From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Bernd Helmle <mailings(at)oopsware(dot)de>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Should we cacheline align PGXACT? |
Date: | 2017-02-15 17:24:44 |
Message-ID: | CA+TgmoazS7wvSq5mHf+9LPtmfeA92LzYWyXHLHrUs38KF2ju1Q@mail.gmail.com |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
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
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
From | Date | Subject | |
---|---|---|---|
Next Message | Robert Haas | 2017-02-15 17:27:11 | Re: Proposal: GetOldestXminExtend for ignoring arbitrary vacuum flags |
Previous Message | Ryan Murphy | 2017-02-15 17:24:14 | Re: Does having a NULL column automatically exclude the table from the tupleDesc cache? |