Messages in this thread | | | Date | Tue, 13 Mar 2007 21:06:14 +1100 | From | Nick Piggin <> | Subject | Re: SMP performance degradation with sysbench |
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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 ;)
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