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

Andres Freund <andres(at)anarazel(dot)de> writes:
> 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(). Postponing the optimization around
> something as trivial as a spinlock around reading an LSN is one thing,
> postponing something we don't know the solution to is anohter.

The message Noah cited mentions only a 4% regression, but this one
seems far worse:

http://www.postgresql.org/message-id/20160413200148.bawmwjdmggbllhha@alap3.anarazel.de

That's more than a 5X penalty, which seems like it would make the
feature unusable; unless there is an argument that that's an extreme
case that wouldn't be representative of most real-world usage.
Which there may well be; I've not been following this thread carefully.

regards, tom lane