Re: Should we cacheline align PGXACT?

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, 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: 2017-02-20 17:19:05
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On 20 February 2017 at 16:53, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Mon, Feb 20, 2017 at 6:02 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> On 15 February 2017 at 19:15, Andres Freund <andres(at)anarazel(dot)de> wrote:
>>> I think I previously
>>> mentioned, even just removing the MyPgXact->xmin assignment in
>>> SnapshotResetXmin() is measurable performance wise and cache-hit ratio
>>> wise.
>> Currently, we issue SnapshotResetXmin() pointlessly at end of xact, so
>> patch attached to remove that call, plus some comments to explain
>> that. This reduces the cause.
>> Also, another patch to reduce the calls to SnapshotResetXmin() using a
>> simple heuristic to reduce the effects.
> I think skip_SnapshotResetXmin_if_idle_timeout.v1.patch isn't a good
> idea, because it could have the surprising result that setting
> idle_in_transaction_timeout to a non-zero value makes bloat worse. I
> don't think users will like that.
> Regarding reduce_pgxact_access_AtEOXact.v1.patch, it took me a few
> minutes to figure out that the comment was referring to
> ProcArrayEndTransaction(), so it might be good to be more explicit
> about that if we go forward with this.

Sure, attached.

> Have you checked whether this
> patch makes any noticeable performance difference?

No, but then we're reducing the number of calls to PgXact directly;
there is no heuristic involved, its just a pure saving.

> It's sure
> surprising that we go to all of this trouble to clean things up in
> AtEOXact_Snapshot() when we've already nuked MyPgXact->xmin from
> orbit. (Instead of changing AtEOXact_Snapshot, should we think about
> removing the xid clear logic from ProcArrayEndTransaction and only
> doing it here, or would that be wrong-headed?)

If anything, I'd move the call to PgXact->xmin = InvalidTransactionId
into a function inside procarray.c, so we only touch snapshots in
snapmgr.c and all procarray stuff is isolated. (Not done here, yet).

Simon Riggs
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
reduce_pgxact_access_AtEOXact.v2.patch application/octet-stream 3.5 KB

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