Re: spinlocks on HP-UX

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Aug 29, 2011 at 11:07 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> IIUC, this is basically total nonsense.

>> It could maybe be rewritten for more clarity, but it's far from being
>> nonsense. The responsibility for having an actual hardware memory fence
>> instruction lies with the author of the TAS macro.

> Right... but the comment implies that you probably don't need one, and
> doesn't even mention that you MIGHT need one.

I think maybe we need to split it into two paragraphs, one addressing
the TAS author and the other for the TAS user. I'll have a go at that.

> I think optimizing spinlocks for machines with only a few CPUs is
> probably pointless. Based on what I've seen so far, spinlock
> contention even at 16 CPUs is negligible pretty much no matter what
> you do.

We did find significant differences several years ago, testing on
machines that probably had no more than four cores; that's where the
existing comments in s_lock.h came from. Whether those tests are
still relevant for today's source code is not obvious though.

regards, tom lane