Re: spinlocks on HP-UX

On Mon, Aug 29, 2011 at 11:42 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> 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.

OK.

>> 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.

Hmm, OK. I guess if you want to put energy into it, I'm not going to
complain too much... just not sure it's the best use of time.

--
Robert Haas
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