Wakeup WALWriter as needed for asynchronous commit performance.

Previously we waited for wal_writer_delay before flushing WAL. Now
we also wake WALWriter as soon as a WAL buffer page has filled.
Significant effect observed on performance of asynchronous commits
by Robert Haas, attributed to the ability to set hint bits on tuples
earlier and so reducing contention caused by clog lookups.

Author: simonat2ndQuadrant

src/backend/access/transam/xlog.c

@@ -432,6 +432,11 @@ typedef struct XLogCtlData
 	 */
 	Latch		recoveryWakeupLatch;
 
+	/*
+	 * WALWriterLatch is used to wake up the WALWriter to write some WAL.
+	 */
+	Latch		WALWriterLatch;
+
 	/*
 	 * During recovery, we keep a copy of the latest checkpoint record here.
 	 * Used by the background writer when it wants to create a restartpoint.
@@ -1916,19 +1921,35 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
 }
 
 /*
- * Record the LSN for an asynchronous transaction commit/abort.
+ * Record the LSN for an asynchronous transaction commit/abort
+ * and nudge the WALWriter if there is a complete page to write.
  * (This should not be called for for synchronous commits.)
  */
 void
 XLogSetAsyncXactLSN(XLogRecPtr asyncXactLSN)
 {
+	XLogRecPtr	WriteRqstPtr = asyncXactLSN;
+
 	/* use volatile pointer to prevent code rearrangement */
 	volatile XLogCtlData *xlogctl = XLogCtl;
 
 	SpinLockAcquire(&xlogctl->info_lck);
+	LogwrtResult = xlogctl->LogwrtResult;
 	if (XLByteLT(xlogctl->asyncXactLSN, asyncXactLSN))
 		xlogctl->asyncXactLSN = asyncXactLSN;
 	SpinLockRelease(&xlogctl->info_lck);
+
+	/* back off to last completed page boundary */
+	WriteRqstPtr.xrecoff -= WriteRqstPtr.xrecoff % XLOG_BLCKSZ;
+
+	/* if we have already flushed that far, we're done */
+	if (XLByteLE(WriteRqstPtr, LogwrtResult.Flush))
+		return;
+
+	/*
+	 * Nudge the WALWriter if we have a full page of WAL to write.
+	 */
+	SetLatch(&XLogCtl->WALWriterLatch);
 }
 
 /*
@@ -5072,6 +5093,7 @@ XLOGShmemInit(void)
 	XLogCtl->Insert.currpage = (XLogPageHeader) (XLogCtl->pages);
 	SpinLockInit(&XLogCtl->info_lck);
 	InitSharedLatch(&XLogCtl->recoveryWakeupLatch);
+	InitSharedLatch(&XLogCtl->WALWriterLatch);
 
 	/*
 	 * If we are not in bootstrap mode, pg_control should already exist. Read
@@ -10013,3 +10035,12 @@ WakeupRecovery(void)
 {
 	SetLatch(&XLogCtl->recoveryWakeupLatch);
 }
+
+/*
+ * Manage the WALWriterLatch
+ */
+Latch *
+WALWriterLatch(void)
+{
+	return &XLogCtl->WALWriterLatch;
+}

src/backend/postmaster/walwriter.c

@@ -11,7 +11,8 @@
  *
  * Note that as with the bgwriter for shared buffers, regular backends are
  * still empowered to issue WAL writes and fsyncs when the walwriter doesn't
- * keep up.
+ * keep up. This means that the WALWriter is not an essential process and
+ * can shutdown quickly when requested.
  *
  * Because the walwriter's cycle is directly linked to the maximum delay
  * before async-commit transactions are guaranteed committed, it's probably
@@ -76,7 +77,6 @@ static void wal_quickdie(SIGNAL_ARGS);
 static void WalSigHupHandler(SIGNAL_ARGS);
 static void WalShutdownHandler(SIGNAL_ARGS);
 
-
 /*
  * Main entry point for walwriter process
  *
@@ -89,6 +89,8 @@ WalWriterMain(void)
 	sigjmp_buf	local_sigjmp_buf;
 	MemoryContext walwriter_context;
 
+	InitLatch(WALWriterLatch()); /* initialize latch used in main loop */
+
 	/*
 	 * If possible, make this process a group leader, so that the postmaster
 	 * can signal any child processes too.	(walwriter probably never has any
@@ -220,7 +222,7 @@ WalWriterMain(void)
 	 */
 	for (;;)
 	{
-		long		udelay;
+		ResetLatch(WALWriterLatch());
 
 		/*
 		 * Emergency bailout if postmaster has died.  This is to avoid the
@@ -248,20 +250,9 @@ WalWriterMain(void)
 		 */
 		XLogBackgroundFlush();
 
-		/*
-		 * Delay until time to do something more, but fall out of delay
-		 * reasonably quickly if signaled.
-		 */
-		udelay = WalWriterDelay * 1000L;
-		while (udelay > 999999L)
-		{
-			if (got_SIGHUP || shutdown_requested)
-				break;
-			pg_usleep(1000000L);
-			udelay -= 1000000L;
-		}
-		if (!(got_SIGHUP || shutdown_requested))
-			pg_usleep(udelay);
+		(void) WaitLatch(WALWriterLatch(),
+							   WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
+							   WalWriterDelay /* ms */);
 	}
 }
 
@@ -308,11 +299,13 @@ static void
 WalSigHupHandler(SIGNAL_ARGS)
 {
 	got_SIGHUP = true;
+	SetLatch(WALWriterLatch());
 }
 
 /* SIGTERM: set flag to exit normally */
 static void
 WalShutdownHandler(SIGNAL_ARGS)
 {
 	shutdown_requested = true;
+	SetLatch(WALWriterLatch());
 }

src/include/access/xlog.h

@@ -16,6 +16,7 @@
 #include "datatype/timestamp.h"
 #include "lib/stringinfo.h"
 #include "storage/buf.h"
+#include "storage/latch.h"
 #include "utils/pg_crc.h"
 
 /*
@@ -319,6 +320,7 @@ extern TimeLineID GetRecoveryTargetTLI(void);
 
 extern bool CheckPromoteSignal(void);
 extern void WakeupRecovery(void);
+extern Latch *WALWriterLatch(void);
 
 /*
  * Starting/stopping a base backup