Spinlock thrashing with libthr

[Kris Kennaway]

On Fri, Apr 14, 2006 at 06:43:18PM -0400, Kris Kennaway wrote:
> I ran a modified version of the thr1 stress test on a quad amd64,
> which spawns 256 threads (using libthr), each of which loops doing
> getpid() 10000 times.  The system spends essentially 100% of the time
> in the kernel.  Profiling with hwpmc shows that the system is spending
> about 60% time in _mtx_lock_spin().

Turns out this is because of contention for the PROC_LOCK, which means
lots of processes sleeping.  Using getuid() instead (which doesn't
acquire locks) does not show contention.  Thanks to rwatson for
figuring it out.

Kris
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