Re: [BENCHMARKS] 2.6.4 vs 2.6.4-mm1

[Nick Piggin]

Ingo Molnar wrote:

> * Nick Piggin <piggin@xxxxxxxxxxxxxxx> wrote:
>
>
> > volanomark (MPS):
> > This one starts getting huge mmap_sem contention at 150+ coming
> > from futexes. Don't know what is taking the mmap_sem for writing.
> > Maybe just brk or mmap.
>
> are you sure it's down_write() contention? down_read() can create
> contention just as much, simply due to the fact that hundreds of threads
> and a dozen CPUs are pounding in on the same poor lock.

No I'm not sure actually, it could be just read lock
contention. IIRC it was all coming from the semaphore's
spinlock, in up_read...

> i do think there should be a rw-semaphore variant that is per-cpu for
> the read path. (This would also fix the 4:4 threading overhead.)

That would be interesting, yes. I have (somewhere) a patch
that wakes up the semaphore's waiters outside its spinlock.
I think that only gave about 5% or so improvement though.

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