Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < - Mailing list pgsql-committers

On 2016-04-13 13:52:15 -0500, Kevin Grittner wrote:
> On Wed, Apr 13, 2016 at 12:25 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>
> > [test results with old_snapshot_threshold = 0 and 10]
>
> From the docs:
>
> | A value of -1 disables this feature, and is the default.

Hm, ok, let me run that as well then. The reason for the massive
performance difference presumably is that
MaintainOldSnapshotTimeMapping() is cut short due to
    /* No further tracking needed for 0 (used for testing). */
    if (old_snapshot_threshold == 0)
        return;
which means that OldSnapshotTimeMap isn't acquired exclusively.


> > Yuck.  Aside from the fact that performance tanks when the feature is
> > turned on, it seems that there is a significant effect even with it
> > turned off.
>
> No evidence of that has been provided.  -1 is off; 0 is for testing
> very fast expiration.

I'll run with -1 once the current (longer) run has finished.

Greetings,

Andres Freund