Re: Fw: HACKERS[PATCH] split ProcArrayLock into multiple parts -- follow-up

On 2017-09-21 15:51:54 -0500, Jim Van Fleet wrote:
> Not to beat on a dead horse, or anything, but this fix was frowned upon
> because in one environment (one socket) it was 6% down and over 15% up in
> the right environment (two sockets).

> So, why not add a configuration parameter which specifies the number of
> parts? Default is 1 which would be "exactly" the same as no parts and
> hence no degradation in the single socket environment -- and with 2, you
> get some positive performance.

Several reasons:

- You'd either add a bunch of branches into a performance critical
parts, or you'd add a compile time flag, which most people would be
unable to toggle.
- It'd be something hard to tune, because even on multi-socket machines
it'll be highly load dependant. E.g. workloads that largely are
bottlenecked in a single backend / few backends will probably regress
as well.

FWIW, you started a new thread with this message, that doesn't seem
helpful?

Greetings,

Andres Freund