Re: Should we cacheline align PGXACT?

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

--
Robert Haas
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