Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Wed, Apr 13, 2016 at 03:21:31PM -0500, Kevin Grittner wrote:
> If 2201d801 was not included in your -1 tests, have you identified
> where the 2% extra run time is going on -1 versus reverted? Since
> several other threads lately have reported bigger variation than
> that based on random memory alignment issues, can we confirm that
> this is a real difference in what is at master's HEAD?

If anyone wishes to confirm that, I recommend this method:
http://www.postgresql.org/message-id/87vbitb2zp.fsf@news-spur.riddles.org.uk

PostgreSQL has not required that from contributors, though. For putative
regressions this small, we've either analyzed them theoretically or just
dismissed them.

The key judgment to finalize here is whether it's okay to release this feature
given its current effect[1], when enabled, on performance. 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. If
anyone else wants to weigh in, now is the time.

[1] http://www.postgresql.org/message-id/20160413192110.fogwesjti3kxycnu@alap3.anarazel.de
[2] http://www.postgresql.org/message-id/20160413140821.GA6568@alvherre.pgsql
[3] http://www.postgresql.org/message-id/CA+TgmoZqN0xevR+1pZ6j-99-ZCBoOphr-23tiREb+QW1Eu=KOA@mail.gmail.com
[4] http://www.postgresql.org/message-id/20160413212356.uv4velailmivnihh@alap3.anarazel.de