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

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.

              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.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION