Re: pgsql: When WalSndCaughtUp, sleep only in WalSndWaitForWal().

On Fri, Apr 17, 2020 at 04:50:38AM +0900, Fujii Masao wrote:
> On 2020/04/12 2:35, Noah Misch wrote:
> >When WalSndCaughtUp, sleep only in WalSndWaitForWal().
> >
> >Before sleeping, WalSndWaitForWal() sends a keepalive if MyWalSnd->write
> >< sentPtr.  That is important in logical replication.  When the latest
> >physical LSN yields no logical replication messages (a common case),
> >that keepalive elicits a reply, and processing the reply updates
> >pg_stat_replication.replay_lsn.  WalSndLoop() lacks that; when
> >WalSndLoop() slept, replay_lsn advancement could stall until
> >wal_receiver_status_interval elapsed.  This sometimes stalled
> >src/test/subscription/t/001_rep_changes.pl for up to 10s.
> 
> Since this commit, walsender started consuming CPU resource too much in my env.

Confirmed.  I have shared this with the main thread and added details there.

>              wakeEvents = WL_LATCH_SET | WL_EXIT_ON_PM_DEATH | WL_TIMEOUT |
> -                WL_SOCKET_READABLE;
> +                WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE;
> 
> I wonder if this change caused WaitLatchOrSocket() in WalSndLoop() to wake up
> frequently more than necessary.

I collected lower wakeup counts after the commit.  The problem is a shortage
of waits.