[Bug 195098] pthread lock performance degradation on single CPU VM

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=195098

--- Comment #3 from Ed Maste <emaste at freebsd.org> ---
It appears that in the "slow" case the repro code is spinning in the
pthread_yield() at line 107 - i.e., trying to allow the contender threads run,
but the main thread immediately resumes.

    98        /* Wait until every thread is ready.  */
    99        dbgprintf ("Main thread before synchronizing for round %d\n",
repeat);
   100        for (;;)
   101          {
   102            int ready_count = 0;
   103            for (i = 0; i < THREAD_COUNT; i++)
   104              ready_count += ready[i];
   105            if (ready_count == THREAD_COUNT)
   106              break;
   107            pthread_yield ();
   108          }

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