Re: SMP performance degradation with sysbench

Nick Piggin
2007-03-13T10:06:26Z

Andrea Arcangeli wrote:
> On Tue, Mar 13, 2007 at 04:11:02PM +1100, Nick Piggin wrote:
>
>>Hi Anton,
>>
>>Very cool. Yeah I had come to the conclusion that it wasn't a kernel
>>issue, and basically was afraid to look into userspace ;)
>
>
> btw, regardless of what glibc is doing, still the cpu shouldn't go
> idle IMHO. Even if we're overscheduling and trashing over the mmap_sem
> with threads (no idea if other OS schedules the task away when they
> find the other cpu in the mmap critical section), or if we've
> overscheduling with futex locking, the cpu usage should remain 100%
> system time in the worst case. The only explanation for going idle
> legitimately could be on HT cpus where HT may hurt more than help but
> on real multicore it shouldn't happen.
>

Well ignoring the HT issue, I was seeing lots of idle time simply
because userspace could not keep up enough load to the scheduler.
There simply were fewer runnable tasks than CPU cores.

But it wasn't a case of all CPUs going idle, just most of them ;)

--
SUSE Labs, Novell Inc.
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