Re: Some interesting results from tweaking spinlocks

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > The difference is small, perhaps 15%.
>
> The thing that gets my attention is not that it's so small, it's that
> it is so large. My expectation was that that code would hardly ever
> be executed at all, and even less seldom (on a multiprocessor) need to
> block via select(). How is it that *increasing* the delay interval
> (which one might reasonably expect to simply waste cycles) can achieve
> a 15% improvement in total throughput? That shouldn't be happening.

OK, I am a little confused now. I thought the spinlock was only done a
few times if we couldn't get a lock, and if we don't we go to sleep, and
the count determines how many times we try. Isn't that expected to
affect SMP machines?

>
> > My feeling is that we may want to start configuring whether we are on
> > a multi-cpu machine and handle thing differently.
>
> That would be more palatable if there were some portable way of
> detecting it. But maybe we'll be forced into an "is_smp" GUC switch.

Yes, that is what I was thinking, but frankly, I am not going to give up
on SMP auto-detection until I am convinced it can't be done portably.

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