On Tue, 3 Apr 2007 14:49:48 -0700
Andrew Morton <[EMAIL PROTECTED]> wrote:
> > int
> > main (void)
> > {
> > pthread_t th[32];
> > int i;
> > for (i = 0; i < 32; i++)
> > if (pthread_create (&th[i], NULL, tf, NULL))
> > exit (4);
> > for (i = 0; i < 32; i++)
> > pthread_join (th[i], NULL);
> > return 0;
> > }
> >
>
> whee. 135,000 context switches/sec on a slow 2-way. mmap_sem, most
> likely. That is ungood.
>
> Did anyone monitor the context switch rate with the mysql test?
>
> Interestingly, your test app (with s/100000/1000) runs to completion in 13
> seocnd on the slow 2-way. On a fast 8-way, it took 52 seconds and
> sustained 40,000 context switches/sec. That's a bit unexpected.
>
> Both machines show ~8% idle time, too :(
Rohit solved this puzzle.
The 2-way is a single package, hyperthreaded.
The 8-way is two-package, four cores in each.
So on the 8-way, that lock is getting transferred between the two packages
like crazy. Running the benchmark on just cpus 0 and 1 (taskset -c 0,1)
took the runtime down to eight seconds (from 52!) and the context switch
rate went up to 200,000/sec (from 45,000).
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