Replace buffer I/O locks with condition variables.

1.  Backends waiting for buffer I/O are now interruptible.

2.  If something goes wrong in a backend that is currently performing
I/O, waiting backends no longer wake up until that backend reaches
AbortBufferIO() and broadcasts on the CV.  Previously, any waiters would
wake up (because the I/O lock was automatically released) and then
busy-loop until AbortBufferIO() cleared BM_IO_IN_PROGRESS.

3.  LWLockMinimallyPadded is removed, as it would now be unused.

Author: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (earlier version, 2016)
Discussion: https://postgr.es/m/CA%2BhUKGJ8nBFrjLuCTuqKN0pd2PQOwj9b_jnsiGFFMDvUxahj_A%40mail.gmail.com
Discussion: https://postgr.es/m/CA+Tgmoaj2aPti0yho7FeEf2qt-JgQPRWb0gci_o1Hfr=C56Xng@mail.gmail.com

Author: macdice

doc/src/sgml/monitoring.sgml

@@ -1586,6 +1586,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry>Waiting for the page number needed to continue a parallel B-tree
        scan to become available.</entry>
      </row>
+     <row>
+      <entry><literal>BufferIO</literal></entry>
+      <entry>Waiting for buffer I/O to complete.</entry>
+     </row>
      <row>
       <entry><literal>CheckpointDone</literal></entry>
       <entry>Waiting for a checkpoint to complete.</entry>
@@ -1876,10 +1880,6 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
       <entry><literal>BufferContent</literal></entry>
       <entry>Waiting to access a data page in memory.</entry>
      </row>
-     <row>
-      <entry><literal>BufferIO</literal></entry>
-      <entry>Waiting for I/O on a data page.</entry>
-     </row>
      <row>
       <entry><literal>BufferMapping</literal></entry>
       <entry>Waiting to associate a data block with a buffer in the buffer

src/backend/postmaster/pgstat.c

@@ -4010,6 +4010,9 @@ pgstat_get_wait_ipc(WaitEventIPC w)
 		case WAIT_EVENT_BTREE_PAGE:
 			event_name = "BtreePage";
 			break;
+		case WAIT_EVENT_BUFFER_IO:
+			event_name = "BufferIO";
+			break;
 		case WAIT_EVENT_CHECKPOINT_DONE:
 			event_name = "CheckpointDone";
 			break;

src/backend/storage/buffer/README

@@ -145,14 +145,11 @@ held within the buffer.  Each buffer header also contains an LWLock, the
 "buffer content lock", that *does* represent the right to access the data
 in the buffer.  It is used per the rules above.
 
-There is yet another set of per-buffer LWLocks, the io_in_progress locks,
-that are used to wait for I/O on a buffer to complete.  The process doing
-a read or write takes exclusive lock for the duration, and processes that
-need to wait for completion try to take shared locks (which they release
-immediately upon obtaining).  XXX on systems where an LWLock represents
-nontrivial resources, it's fairly annoying to need so many locks.  Possibly
-we could use per-backend LWLocks instead (a buffer header would then contain
-a field to show which backend is doing its I/O).
+* The BM_IO_IN_PROGRESS flag acts as a kind of lock, used to wait for I/O on a
+buffer to complete (and in releases before 14, it was accompanied by a
+per-buffer LWLock).  The process doing a read or write sets the flag for the
+duration, and processes that need to wait for it to be cleared sleep on a
+condition variable.
 
 
 Normal Buffer Replacement Strategy

src/backend/storage/buffer/buf_init.c

@@ -19,7 +19,7 @@
 
 BufferDescPadded *BufferDescriptors;
 char	   *BufferBlocks;
-LWLockMinimallyPadded *BufferIOLWLockArray = NULL;
+ConditionVariableMinimallyPadded *BufferIOCVArray;
 WritebackContext BackendWritebackContext;
 CkptSortItem *CkptBufferIds;
 
@@ -68,7 +68,7 @@ InitBufferPool(void)
 {
 	bool		foundBufs,
 				foundDescs,
-				foundIOLocks,
+				foundIOCV,
 				foundBufCkpt;
 
 	/* Align descriptors to a cacheline boundary. */
@@ -81,11 +81,11 @@ InitBufferPool(void)
 		ShmemInitStruct("Buffer Blocks",
 						NBuffers * (Size) BLCKSZ, &foundBufs);
 
-	/* Align lwlocks to cacheline boundary */
-	BufferIOLWLockArray = (LWLockMinimallyPadded *)
-		ShmemInitStruct("Buffer IO Locks",
-						NBuffers * (Size) sizeof(LWLockMinimallyPadded),
-						&foundIOLocks);
+	/* Align condition variables to cacheline boundary. */
+	BufferIOCVArray = (ConditionVariableMinimallyPadded *)
+		ShmemInitStruct("Buffer IO Condition Variables",
+						NBuffers * sizeof(ConditionVariableMinimallyPadded),
+						&foundIOCV);
 
 	/*
 	 * The array used to sort to-be-checkpointed buffer ids is located in
@@ -98,10 +98,10 @@ InitBufferPool(void)
 		ShmemInitStruct("Checkpoint BufferIds",
 						NBuffers * sizeof(CkptSortItem), &foundBufCkpt);
 
-	if (foundDescs || foundBufs || foundIOLocks || foundBufCkpt)
+	if (foundDescs || foundBufs || foundIOCV || foundBufCkpt)
 	{
 		/* should find all of these, or none of them */
-		Assert(foundDescs && foundBufs && foundIOLocks && foundBufCkpt);
+		Assert(foundDescs && foundBufs && foundIOCV && foundBufCkpt);
 		/* note: this path is only taken in EXEC_BACKEND case */
 	}
 	else
@@ -131,8 +131,7 @@ InitBufferPool(void)
 			LWLockInitialize(BufferDescriptorGetContentLock(buf),
 							 LWTRANCHE_BUFFER_CONTENT);
 
-			LWLockInitialize(BufferDescriptorGetIOLock(buf),
-							 LWTRANCHE_BUFFER_IO);
+			ConditionVariableInit(BufferDescriptorGetIOCV(buf));
 		}
 
 		/* Correct last entry of linked list */
@@ -169,16 +168,9 @@ BufferShmemSize(void)
 	/* size of stuff controlled by freelist.c */
 	size = add_size(size, StrategyShmemSize());
 
-	/*
-	 * It would be nice to include the I/O locks in the BufferDesc, but that
-	 * would increase the size of a BufferDesc to more than one cache line,
-	 * and benchmarking has shown that keeping every BufferDesc aligned on a
-	 * cache line boundary is important for performance.  So, instead, the
-	 * array of I/O locks is allocated in a separate tranche.  Because those
-	 * locks are not highly contended, we lay out the array with minimal
-	 * padding.
-	 */
-	size = add_size(size, mul_size(NBuffers, sizeof(LWLockMinimallyPadded)));
+	/* size of I/O condition variables */
+	size = add_size(size, mul_size(NBuffers,
+								   sizeof(ConditionVariableMinimallyPadded)));
 	/* to allow aligning the above */
 	size = add_size(size, PG_CACHE_LINE_SIZE);
 

src/backend/storage/buffer/bufmgr.c

@@ -1352,9 +1352,10 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 	LWLockRelease(newPartitionLock);
 
 	/*
-	 * Buffer contents are currently invalid.  Try to get the io_in_progress
-	 * lock.  If StartBufferIO returns false, then someone else managed to
-	 * read it before we did, so there's nothing left for BufferAlloc() to do.
+	 * Buffer contents are currently invalid.  Try to obtain the right to
+	 * start I/O.  If StartBufferIO returns false, then someone else managed
+	 * to read it before we did, so there's nothing left for BufferAlloc() to
+	 * do.
 	 */
 	if (StartBufferIO(buf, true))
 		*foundPtr = false;
@@ -1777,9 +1778,8 @@ UnpinBuffer(BufferDesc *buf, bool fixOwner)
 		 */
 		VALGRIND_MAKE_MEM_NOACCESS(BufHdrGetBlock(buf), BLCKSZ);
 
-		/* I'd better not still hold any locks on the buffer */
+		/* I'd better not still hold the buffer content lock */
 		Assert(!LWLockHeldByMe(BufferDescriptorGetContentLock(buf)));
-		Assert(!LWLockHeldByMe(BufferDescriptorGetIOLock(buf)));
 
 		/*
 		 * Decrement the shared reference count.
@@ -2742,9 +2742,9 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln)
 	uint32		buf_state;
 
 	/*
-	 * Acquire the buffer's io_in_progress lock.  If StartBufferIO returns
-	 * false, then someone else flushed the buffer before we could, so we need
-	 * not do anything.
+	 * Try to start an I/O operation.  If StartBufferIO returns false, then
+	 * someone else flushed the buffer before we could, so we need not do
+	 * anything.
 	 */
 	if (!StartBufferIO(buf, false))
 		return;
@@ -2800,7 +2800,7 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln)
 	/*
 	 * Now it's safe to write buffer to disk. Note that no one else should
 	 * have been able to write it while we were busy with log flushing because
-	 * we have the io_in_progress lock.
+	 * only one process at a time can set the BM_IO_IN_PROGRESS bit.
 	 */
 	bufBlock = BufHdrGetBlock(buf);
 
@@ -2835,7 +2835,7 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln)
 
 	/*
 	 * Mark the buffer as clean (unless BM_JUST_DIRTIED has become set) and
-	 * end the io_in_progress state.
+	 * end the BM_IO_IN_PROGRESS state.
 	 */
 	TerminateBufferIO(buf, true, 0);
 
@@ -4271,7 +4271,7 @@ IsBufferCleanupOK(Buffer buffer)
  *	Functions for buffer I/O handling
  *
  *	Note: We assume that nested buffer I/O never occurs.
- *	i.e at most one io_in_progress lock is held per proc.
+ *	i.e at most one BM_IO_IN_PROGRESS bit is set per proc.
  *
  *	Also note that these are used only for shared buffers, not local ones.
  */
@@ -4282,13 +4282,9 @@ IsBufferCleanupOK(Buffer buffer)
 static void
 WaitIO(BufferDesc *buf)
 {
-	/*
-	 * Changed to wait until there's no IO - Inoue 01/13/2000
-	 *
-	 * Note this is *necessary* because an error abort in the process doing
-	 * I/O could release the io_in_progress_lock prematurely. See
-	 * AbortBufferIO.
-	 */
+	ConditionVariable *cv = BufferDescriptorGetIOCV(buf);
+
+	ConditionVariablePrepareToSleep(cv);
 	for (;;)
 	{
 		uint32		buf_state;
@@ -4303,9 +4299,9 @@ WaitIO(BufferDesc *buf)
 
 		if (!(buf_state & BM_IO_IN_PROGRESS))
 			break;
-		LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_SHARED);
-		LWLockRelease(BufferDescriptorGetIOLock(buf));
+		ConditionVariableSleep(cv, WAIT_EVENT_BUFFER_IO);
 	}
+	ConditionVariableCancelSleep();
 }
 
 /*
@@ -4317,7 +4313,7 @@ WaitIO(BufferDesc *buf)
  * In some scenarios there are race conditions in which multiple backends
  * could attempt the same I/O operation concurrently.  If someone else
  * has already started I/O on this buffer then we will block on the
- * io_in_progress lock until he's done.
+ * I/O condition variable until he's done.
  *
  * Input operations are only attempted on buffers that are not BM_VALID,
  * and output operations only on buffers that are BM_VALID and BM_DIRTY,
@@ -4335,25 +4331,11 @@ StartBufferIO(BufferDesc *buf, bool forInput)
 
 	for (;;)
 	{
-		/*
-		 * Grab the io_in_progress lock so that other processes can wait for
-		 * me to finish the I/O.
-		 */
-		LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_EXCLUSIVE);
-
 		buf_state = LockBufHdr(buf);
 
 		if (!(buf_state & BM_IO_IN_PROGRESS))
 			break;
-
-		/*
-		 * The only way BM_IO_IN_PROGRESS could be set when the io_in_progress
-		 * lock isn't held is if the process doing the I/O is recovering from
-		 * an error (see AbortBufferIO).  If that's the case, we must wait for
-		 * him to get unwedged.
-		 */
 		UnlockBufHdr(buf, buf_state);
-		LWLockRelease(BufferDescriptorGetIOLock(buf));
 		WaitIO(buf);
 	}
 
@@ -4363,7 +4345,6 @@ StartBufferIO(BufferDesc *buf, bool forInput)
 	{
 		/* someone else already did the I/O */
 		UnlockBufHdr(buf, buf_state);
-		LWLockRelease(BufferDescriptorGetIOLock(buf));
 		return false;
 	}
 
@@ -4381,7 +4362,6 @@ StartBufferIO(BufferDesc *buf, bool forInput)
  *	(Assumptions)
  *	My process is executing IO for the buffer
  *	BM_IO_IN_PROGRESS bit is set for the buffer
- *	We hold the buffer's io_in_progress lock
  *	The buffer is Pinned
  *
  * If clear_dirty is true and BM_JUST_DIRTIED is not set, we clear the
@@ -4413,7 +4393,7 @@ TerminateBufferIO(BufferDesc *buf, bool clear_dirty, uint32 set_flag_bits)
 
 	InProgressBuf = NULL;
 
-	LWLockRelease(BufferDescriptorGetIOLock(buf));
+	ConditionVariableBroadcast(BufferDescriptorGetIOCV(buf));
 }
 
 /*
@@ -4434,14 +4414,6 @@ AbortBufferIO(void)
 	{
 		uint32		buf_state;
 
-		/*
-		 * Since LWLockReleaseAll has already been called, we're not holding
-		 * the buffer's io_in_progress_lock. We have to re-acquire it so that
-		 * we can use TerminateBufferIO. Anyone who's executing WaitIO on the
-		 * buffer will be in a busy spin until we succeed in doing this.
-		 */
-		LWLockAcquire(BufferDescriptorGetIOLock(buf), LW_EXCLUSIVE);
-
 		buf_state = LockBufHdr(buf);
 		Assert(buf_state & BM_IO_IN_PROGRESS);
 		if (IsForInput)

src/backend/storage/lmgr/lwlock.c

@@ -146,8 +146,6 @@ static const char *const BuiltinTrancheNames[] = {
 	"WALInsert",
 	/* LWTRANCHE_BUFFER_CONTENT: */
 	"BufferContent",
-	/* LWTRANCHE_BUFFER_IO: */
-	"BufferIO",
 	/* LWTRANCHE_REPLICATION_ORIGIN_STATE: */
 	"ReplicationOriginState",
 	/* LWTRANCHE_REPLICATION_SLOT_IO: */
@@ -469,8 +467,7 @@ CreateLWLocks(void)
 	StaticAssertStmt(LW_VAL_EXCLUSIVE > (uint32) MAX_BACKENDS,
 					 "MAX_BACKENDS too big for lwlock.c");
 
-	StaticAssertStmt(sizeof(LWLock) <= LWLOCK_MINIMAL_SIZE &&
-					 sizeof(LWLock) <= LWLOCK_PADDED_SIZE,
+	StaticAssertStmt(sizeof(LWLock) <= LWLOCK_PADDED_SIZE,
 					 "Miscalculated LWLock padding");
 
 	if (!IsUnderPostmaster)

src/include/pgstat.h

@@ -971,6 +971,7 @@ typedef enum
 	WAIT_EVENT_BGWORKER_SHUTDOWN,
 	WAIT_EVENT_BGWORKER_STARTUP,
 	WAIT_EVENT_BTREE_PAGE,
+	WAIT_EVENT_BUFFER_IO,
 	WAIT_EVENT_CHECKPOINT_DONE,
 	WAIT_EVENT_CHECKPOINT_START,
 	WAIT_EVENT_EXECUTE_GATHER,

src/include/storage/buf_internals.h

@@ -18,6 +18,7 @@
 #include "port/atomics.h"
 #include "storage/buf.h"
 #include "storage/bufmgr.h"
+#include "storage/condition_variable.h"
 #include "storage/latch.h"
 #include "storage/lwlock.h"
 #include "storage/shmem.h"
@@ -184,7 +185,6 @@ typedef struct BufferDesc
 
 	int			wait_backend_pid;	/* backend PID of pin-count waiter */
 	int			freeNext;		/* link in freelist chain */
-
 	LWLock		content_lock;	/* to lock access to buffer contents */
 } BufferDesc;
 
@@ -221,12 +221,12 @@ typedef union BufferDescPadded
 
 #define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
 
-#define BufferDescriptorGetIOLock(bdesc) \
-	(&(BufferIOLWLockArray[(bdesc)->buf_id]).lock)
+#define BufferDescriptorGetIOCV(bdesc) \
+	(&(BufferIOCVArray[(bdesc)->buf_id]).cv)
 #define BufferDescriptorGetContentLock(bdesc) \
 	((LWLock*) (&(bdesc)->content_lock))
 
-extern PGDLLIMPORT LWLockMinimallyPadded *BufferIOLWLockArray;
+extern PGDLLIMPORT ConditionVariableMinimallyPadded *BufferIOCVArray;
 
 /*
  * The freeNext field is either the index of the next freelist entry,

src/include/storage/condition_variable.h

@@ -31,6 +31,17 @@ typedef struct
 	proclist_head wakeup;		/* list of wake-able processes */
 } ConditionVariable;
 
+/*
+ * Pad a condition variable to a power-of-two size so that an array of
+ * condition variables does not cross a cache line boundary.
+ */
+#define CV_MINIMAL_SIZE		(sizeof(ConditionVariable) <= 16 ? 16 : 32)
+typedef union ConditionVariableMinimallyPadded
+{
+	ConditionVariable cv;
+	char		pad[CV_MINIMAL_SIZE];
+} ConditionVariableMinimallyPadded;
+
 /* Initialize a condition variable. */
 extern void ConditionVariableInit(ConditionVariable *cv);