Re: heavily contended lwlocks with long wait queues scale badly

On Tue, Nov 1, 2022 at 3:17 AM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
> Below are test results with v3 patch. +1 for back-patching it.

The problem with back-patching stuff like this is that it can have
unanticipated consequences. I think that the chances of something like
this backfiring are less than for a patch that changes plans, but I
don't think that they're nil, either. It could turn out that this
patch, which has really promising results on the workloads we've
tested, harms some other workload due to some other contention pattern
we can't foresee. It could also turn out that improving performance at
the database level actually has negative consequences for some
application using the database, because the application could be
unknowingly relying on the database to throttle its activity.

It's hard for me to estimate exactly what the risk of a patch like
this is. I think that if we back-patched this, and only this, perhaps
the chances of something bad happening aren't incredibly high. But if
we get into the habit of back-patching seemingly-innocuous performance
improvements, it's only a matter of time before one of them turns out
not to be so innocuous as we thought. I would guess that the number of
times we have to back-patch something like this before somebody starts
complaining about a regression is likely to be somewhere between 3 and
5.

It's possible that I'm too pessimistic, though.

--
Robert Haas
EDB: http://www.enterprisedb.com