On 20 February 2017 at 16:53, Robert Haas <robertmh...@gmail.com> wrote:
> On Mon, Feb 20, 2017 at 6:02 PM, Simon Riggs <si...@2ndquadrant.com> wrote:
>> On 15 February 2017 at 19:15, Andres Freund <and...@anarazel.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                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment: reduce_pgxact_access_AtEOXact.v2.patch
Description: Binary data

Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to