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

Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> 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;
> }

Multiplied by how many platforms? Ewww...

I was wondering about some sort of dynamic adaptation, roughly along the
lines of "whenever a spin loop successfully gets the lock after
spinning, decrease the allowed loop count by one; whenever we fail to
get the lock after spinning, increase by 100; if the loop count reaches,
say, 10000, decide we are on a uniprocessor and irreversibly set it to
1." As written this would tend to incur a select() delay once per
hundred spinlock acquisitions, which is way too much, but I think we
could make it work with a sufficiently slow adaptation rate. The tricky
part is that a slow adaptation rate means we can't have every backend
figuring this out for itself --- the right value would have to be
maintained globally, and I'm not sure how to do that without adding a
lot of overhead.

regards, tom lane