Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 hugeregression in performance

[Ingo Molnar]

* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Tue, 23 Mar 2010 18:34:09 +0100
> Ingo Molnar <mingo@xxxxxxx> 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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