Re: Re: [COMMITTERS] pgsql: Tune GetSnapshotData() during Hot Standby by avoiding loop

On Mon, 2010-04-19 at 10:55 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > On Mon, 2010-04-19 at 17:44 +0300, Heikki Linnakangas wrote:
> >>> Choices are
> >>> 1. Check RecoveryInProgress() once outside of lock, plus wild rumour of
> >>> Murphy
> >>> 2. Check RecoveryInProgress() before and after holding lock
> >>> 3. Check RecoveryInProgress() while holding lock
> >>
> >> 4. Check RecoveryInProgress() once outside of lock, and scan the
> >> ProcArray anyway, just in case. That's what we did before this patch.
> >> Document that takenDuringRecovery == true means that the snapshot was
> >> most likely taken during recovery, but there is some race conditions
> >> where takenDuringRecovery is true even though the snapshot was taken
> >> just after recovery finished. AFAICS all of the other current uses of
> >> takenDuringRecovery work fine with that.
>
> > Checking RecoveryInProgress() is much cheaper than scanning the whole
> > ProcArray, so (4) is definitely worse than 1-3.
>
> If the lock we're talking about is an LWLock, #3 is okay.  If it's a
> spinlock, not so much.

Just committed the following patch to implement option #3.

We test RecoveryInProgress() after the LWLockAcquire rather than before.

--
 Simon Riggs           www.2ndQuadrant.com