3 files changed, 75 insertions, 25 deletions

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 7915ed624c1..8243f4b2445 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -211,6 +211,8 @@ static int32 PrivateRefCountOverflowed = 0;
 static uint32 PrivateRefCountClock = 0;
 static PrivateRefCountEntry *ReservedRefCountEntry = NULL;
 
+static uint32 MaxProportionalPins;
+
 static void ReservePrivateRefCountEntry(void);
 static PrivateRefCountEntry *NewPrivateRefCountEntry(Buffer buffer);
 static PrivateRefCountEntry *GetPrivateRefCountEntry(Buffer buffer, bool do_move);
@@ -2098,42 +2100,61 @@ again:
 }
 
 /*
+ * Return the maximum number of buffers that a backend should try to pin once,
+ * to avoid exceeding its fair share.  This is the highest value that
+ * GetAdditionalPinLimit() could ever return.  Note that it may be zero on a
+ * system with a very small buffer pool relative to max_connections.
+ */
+uint32
+GetPinLimit(void)
+{
+	return MaxProportionalPins;
+}
+
+/*
+ * Return the maximum number of additional buffers that this backend should
+ * pin if it wants to stay under the per-backend limit, considering the number
+ * of buffers it has already pinned.  Unlike LimitAdditionalPins(), the limit
+ * return by this function can be zero.
+ */
+uint32
+GetAdditionalPinLimit(void)
+{
+	uint32		estimated_pins_held;
+
+	/*
+	 * We get the number of "overflowed" pins for free, but don't know the
+	 * number of pins in PrivateRefCountArray.  The cost of calculating that
+	 * exactly doesn't seem worth it, so just assume the max.
+	 */
+	estimated_pins_held = PrivateRefCountOverflowed + REFCOUNT_ARRAY_ENTRIES;
+
+	/* Is this backend already holding more than its fair share? */
+	if (estimated_pins_held > MaxProportionalPins)
+		return 0;
+
+	return MaxProportionalPins - estimated_pins_held;
+}
+
+/*
  * Limit the number of pins a batch operation may additionally acquire, to
  * avoid running out of pinnable buffers.
  *
- * One additional pin is always allowed, as otherwise the operation likely
- * cannot be performed at all.
- *
- * The number of allowed pins for a backend is computed based on
- * shared_buffers and the maximum number of connections possible. That's very
- * pessimistic, but outside of toy-sized shared_buffers it should allow
- * sufficient pins.
+ * One additional pin is always allowed, on the assumption that the operation
+ * requires at least one to make progress.
  */
 void
 LimitAdditionalPins(uint32 *additional_pins)
 {
-	uint32		max_backends;
-	int			max_proportional_pins;
+	uint32		limit;
 
 	if (*additional_pins <= 1)
 		return;
 
-	max_backends = MaxBackends + NUM_AUXILIARY_PROCS;
-	max_proportional_pins = NBuffers / max_backends;
-
-	/*
-	 * Subtract the approximate number of buffers already pinned by this
-	 * backend. We get the number of "overflowed" pins for free, but don't
-	 * know the number of pins in PrivateRefCountArray. The cost of
-	 * calculating that exactly doesn't seem worth it, so just assume the max.
-	 */
-	max_proportional_pins -= PrivateRefCountOverflowed + REFCOUNT_ARRAY_ENTRIES;
-
-	if (max_proportional_pins <= 0)
-		max_proportional_pins = 1;
-
-	if (*additional_pins > max_proportional_pins)
-		*additional_pins = max_proportional_pins;
+	limit = GetAdditionalPinLimit();
+	limit = Max(limit, 1);
+	if (limit < *additional_pins)
+		*additional_pins = limit;
 }
 
 /*
@@ -3575,6 +3596,15 @@ InitBufferManagerAccess(void)
 {
 	HASHCTL		hash_ctl;
 
+	/*
+	 * An advisory limit on the number of pins each backend should hold, based
+	 * on shared_buffers and the maximum number of connections possible.
+	 * That's very pessimistic, but outside toy-sized shared_buffers it should
+	 * allow plenty of pins.  LimitAdditionalPins() and
+	 * GetAdditionalPinLimit() can be used to check the remaining balance.
+	 */
+	MaxProportionalPins = NBuffers / (MaxBackends + NUM_AUXILIARY_PROCS);
+
 	memset(&PrivateRefCountArray, 0, sizeof(PrivateRefCountArray));
 
 	hash_ctl.keysize = sizeof(int32);
diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c
index 80b83444eb2..919f600ed41 100644
--- a/src/backend/storage/buffer/localbuf.c
+++ b/src/backend/storage/buffer/localbuf.c
@@ -286,6 +286,22 @@ GetLocalVictimBuffer(void)
 	return BufferDescriptorGetBuffer(bufHdr);
 }
 
+/* see GetPinLimit() */
+uint32
+GetLocalPinLimit(void)
+{
+	/* Every backend has its own temporary buffers, and can pin them all. */
+	return num_temp_buffers;
+}
+
+/* see GetAdditionalPinLimit() */
+uint32
+GetAdditionalLocalPinLimit(void)
+{
+	Assert(NLocalPinnedBuffers <= num_temp_buffers);
+	return num_temp_buffers - NLocalPinnedBuffers;
+}
+
 /* see LimitAdditionalPins() */
 void
 LimitAdditionalLocalPins(uint32 *additional_pins)
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index b204e4731c1..79a89f87fcc 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -290,6 +290,10 @@ extern bool HoldingBufferPinThatDelaysRecovery(void);
 
 extern bool BgBufferSync(struct WritebackContext *wb_context);
 
+extern uint32 GetPinLimit(void);
+extern uint32 GetLocalPinLimit(void);
+extern uint32 GetAdditionalPinLimit(void);
+extern uint32 GetAdditionalLocalPinLimit(void);
 extern void LimitAdditionalPins(uint32 *additional_pins);
 extern void LimitAdditionalLocalPins(uint32 *additional_pins);