Wake up for latches in CheckpointWriteDelay().

The checkpointer shouldn't ignore its latch.  Other backends may be
waiting for it to drain the request queue.  Hopefully real systems don't
have a full queue often, but the condition is reached easily when
shared_buffers is small.

This involves defining a new wait event, which will appear in the
pg_stat_activity view often due to spread checkpoints.

Back-patch only to 14.  Even though the problem exists in earlier
branches too, it's hard to hit there.  In 14 we stopped using signal
handlers for latches on Linux, *BSD and macOS, which were previously
hiding this problem by interrupting the sleep (though not reliably, as
the signal could arrive before the sleep begins; precisely the problem
latches address).

Reported-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20220226213942.nb7uvb2pamyu26dj%40alap3.anarazel.de

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 9fb62fec8ec5822d6fc3f3669394369d7bbd902d..8620aaddc79e68b3926f08b9ee50d6e0598cff1d 100644 (file)
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -2235,6 +2235,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry><literal>BaseBackupThrottle</literal></entry>
       <entry>Waiting during base backup when throttling activity.</entry>
      </row>
+     <row>
+      <entry><literal>CheckpointerWriteDelay</literal></entry>
+      <entry>Waiting between writes while performing a checkpoint.</entry>
+     </row>
      <row>
       <entry><literal>PgSleep</literal></entry>
       <entry>Waiting due to a call to <function>pg_sleep</function> or

diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c
index 4488e3a44357682a44928bb0bcbb5471ceb5a857..a59c3cf02015267df7f6bdcac18156dcdf943db2 100644 (file)
--- a/src/backend/postmaster/checkpointer.c
+++ b/src/backend/postmaster/checkpointer.c
@@ -484,6 +484,9 @@ CheckpointerMain(void)
            }
 
            ckpt_active = false;
+
+           /* We may have received an interrupt during the checkpoint. */
+           HandleCheckpointerInterrupts();
        }
 
        /* Check for archive_timeout and switch xlog files if necessary. */
@@ -726,7 +729,10 @@ CheckpointWriteDelay(int flags, double progress)
         * Checkpointer and bgwriter are no longer related so take the Big
         * Sleep.
         */
-       pg_usleep(100000L);
+       WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH | WL_TIMEOUT,
+                 100,
+                 WAIT_EVENT_CHECKPOINT_WRITE_DELAY);
+       ResetLatch(MyLatch);
    }
    else if (--absorb_counter <= 0)
    {

diff --git a/src/backend/utils/activity/wait_event.c b/src/backend/utils/activity/wait_event.c
index 60972c3a750babfa7fff445cf4e730b432674f52..0706e922b5369686c47de3cab9f3af0f70e6d674 100644 (file)
--- a/src/backend/utils/activity/wait_event.c
+++ b/src/backend/utils/activity/wait_event.c
@@ -485,6 +485,9 @@ pgstat_get_wait_timeout(WaitEventTimeout w)
        case WAIT_EVENT_BASE_BACKUP_THROTTLE:
            event_name = "BaseBackupThrottle";
            break;
+       case WAIT_EVENT_CHECKPOINT_WRITE_DELAY:
+           event_name = "CheckpointWriteDelay";
+           break;
        case WAIT_EVENT_PG_SLEEP:
            event_name = "PgSleep";
            break;

diff --git a/src/include/utils/wait_event.h b/src/include/utils/wait_event.h
index 395d325c5fe769e71d766f1db658ba9d794d8c59..d0345c6b49e85f60b3505b55f63752230e0de4e2 100644 (file)
--- a/src/include/utils/wait_event.h
+++ b/src/include/utils/wait_event.h
@@ -141,6 +141,7 @@ typedef enum
 typedef enum
 {
    WAIT_EVENT_BASE_BACKUP_THROTTLE = PG_WAIT_TIMEOUT,
+   WAIT_EVENT_CHECKPOINT_WRITE_DELAY,
    WAIT_EVENT_PG_SLEEP,
    WAIT_EVENT_RECOVERY_APPLY_DELAY,
    WAIT_EVENT_RECOVERY_RETRIEVE_RETRY_INTERVAL,