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

Robert Haas wrote:
> On Tue, Apr 12, 2016 at 11:05 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:

> > I didn't plan to do anything without a few +1's. I don't think we can
> > release with the state of things as is though. I don't see a less
> > intrusive way than to get rid of that spinlock on all platforms capable
> > of significant concurrency.
> >
> > So, RMT, what are your thoughts on this?
>
> I think that a significant performance regression which affects people
> not using snapshot_too_old would be a stop-ship issue,

Agreed.

> but I disagree that an issue which only affects people using the
> feature is a must-fix.

Agreed.

> It's reasonable to fix some kinds of
> issues after feature freeze, but not at the price of accepting
> arbitrary amounts of new code that may have problems of its own.
> Every release will have some warts.

Agreed.

The patch being proposed for commit is fiddly architecture-specific
stuff which is likely to destabilize the tree for quite some time, and
cause lots of additional work to Andres and anyone else likely to work
on such low-level details, such as Robert, both of which already have
plenty to do.

The snapshot-too-old feature is said to be great and shows lots of
improvement in certain cases, and no regression can be measured for
those who have it turned off. The regression only seems to show up if
you turn it on and have a crazily high rate of read-only transactions.
I think this can wait for 9.7.

--
Álvaro Herrera http://www.2ndQuadrant.com/
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