Make walsender more responsive.

Per testing by Andres Freund, this improves replication performance
and reduces replication latency and latency jitter.  I was a bit
concerned about moving more work into XLogInsert, but testing seems
to show that it's not a problem in practice.

Along the way, improve comments for WaitLatchOrSocket.

Andres Freund.  Review and stylistic cleanup by me.

src/backend/access/transam/twophase.c

@@ -1042,13 +1042,6 @@ EndPrepare(GlobalTransaction gxact)
 
 	/* If we crash now, we have prepared: WAL replay will fix things */
 
-	/*
-	 * Wake up all walsenders to send WAL up to the PREPARE record immediately
-	 * if replication is enabled
-	 */
-	if (max_wal_senders > 0)
-		WalSndWakeup();
-
 	/* write correct CRC and close file */
 	if ((write(fd, &statefile_crc, sizeof(pg_crc32))) != sizeof(pg_crc32))
 	{
@@ -2045,13 +2038,6 @@ RecordTransactionCommitPrepared(TransactionId xid,
 	/* Flush XLOG to disk */
 	XLogFlush(recptr);
 
-	/*
-	 * Wake up all walsenders to send WAL up to the COMMIT PREPARED record
-	 * immediately if replication is enabled
-	 */
-	if (max_wal_senders > 0)
-		WalSndWakeup();
-
 	/* Mark the transaction committed in pg_clog */
 	TransactionIdCommitTree(xid, nchildren, children);
 
@@ -2132,13 +2118,6 @@ RecordTransactionAbortPrepared(TransactionId xid,
 	/* Always flush, since we're about to remove the 2PC state file */
 	XLogFlush(recptr);
 
-	/*
-	 * Wake up all walsenders to send WAL up to the ABORT PREPARED record
-	 * immediately if replication is enabled
-	 */
-	if (max_wal_senders > 0)
-		WalSndWakeup();
-
 	/*
 	 * Mark the transaction aborted in clog.  This is not absolutely necessary
 	 * but we may as well do it while we are here.

src/backend/access/transam/xact.c

@@ -1141,13 +1141,6 @@ RecordTransactionCommit(void)
 
 		XLogFlush(XactLastRecEnd);
 
-		/*
-		 * Wake up all walsenders to send WAL up to the COMMIT record
-		 * immediately if replication is enabled
-		 */
-		if (max_wal_senders > 0)
-			WalSndWakeup();
-
 		/*
 		 * Now we may update the CLOG, if we wrote a COMMIT record above
 		 */

src/backend/access/transam/xlog.c

@@ -1025,6 +1025,8 @@ begin:;
 
 		END_CRIT_SECTION();
 
+		/* wakeup the WalSnd now that we released the WALWriteLock */
+		WalSndWakeupProcessRequests();
 		return RecPtr;
 	}
 
@@ -1208,6 +1210,9 @@ begin:;
 
 	END_CRIT_SECTION();
 
+	/* wakeup the WalSnd now that we outside contented locks */
+	WalSndWakeupProcessRequests();
+
 	return RecPtr;
 }
 
@@ -1792,6 +1797,10 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
 			if (finishing_seg || (xlog_switch && last_iteration))
 			{
 				issue_xlog_fsync(openLogFile, openLogSegNo);
+
+				/* signal that we need to wakeup WalSnd later */
+				WalSndWakeupRequest();
+
 				LogwrtResult.Flush = LogwrtResult.Write;		/* end of page */
 
 				if (XLogArchivingActive())
@@ -1854,7 +1863,11 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
 				openLogFile = XLogFileOpen(openLogSegNo);
 				openLogOff = 0;
 			}
+
 			issue_xlog_fsync(openLogFile, openLogSegNo);
+
+			/* signal that we need to wakeup WalSnd later */
+			WalSndWakeupRequest();
 		}
 		LogwrtResult.Flush = LogwrtResult.Write;
 	}
@@ -2120,6 +2133,9 @@ XLogFlush(XLogRecPtr record)
 
 	END_CRIT_SECTION();
 
+	/* wakeup the WalSnd now that we released the WALWriteLock */
+	WalSndWakeupProcessRequests();
+
 	/*
 	 * If we still haven't flushed to the request point then we have a
 	 * problem; most likely, the requested flush point is past end of XLOG.
@@ -2245,13 +2261,8 @@ XLogBackgroundFlush(void)
 
 	END_CRIT_SECTION();
 
-	/*
-	 * If we wrote something then we have something to send to standbys also,
-	 * otherwise the replication delay become around 7s with just async
-	 * commit.
-	 */
-	if (wrote_something)
-		WalSndWakeup();
+	/* wakeup the WalSnd now that we released the WALWriteLock */
+	WalSndWakeupProcessRequests();
 
 	return wrote_something;
 }

src/backend/port/unix_latch.c

@@ -418,6 +418,9 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
  * NB: when calling this in a signal handler, be sure to save and restore
  * errno around it.  (That's standard practice in most signal handlers, of
  * course, but we used to omit it in handlers that only set a flag.)
+ *
+ * NB: this function is called from critical sections and signal handlers so
+ * throwing an error is not a good idea.
  */
 void
 SetLatch(volatile Latch *latch)

src/backend/port/win32_latch.c

@@ -247,6 +247,10 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 	return result;
 }
 
+/*
+ * The comments above the unix implementation (unix_latch.c) of this function
+ * apply here as well.
+ */
 void
 SetLatch(volatile Latch *latch)
 {

src/backend/replication/walsender.c

@@ -81,6 +81,10 @@ bool		am_cascading_walsender = false;		/* Am I cascading WAL to
 int			max_wal_senders = 0;	/* the maximum number of concurrent walsenders */
 int			replication_timeout = 60 * 1000;	/* maximum time to send one
 												 * WAL data message */
+/*
+ * State for WalSndWakeupRequest
+ */
+bool wake_wal_senders = false;
 
 /*
  * These variables are used similarly to openLogFile/Id/Seg/Off,
@@ -1395,7 +1399,12 @@ WalSndShmemInit(void)
 	}
 }
 
-/* Wake up all walsenders */
+/*
+ * Wake up all walsenders
+ *
+ * This will be called inside critical sections, so throwing an error is not
+ * adviseable.
+ */
 void
 WalSndWakeup(void)
 {

src/include/replication/walsender.h

@@ -21,6 +21,7 @@ extern bool am_walsender;
 extern bool am_cascading_walsender;
 extern volatile sig_atomic_t walsender_shutdown_requested;
 extern volatile sig_atomic_t walsender_ready_to_stop;
+extern bool wake_wal_senders;
 
 /* user-settable parameters */
 extern int	max_wal_senders;
@@ -35,4 +36,27 @@ extern void WalSndRqstFileReload(void);
 
 extern Datum pg_stat_get_wal_senders(PG_FUNCTION_ARGS);
 
+/*
+ * Remember that we want to wakeup walsenders later
+ *
+ * This is separated from doing the actual wakeup because the writeout is done
+ * while holding contended locks.
+ */
+#define WalSndWakeupRequest() \
+	do { wake_wal_senders = true; } while (0)
+
+/*
+ * wakeup walsenders if there is work to be done
+ */
+#define WalSndWakeupProcessRequests()		\
+	do										\
+	{										\
+		if (wake_wal_senders)				\
+		{									\
+			wake_wal_senders = false;		\
+			if (max_wal_senders > 0)		\
+				WalSndWakeup();				\
+		}									\
+	} while (0)
+
 #endif   /* _WALSENDER_H */