Horton

Re: track_planning causing performance regression

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Ants Aasma <ants@cybertec.at>
Cc: Fujii Masao <masao.fujii@oss.nttdata.com>, Julien Rouhaud <rjuju123@gmail.com>, "Tharakan, Robins" <tharar@amazon.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-06-30T19:06:13Z
Lists: pgsql-hackers 
Hi,

On 2020-06-30 14:30:03 +0300, Ants Aasma wrote:
> Futex is a Linux kernel call that allows to build a lock that has
> uncontended cases work fully in user space almost exactly like a spinlock,
> while falling back to syscalls that wait for wakeup in case of contention.
> It's not portable, but probably something similar could be implemented for
> other operating systems. I did not pursue this further because it became
> apparent that every performance critical spinlock had already been removed.

Our lwlock implementation does have that property already, though. While
the kernel wait is implemented using semaphores, those are implemented
using futexes internally (posix ones, not sysv ones, so only after
whatever version we switched the default to posix semas on linux).

I'd rather move towards removing spinlocks from postgres than making
their implementation more complicated...

Greetings,

Andres Freund

Commits

  1. doc: Fix description about pg_stat_statements.track_planning.

  2. doc: Add note about possible performance overhead by enabling track_planning.

  3. Change default of pg_stat_statements.track_planning to off.