Re: Wait free LW_SHARED acquisition - v0.9

On Wed, Oct 8, 2014 at 7:05 PM, Andres Freund <andres@2ndquadrant.com> wrote:
>
> Hi,
>
> Attached you can find the next version of my LW_SHARED patchset. Now
> that atomics are committed, it seems like a good idea to also add their
> raison d'être.
>
> Since the last public version I have:
> * Addressed lots of Amit's comments. Thanks!
> * Peformed a fair amount of testing.
> * Rebased the code. The volatile removal made that not entirely
>   trivial...
> * Significantly cleaned up and simplified the code.
> * Updated comments and such
> * Fixed a minor bug (unpaired HOLD/RESUME_INTERRUPTS in a corner case)
>
> The feature currently consists out of two patches:
> 1) Convert PGPROC->lwWaitLink into a dlist. The old code was frail and
>    verbose. This also does:
>     * changes the logic in LWLockRelease() to release all shared lockers
>       when waking up any. This can yield some significant performance
>       improvements - and the fairness isn't really much worse than
>       before,
>       as we always allowed new shared lockers to jump the queue.
>
>     * adds a memory pg_write_barrier() in the wakeup paths between
>       dequeuing and unsetting ->lwWaiting. That was always required on
>       weakly ordered machines, but f4077cda2 made it more urgent. I can
>       reproduce crashes without it.
> 2) Implement the wait free LW_SHARED algorithm.


I have done few performance tests for above patches and results of