Messages in this thread | | | Date | Tue, 23 Mar 2010 19:27:35 +0100 | From | Ingo Molnar <> | Subject | Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge regression in performance |
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* 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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