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    SubjectRe: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge regression in performance

    * Andrew Morton <akpm@linux-foundation.org> wrote:

    > On Tue, 23 Mar 2010 18:34:09 +0100
    > Ingo Molnar <mingo@elte.hu> wrote:
    >
    > >
    > > It shows a very brutal amount of page fault invoked mmap_sem spinning
    > > overhead.
    > >
    >
    > Yes. Note that we fall off a cliff at nine threads on a 16-way. As soon as
    > a core gets two threads scheduled onto it?

    it's AMD Opterons so no SMT.

    My (wild) guess would be that 8 cpus can still do cacheline ping-pong
    reasonably efficiently, but it starts breaking down very seriously with 9 or
    more cores bouncing the same single cache-line.

    Breakdowns in scalability are usually very non-linear, for hardware and
    software reasons. '8 threads' sounds like a hw limit to me. From the scheduler
    POV there's no big difference between 8 or 9 CPUs used [this is non-HT] - with
    8 or 7 cores still idle.

    Ingo


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