Re: Is the unfair lwlock behavior intended?

On 2016-05-25 17:24:22 -0400, Robert Haas wrote:
> On Wed, May 25, 2016 at 3:26 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2016-05-25 11:15:37 -0700, Andres Freund wrote:
> >> On 2016-05-25 14:09:43 -0400, Robert Haas wrote:
> >> I don't think anybody was doing that? The first questions on this thread
> >> were about upgrading and retesting...
> >
> > Something I've repeatedly wondered about around this topic is whether we
> > could split ProcArrayLock into one that governs entering or leaving the
> > procarray from the one that's for consistent snapshots. I think there's
> > no need for ProcArrayAdd/ProcArrayRemove/CountDBBackends()/CancelDBBackends()/
> > CountUserBackends()/CountOtherDBBackends() (and potentially some more)
> > to conflict with GetSnapshotData()/ProcArrayEndTransaction()/
> > TransactionIdIsInProgress()/TransactionIdIsActive()/GetOldestXmin()/...
> > as long as we're careful to ensure that by the time a entry is removed
> > ProcArrayEndTransaction() has been called.
>
> I'm doubtful about how much that would reduce contention,

Yea, I don't think it'll usually will make a huge difference. It'd
likely have solved this specific complaint though; and it'd remove
nearly all other exclusive acquisitions of ProcArrayLock other than
ProcArrayEndTransaction().

> However, I think it might be worth doing anyway, because redesigning
> the whole mechanism might be easier if that lock weren't doing so many
> only-semi-related things.

Very much agreed upon this. Having a distinct 'SnapshotLock', for
EndTransaction() and GetSnapshotData() would be a first step in making
the locking more granular.

Andres