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

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.
        regards, tom lane