Re: Should we cacheline align PGXACT?

On Wed, Feb 15, 2017 at 12:14 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Mon, Feb 13, 2017 at 11:07 AM, Alvaro Herrera
>> <alvherre(at)2ndquadrant(dot)com> wrote:
>>> It seems to me that Andres comments here were largely ignored:
>>> https://www.postgresql.org/message-id/20160822021747.u5bqx2xwwjzac5u5@alap3.anarazel.de
>>> He was suggesting to increase the struct size to 16 bytes rather than
>>> going all the way up to 128. Did anybody test this?
>
>> So, I think that going up to 128 bytes can't really make sense. If
>> that's the best-performing solution here, then maybe what we ought to
>> be doing is reverting the PGXACT/PGPROC separation, sticking these
>> critical members at the beginning, and padding the whole PGXACT out to
>> a multiple of the cache line size.
>
> Yes. That separation was never more than a horribly ugly kluge.
> I would love to see it go away. But keeping it *and* padding
> PGXACT to something >= the size of PGPROC borders on insanity.

I don't think it would be bigger than a PGPROC. PGPROCs are really
big, 816 bytes on my MacBook Pro. But if you did what I suggested,
you could take a snapshot by touching 1 cache line per backend. They
wouldn't be consecutive; it would be an upward pattern, with skips.
If you pad PGXACT out to one cache line, you could likewise take a
snapshot by touching 1 cache line per backend, and they'd be
consecutive. Maybe that difference matters to the memory prefetching
controller, I dunno, but it seems funny that we did the PGXACT work to
reduce the number of cache lines that had to be touched in order to
take a snapshot to improve performance, and now we're talking about
increasing it again, also to improve performance.

--
Robert Haas
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