Re: Wierd context-switching issue on Xeon patch for 7.4.1

Tom Lane wrote:
> Dave Cramer <pg@fastcrypt.com> writes:
> > I tried increasing the NUM_SPINS to 1000 and it works better.
>
> Doesn't surprise me.  The value of 100 is about right on the assumption
> that the spinlock instruction per se is not too much more expensive than
> any other instruction.  What I was seeing from oprofile suggested that
> the spinlock instruction cost about 100x more than an ordinary
> instruction :-( ... so maybe 200 or so would be good on a Xeon.
>
> > This is certainly heading in the right direction ? Although it looks
> > like it is highly dependent on the system you are running on.
>
> Yeah.  I don't know a reasonable way to tune this number automatically
> for particular systems ... but at the very least we'd need to find a way
> to distinguish uniprocessor from multiprocessor, because on a
> uniprocessor the optimal value is surely 1.

Have you looked at the code pointed to by our TODO item:

    * Add code to detect an SMP machine and handle spinlocks accordingly
      from distributted.net, http://www1.distributed.net/source,
      in client/common/cpucheck.cpp

For BSDOS it has:

    #if (CLIENT_OS == OS_FREEBSD) || (CLIENT_OS == OS_BSDOS) || \
        (CLIENT_OS == OS_OPENBSD) || (CLIENT_OS == OS_NETBSD)
    { /* comment out if inappropriate for your *bsd - cyp (25/may/1999) */
      int ncpus; size_t len = sizeof(ncpus);
      int mib[2]; mib[0] = CTL_HW; mib[1] = HW_NCPU;
      if (sysctl( &mib[0], 2, &ncpus, &len, NULL, 0 ) == 0)
      //if (sysctlbyname("hw.ncpu", &ncpus, &len, NULL, 0 ) == 0)
        cpucount = ncpus;
    }

and I can confirm that on my computer it works:

    hw.ncpu = 2

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