Lockless StrategyGetBuffer clock sweep hot path.

StrategyGetBuffer() has proven to be a bottleneck in a number of
buffer acquisition heavy workloads. To some degree this has already
been alleviated by 5d7962c6, but it still can be quite a heavy
bottleneck.  The problem is that in unfortunate usage patterns a
single StrategyGetBuffer() call will have to look at a large number of
buffers - in turn making it likely that the process will be put to
sleep while still holding the spinlock.

Replace most of the usage of the buffer_strategy_lock spinlock for the
clock sweep by a atomic nextVictimBuffer variable. That variable,
modulo NBuffers, is the current hand of the clock sweep. The buffer
clock-sweep then only needs to acquire the spinlock after a
wraparound. And even then only in the process that did the wrapping
around. That alleviates nearly all the contention on the relevant
spinlock, although significant contention on the cacheline can still
exist.

Reviewed-By: Robert Haas and Amit Kapila

Discussion: 20141010160020.GG6670@alap3.anarazel.de,
    20141027133218.GA2639@awork2.anarazel.de

src/backend/postmaster/bgwriter.c

@@ -373,13 +373,13 @@ BackgroundWriterMain(void)
 		if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
 		{
 			/* Ask for notification at next buffer allocation */
-			StrategyNotifyBgWriter(&MyProc->procLatch);
+			StrategyNotifyBgWriter(MyProc->pgprocno);
 			/* Sleep ... */
 			rc = WaitLatch(&MyProc->procLatch,
 						   WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
 						   BgWriterDelay * HIBERNATE_FACTOR);
 			/* Reset the notification request in case we timed out */
-			StrategyNotifyBgWriter(NULL);
+			StrategyNotifyBgWriter(-1);
 		}
 
 		/*

src/backend/storage/buffer/freelist.c

@@ -15,8 +15,12 @@
  */
 #include "postgres.h"
 
+#include "port/atomics.h"
 #include "storage/buf_internals.h"
 #include "storage/bufmgr.h"
+#include "storage/proc.h"
+
+#define INT_ACCESS_ONCE(var)	((int)(*((volatile int *)&(var))))
 
 
 /*
@@ -27,8 +31,12 @@ typedef struct
 	/* Spinlock: protects the values below */
 	slock_t		buffer_strategy_lock;
 
-	/* Clock sweep hand: index of next buffer to consider grabbing */
-	int			nextVictimBuffer;
+	/*
+	 * Clock sweep hand: index of next buffer to consider grabbing. Note that
+	 * this isn't a concrete buffer - we only ever increase the value. So, to
+	 * get an actual buffer, it needs to be used modulo NBuffers.
+	 */
+	pg_atomic_uint32 nextVictimBuffer;
 
 	int			firstFreeBuffer;	/* Head of list of unused buffers */
 	int			lastFreeBuffer; /* Tail of list of unused buffers */
@@ -42,13 +50,14 @@ typedef struct
 	 * Statistics.  These counters should be wide enough that they can't
 	 * overflow during a single bgwriter cycle.
 	 */
-	uint32		completePasses; /* Complete cycles of the clock sweep */
-	uint32		numBufferAllocs;	/* Buffers allocated since last reset */
+	uint32		 completePasses; /* Complete cycles of the clock sweep */
+	pg_atomic_uint32 numBufferAllocs;	/* Buffers allocated since last reset */
 
 	/*
-	 * Notification latch, or NULL if none.  See StrategyNotifyBgWriter.
+	 * Bgworker process to be notified upon activity or -1 if none. See
+	 * StrategyNotifyBgWriter.
 	 */
-	Latch	   *bgwriterLatch;
+	int			bgwprocno;
 } BufferStrategyControl;
 
 /* Pointers to shared state */
@@ -93,6 +102,70 @@ static volatile BufferDesc *GetBufferFromRing(BufferAccessStrategy strategy);
 static void AddBufferToRing(BufferAccessStrategy strategy,
 				volatile BufferDesc *buf);
 
+/*
+ * ClockSweepTick - Helper routine for StrategyGetBuffer()
+ *
+ * Move the clock hand one buffer ahead of its current position and return the
+ * id of the buffer now under the hand.
+ */
+static inline uint32
+ClockSweepTick(void)
+{
+	uint32 victim;
+
+	/*
+	 * Atomically move hand ahead one buffer - if there's several processes
+	 * doing this, this can lead to buffers being returned slightly out of
+	 * apparent order.
+	 */
+	victim =
+		pg_atomic_fetch_add_u32(&StrategyControl->nextVictimBuffer, 1);
+
+	if (victim >= NBuffers)
+	{
+		uint32 originalVictim = victim;
+
+		/* always wrap what we look up in BufferDescriptors */
+		victim = victim % NBuffers;
+
+		/*
+		 * If we're the one that just caused a wraparound, force
+		 * completePasses to be incremented while holding the spinlock. We
+		 * need the spinlock so StrategySyncStart() can return a consistent
+		 * value consisting of nextVictimBuffer and completePasses.
+		 */
+		if (victim == 0)
+		{
+			uint32 expected;
+			uint32 wrapped;
+			bool success = false;
+
+			expected = originalVictim + 1;
+
+			while (!success)
+			{
+				/*
+				 * Acquire the spinlock while increasing completePasses. That
+				 * allows other readers to read nextVictimBuffer and
+				 * completePasses in a consistent manner which is required for
+				 * StrategySyncStart().  In theory delaying the increment
+				 * could lead to a overflow of nextVictimBuffers, but that's
+				 * highly unlikely and wouldn't be particularly harmful.
+				 */
+				SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
+
+				wrapped = expected % NBuffers;
+
+				success = pg_atomic_compare_exchange_u32(&StrategyControl->nextVictimBuffer,
+														 &expected, wrapped);
+				if (success)
+					StrategyControl->completePasses++;
+				SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+			}
+		}
+	}
+	return victim;
+}
 
 /*
  * StrategyGetBuffer
@@ -110,7 +183,7 @@ volatile BufferDesc *
 StrategyGetBuffer(BufferAccessStrategy strategy)
 {
 	volatile BufferDesc *buf;
-	Latch	   *bgwriterLatch;
+	int			bgwprocno;
 	int			trycounter;
 
 	/*
@@ -124,86 +197,107 @@ StrategyGetBuffer(BufferAccessStrategy strategy)
 			return buf;
 	}
 
-	/* Nope, so lock the freelist */
-	SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
+	/*
+	 * If asked, we need to waken the bgwriter. Since we don't want to rely on
+	 * a spinlock for this we force a read from shared memory once, and then
+	 * set the latch based on that value. We need to go through that length
+	 * because otherwise bgprocno might be reset while/after we check because
+	 * the compiler might just reread from memory.
+	 *
+	 * This can possibly set the latch of the wrong process if the bgwriter
+	 * dies in the wrong moment. But since PGPROC->procLatch is never
+	 * deallocated the worst consequence of that is that we set the latch of
+	 * some arbitrary process.
+	 */
+	bgwprocno = INT_ACCESS_ONCE(StrategyControl->bgwprocno);
+	if (bgwprocno != -1)
+	{
+		/* reset bgwprocno first, before setting the latch */
+		StrategyControl->bgwprocno = -1;
+		pg_write_barrier();
+
+		/*
+		 * Not acquiring ProcArrayLock here which is slightly icky. It's
+		 * actually fine because procLatch isn't ever freed, so we just can
+		 * potentially set the wrong process' (or no process') latch.
+		 */
+		SetLatch(&ProcGlobal->allProcs[bgwprocno].procLatch);
+	}
 
 	/*
 	 * We count buffer allocation requests so that the bgwriter can estimate
 	 * the rate of buffer consumption.  Note that buffers recycled by a
 	 * strategy object are intentionally not counted here.
 	 */
-	StrategyControl->numBufferAllocs++;
+	pg_atomic_fetch_add_u32(&StrategyControl->numBufferAllocs, 1);
 
 	/*
-	 * If bgwriterLatch is set, we need to waken the bgwriter, but we should
-	 * not do so while holding buffer_strategy_lock; so release and re-grab.
-	 * This is annoyingly tedious, but it happens at most once per bgwriter
-	 * cycle, so the performance hit is minimal.
+	 * First check, without acquiring the lock, whether there's buffers in the
+	 * freelist. Since we otherwise don't require the spinlock in every
+	 * StrategyGetBuffer() invocation, it'd be sad to acquire it here -
+	 * uselessly in most cases. That obviously leaves a race where a buffer is
+	 * put on the freelist but we don't see the store yet - but that's pretty
+	 * harmless, it'll just get used during the next buffer acquisition.
+	 *
+	 * If there's buffers on the freelist, acquire the spinlock to pop one
+	 * buffer of the freelist. Then check whether that buffer is usable and
+	 * repeat if not.
+	 *
+	 * Note that the freeNext fields are considered to be protected by the
+	 * buffer_strategy_lock not the individual buffer spinlocks, so it's OK to
+	 * manipulate them without holding the spinlock.
 	 */
-	bgwriterLatch = StrategyControl->bgwriterLatch;
-	if (bgwriterLatch)
+	if (StrategyControl->firstFreeBuffer >= 0)
 	{
-		StrategyControl->bgwriterLatch = NULL;
-		SpinLockRelease(&StrategyControl->buffer_strategy_lock);
-		SetLatch(bgwriterLatch);
-		SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
-	}
+		while (true)
+		{
+			/* Acquire the spinlock to remove element from the freelist */
+			SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
 
-	/*
-	 * Try to get a buffer from the freelist.  Note that the freeNext fields
-	 * are considered to be protected by the buffer_strategy_lock not the
-	 * individual buffer spinlocks, so it's OK to manipulate them without
-	 * holding the spinlock.
-	 */
-	while (StrategyControl->firstFreeBuffer >= 0)
-	{
-		buf = &BufferDescriptors[StrategyControl->firstFreeBuffer];
-		Assert(buf->freeNext != FREENEXT_NOT_IN_LIST);
+			if (StrategyControl->firstFreeBuffer < 0)
+			{
+				SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+				break;
+			}
 
-		/* Unconditionally remove buffer from freelist */
-		StrategyControl->firstFreeBuffer = buf->freeNext;
-		buf->freeNext = FREENEXT_NOT_IN_LIST;
+			buf = &BufferDescriptors[StrategyControl->firstFreeBuffer];
+			Assert(buf->freeNext != FREENEXT_NOT_IN_LIST);
 
-		/*
-		 * Release the lock so someone else can access the freelist (or run
-		 * the clocksweep) while we check out this buffer.
-		 */
-		SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+			/* Unconditionally remove buffer from freelist */
+			StrategyControl->firstFreeBuffer = buf->freeNext;
+			buf->freeNext = FREENEXT_NOT_IN_LIST;
 
-		/*
-		 * If the buffer is pinned or has a nonzero usage_count, we cannot use
-		 * it; discard it and retry.  (This can only happen if VACUUM put a
-		 * valid buffer in the freelist and then someone else used it before
-		 * we got to it.  It's probably impossible altogether as of 8.3, but
-		 * we'd better check anyway.)
-		 */
-		LockBufHdr(buf);
-		if (buf->refcount == 0 && buf->usage_count == 0)
-		{
-			if (strategy != NULL)
-				AddBufferToRing(strategy, buf);
-			return buf;
-		}
-		UnlockBufHdr(buf);
+			/*
+			 * Release the lock so someone else can access the freelist while
+			 * we check out this buffer.
+			 */
+			SpinLockRelease(&StrategyControl->buffer_strategy_lock);
 
-		/* Reacquire the lock and go around for another pass. */
-		SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
+			/*
+			 * If the buffer is pinned or has a nonzero usage_count, we cannot
+			 * use it; discard it and retry.  (This can only happen if VACUUM
+			 * put a valid buffer in the freelist and then someone else used
+			 * it before we got to it.  It's probably impossible altogether as
+			 * of 8.3, but we'd better check anyway.)
+			 */
+			LockBufHdr(buf);
+			if (buf->refcount == 0 && buf->usage_count == 0)
+			{
+				if (strategy != NULL)
+					AddBufferToRing(strategy, buf);
+				return buf;
+			}
+			UnlockBufHdr(buf);
+
+		}
 	}
 
 	/* Nothing on the freelist, so run the "clock sweep" algorithm */
 	trycounter = NBuffers;
 	for (;;)
 	{
-		buf = &BufferDescriptors[StrategyControl->nextVictimBuffer];
-
-		if (++StrategyControl->nextVictimBuffer >= NBuffers)
-		{
-			StrategyControl->nextVictimBuffer = 0;
-			StrategyControl->completePasses++;
-		}
 
-		/* Release the lock before manipulating the candidate buffer. */
-		SpinLockRelease(&StrategyControl->buffer_strategy_lock);
+		buf = &BufferDescriptors[ClockSweepTick()];
 
 		/*
 		 * If the buffer is pinned or has a nonzero usage_count, we cannot use
@@ -238,9 +332,6 @@ StrategyGetBuffer(BufferAccessStrategy strategy)
 			elog(ERROR, "no unpinned buffers available");
 		}
 		UnlockBufHdr(buf);
-
-		/* Reacquire the lock and get a new candidate buffer. */
-		SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
 	}
 }
 
@@ -281,16 +372,26 @@ StrategyFreeBuffer(volatile BufferDesc *buf)
 int
 StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc)
 {
+	uint32		nextVictimBuffer;
 	int			result;
 
 	SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
-	result = StrategyControl->nextVictimBuffer;
+	nextVictimBuffer = pg_atomic_read_u32(&StrategyControl->nextVictimBuffer);
+	result = nextVictimBuffer % NBuffers;
+
 	if (complete_passes)
+	{
 		*complete_passes = StrategyControl->completePasses;
+		/*
+		 * Additionally add the number of wraparounds that happened before
+		 * completePasses could be incremented. C.f. ClockSweepTick().
+		 */
+		*complete_passes += nextVictimBuffer / NBuffers;
+	}
+
 	if (num_buf_alloc)
 	{
-		*num_buf_alloc = StrategyControl->numBufferAllocs;
-		StrategyControl->numBufferAllocs = 0;
+		*num_buf_alloc = pg_atomic_exchange_u32(&StrategyControl->numBufferAllocs, 0);
 	}
 	SpinLockRelease(&StrategyControl->buffer_strategy_lock);
 	return result;
@@ -305,7 +406,7 @@ StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc)
  * from hibernation, and is not meant for anybody else to use.
  */
 void
-StrategyNotifyBgWriter(Latch *bgwriterLatch)
+StrategyNotifyBgWriter(int bgwprocno)
 {
 	/*
 	 * We acquire buffer_strategy_lock just to ensure that the store appears
@@ -313,7 +414,7 @@ StrategyNotifyBgWriter(Latch *bgwriterLatch)
 	 * infrequently, so there's no performance penalty from being safe.
 	 */
 	SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
-	StrategyControl->bgwriterLatch = bgwriterLatch;
+	StrategyControl->bgwprocno = bgwprocno;
 	SpinLockRelease(&StrategyControl->buffer_strategy_lock);
 }
 
@@ -389,14 +490,14 @@ StrategyInitialize(bool init)
 		StrategyControl->lastFreeBuffer = NBuffers - 1;
 
 		/* Initialize the clock sweep pointer */
-		StrategyControl->nextVictimBuffer = 0;
+		pg_atomic_init_u32(&StrategyControl->nextVictimBuffer, 0);
 
 		/* Clear statistics */
 		StrategyControl->completePasses = 0;
-		StrategyControl->numBufferAllocs = 0;
+		pg_atomic_init_u32(&StrategyControl->numBufferAllocs, 0);
 
 		/* No pending notification */
-		StrategyControl->bgwriterLatch = NULL;
+		StrategyControl->bgwprocno = -1;
 	}
 	else
 		Assert(!init);

src/include/storage/buf_internals.h

@@ -191,7 +191,7 @@ extern bool StrategyRejectBuffer(BufferAccessStrategy strategy,
 					 volatile BufferDesc *buf);
 
 extern int	StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc);
-extern void StrategyNotifyBgWriter(Latch *bgwriterLatch);
+extern void StrategyNotifyBgWriter(int bgwprocno);
 
 extern Size StrategyShmemSize(void);
 extern void StrategyInitialize(bool init);