Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On 2016-04-19 20:27:31 +0530, Amit Kapila wrote:
> On Sun, Apr 17, 2016 at 2:26 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >
> > On 2016-04-16 16:44:52 -0400, Noah Misch wrote:
> > > That is more controversial than the potential ~2% regression for
> > > old_snapshot_threshold=-1. Alvaro[2] and Robert[3] are okay releasing
> > > that way, and Andres[4] is not.
> >
> > FWIW, I could be kinda convinced that it's temporarily ok, if there'd be
> > a clear proposal on the table how to solve the scalability issue around
> > MaintainOldSnapshotTimeMapping().
> >
>
> It seems that for read-only workloads, MaintainOldSnapshotTimeMapping()
> takes EXCLUSIVE LWLock which seems to be a probable reason for a
> performance regression.

Yes, that's the major problem.

> Now, here the question is do we need to acquire that lock if xmin is
> not changed since the last time value of
> oldSnapshotControl->latest_xmin is updated or xmin is lesser than
> equal to oldSnapshotControl->latest_xmin? If we don't need it for
> above cases, I think it can address the performance regression to a
> good degree for read-only workloads when the feature is enabled.

I think the more fundamental issue is that the time->xid mapping is
built at GetSnapshotData() time (via MaintainOldSnapshotTimeMapping()),
and not when xids are assigned. Snapshots are created a lot more
frequently in nearly all use-cases than xids are assigned. That's what
forces the exclusive lock to be in the read path, rather than the write
path.

What's the reason for this?

Greetings,

Andres Freund