Thread: Idle idea for improving concurrency of LISTEN/NOTIFY

Sooner or later we ought to completely reimplement LISTEN/NOTIFY,
but while thinking about Joel Stevenson's performance issue I had a
sudden idea for a very simple change that would buy some improvement.
Currently, all operations in async.c take ExclusiveLock on pg_listener,
but it strikes me that that is overkill.  Wouldn't it work fine to
take a plain writer's lock (ie, RowExclusiveLock) for operations that
are touching only the current backend's pg_listener entries?  We would
only need ExclusiveLock when we want to scribble on *other* backends'
entries, ie, only in AtCommit_Notify().  What this would mean is that
when a NOTIFY awakens multiple listeners, the listeners don't serialize
on the pg_listener lock in order to find and clear their table entries.
        regards, tom lane

Added to TODO:
       o Improve LISTEN concurrency
         http://archives.postgresql.org/pgsql-hackers/2008-02/msg01106.php


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Tom Lane wrote:
> Sooner or later we ought to completely reimplement LISTEN/NOTIFY,
> but while thinking about Joel Stevenson's performance issue I had a
> sudden idea for a very simple change that would buy some improvement.
> Currently, all operations in async.c take ExclusiveLock on pg_listener,
> but it strikes me that that is overkill.  Wouldn't it work fine to
> take a plain writer's lock (ie, RowExclusiveLock) for operations that
> are touching only the current backend's pg_listener entries?  We would
> only need ExclusiveLock when we want to scribble on *other* backends'
> entries, ie, only in AtCommit_Notify().  What this would mean is that
> when a NOTIFY awakens multiple listeners, the listeners don't serialize
> on the pg_listener lock in order to find and clear their table entries.
> 
>             regards, tom lane
> 
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