Re: Move PinBuffer and UnpinBuffer to atomics - Mailing list pgsql-hackers

On 2015-12-08 12:53:49 +0300, Alexander Korotkov wrote:
> ​This is why atomic increment *could be* cheaper than loop over CAS and, it
> worth having experiments. ​Another idea is that we can put arbitrary logic
> between lwarx and stwcx. Thus, we can implement PinBuffer using single loop
> of lwarx and stwcx which could be better than loop of CAS.

You can't really put that much between an ll/sc - the hardware is only
able to track a very limited number of cacheline references.

> 3) lwlock-increment.patch – LWLockAttemptLock change state using atomic
> increment operation instead of loop of CAS. This patch does it for
> LWLockAttemptLock like pinunpin-increment.patch does for PinBuffer.
> Actually, this patch is not directly related to buffer manager. However,
> it's nice to test loop of CAS vs atomic increment in different places.

Yea, that's a worthwhile improvement. Actually it's how the first
versions of the lwlock patches worked - unfortunately I couldn't see big
differences on hardware I had available at the time.

There's some more trickyness required than what you have in your patch
(afaics at least). The problem is that when you 'optimistically'
increment by LW_VAL_SHARED and notice that there actually was another
locker, you possibly, until you've 'fixed' the state, are blocking new
exclusive lockers from acquiring the locks.  So you additionally need to
do special handling in these cases, and check the queue more.