'ing Mel in case he has some suggestions how to test
    > it.

    netperf and tbench will both pound the sl*b allocator far more than sysbench
    will in my opinion although I don't have figures on-hand to back that up. In
    the case of netperf, it might be particular obvious if the client is on one
    CPU and the server on another because I believe that means all allocs happen
    on one CPU and all frees on another.

    I have a vague concern that such a tunable needs to exist at all though
    and wonder what workloads it can hurt when set to 20 for example versus any
    other value.

    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab

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