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    SubjectRe: [RFC][PATCH 00/26] sched/numa

    * Christoph Lameter <cl@linux.com> wrote:

    > On Mon, 19 Mar 2012, Ingo Molnar wrote:
    >
    > > > I wonder how we can verify that the automatic migration
    > > > schemes are a real benefit to the application? We have a
    > > > history of developing a kernel that decreases in
    > > > performance as development proceeds. How can we make sure
    > > > that these schemes are actually beneficial overall for all
    > > > loads and do not cause regressions elsewhere? [...]
    > >
    > > The usual way?
    >
    > Which is merge after a couple of benchmarks and then deal with
    > the regressions for a couple of years?
    >
    > [...]

    No, and I gave you my answer:

    > Obviously any such scheme must be a win in general for it to be
    > default on. We don't have the numbers to justify that - and I'm
    > sceptical whether it will be possible, but I'm willing to be
    > surprised.
    >
    > I'm especially sceptical since most mainstream NUMA systems tend
    > to have a low NUMA factor. Thus the actual cost of being NUMA is
    > pretty low.
    >
    > That having said PeterZ's numbers showed some pretty good
    > improvement for the streams workload:
    >
    > before: 512.8M
    > after: 615.7M
    >
    > i.e. a +20% improvement on a not very heavily NUMA box.
    >
    > That kind of raw speedup of a CPU execution workload like
    > streams is definitely not something to ignore out of hand. *IF*
    > there is a good automatism that can activate it for the apps
    > that are very likely to benefit from it then we can possibly do
    > it.
    >
    > But a lot more measurements have to be done, and I'd be also
    > very interested in the areas that regress.
    >
    > Otherwise, if no robust automation is possible, it will have to
    > be opt-in, on a per app basis, with both programmatic and
    > sysadmin knobs available. (who will hopefully make use if it...)
    >
    > That's the best we can do I think.

    Thanks,

    Ingo


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