Move the replication lag tracker into heap memory.

Andres Freund complained about the 128KB of .bss occupied by LagTracker.
It's only needed in the walsender process, so allocate it in heap
memory there.

Author: Thomas Munro
Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya%40alap3.anarazel.de

src/backend/replication/walsender.c

@@ -211,7 +211,7 @@ typedef struct
 #define LAG_TRACKER_BUFFER_SIZE 8192
 
 /* A mechanism for tracking replication lag. */
-static struct
+typedef struct
 {
 	XLogRecPtr	last_lsn;
 	WalTimeSample buffer[LAG_TRACKER_BUFFER_SIZE];
@@ -220,6 +220,8 @@ static struct
 	WalTimeSample last_read[NUM_SYNC_REP_WAIT_MODE];
 }			LagTracker;
 
+static LagTracker *lag_tracker;
+
 /* Signal handlers */
 static void WalSndLastCycleHandler(SIGNAL_ARGS);
 
@@ -282,7 +284,7 @@ InitWalSender(void)
 	SendPostmasterSignal(PMSIGNAL_ADVANCE_STATE_MACHINE);
 
 	/* Initialize empty timestamp buffer for lag tracking. */
-	memset(&LagTracker, 0, sizeof(LagTracker));
+	lag_tracker = MemoryContextAllocZero(TopMemoryContext, sizeof(LagTracker));
 }
 
 /*
@@ -3439,9 +3441,9 @@ LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time)
 	 * If the lsn hasn't advanced since last time, then do nothing.  This way
 	 * we only record a new sample when new WAL has been written.
 	 */
-	if (LagTracker.last_lsn == lsn)
+	if (lag_tracker->last_lsn == lsn)
 		return;
-	LagTracker.last_lsn = lsn;
+	lag_tracker->last_lsn = lsn;
 
 	/*
 	 * If advancing the write head of the circular buffer would crash into any
@@ -3449,11 +3451,11 @@ LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time)
 	 * slowest reader (presumably apply) is the one that controls the release
 	 * of space.
 	 */
-	new_write_head = (LagTracker.write_head + 1) % LAG_TRACKER_BUFFER_SIZE;
+	new_write_head = (lag_tracker->write_head + 1) % LAG_TRACKER_BUFFER_SIZE;
 	buffer_full = false;
 	for (i = 0; i < NUM_SYNC_REP_WAIT_MODE; ++i)
 	{
-		if (new_write_head == LagTracker.read_heads[i])
+		if (new_write_head == lag_tracker->read_heads[i])
 			buffer_full = true;
 	}
 
@@ -3464,17 +3466,17 @@ LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time)
 	 */
 	if (buffer_full)
 	{
-		new_write_head = LagTracker.write_head;
-		if (LagTracker.write_head > 0)
-			LagTracker.write_head--;
+		new_write_head = lag_tracker->write_head;
+		if (lag_tracker->write_head > 0)
+			lag_tracker->write_head--;
 		else
-			LagTracker.write_head = LAG_TRACKER_BUFFER_SIZE - 1;
+			lag_tracker->write_head = LAG_TRACKER_BUFFER_SIZE - 1;
 	}
 
 	/* Store a sample at the current write head position. */
-	LagTracker.buffer[LagTracker.write_head].lsn = lsn;
-	LagTracker.buffer[LagTracker.write_head].time = local_flush_time;
-	LagTracker.write_head = new_write_head;
+	lag_tracker->buffer[lag_tracker->write_head].lsn = lsn;
+	lag_tracker->buffer[lag_tracker->write_head].time = local_flush_time;
+	lag_tracker->write_head = new_write_head;
 }
 
 /*
@@ -3496,14 +3498,14 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
 	TimestampTz time = 0;
 
 	/* Read all unread samples up to this LSN or end of buffer. */
-	while (LagTracker.read_heads[head] != LagTracker.write_head &&
-		   LagTracker.buffer[LagTracker.read_heads[head]].lsn <= lsn)
+	while (lag_tracker->read_heads[head] != lag_tracker->write_head &&
+		   lag_tracker->buffer[lag_tracker->read_heads[head]].lsn <= lsn)
 	{
-		time = LagTracker.buffer[LagTracker.read_heads[head]].time;
-		LagTracker.last_read[head] =
-			LagTracker.buffer[LagTracker.read_heads[head]];
-		LagTracker.read_heads[head] =
-			(LagTracker.read_heads[head] + 1) % LAG_TRACKER_BUFFER_SIZE;
+		time = lag_tracker->buffer[lag_tracker->read_heads[head]].time;
+		lag_tracker->last_read[head] =
+			lag_tracker->buffer[lag_tracker->read_heads[head]];
+		lag_tracker->read_heads[head] =
+			(lag_tracker->read_heads[head] + 1) % LAG_TRACKER_BUFFER_SIZE;
 	}
 
 	/*
@@ -3513,8 +3515,8 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
 	 * interpolation at the beginning of the next burst of WAL after a period
 	 * of idleness.
 	 */
-	if (LagTracker.read_heads[head] == LagTracker.write_head)
-		LagTracker.last_read[head].time = 0;
+	if (lag_tracker->read_heads[head] == lag_tracker->write_head)
+		lag_tracker->last_read[head].time = 0;
 
 	if (time > now)
 	{
@@ -3532,17 +3534,17 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
 		 * eventually start moving again and cross one of our samples before
 		 * we can show the lag increasing.
 		 */
-		if (LagTracker.read_heads[head] == LagTracker.write_head)
+		if (lag_tracker->read_heads[head] == lag_tracker->write_head)
 		{
 			/* There are no future samples, so we can't interpolate. */
 			return -1;
 		}
-		else if (LagTracker.last_read[head].time != 0)
+		else if (lag_tracker->last_read[head].time != 0)
 		{
 			/* We can interpolate between last_read and the next sample. */
 			double		fraction;
-			WalTimeSample prev = LagTracker.last_read[head];
-			WalTimeSample next = LagTracker.buffer[LagTracker.read_heads[head]];
+			WalTimeSample prev = lag_tracker->last_read[head];
+			WalTimeSample next = lag_tracker->buffer[lag_tracker->read_heads[head]];
 
 			if (lsn < prev.lsn)
 			{
@@ -3579,7 +3581,7 @@ LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
 			 * standby reaches the future sample the best we can do is report
 			 * the hypothetical lag if that sample were to be replayed now.
 			 */
-			time = LagTracker.buffer[LagTracker.read_heads[head]].time;
+			time = lag_tracker->buffer[lag_tracker->read_heads[head]].time;
 		}
 	}