shm_mq: Have the receiver set the sender's less frequently.

Instead of marking data from the ringer buffer consumed and setting the
sender's latch for every message, do it only when the amount of data we
can consume is at least 1/4 of the size of the ring buffer, or when no
data remains in the ring buffer.  This is dramatically faster in my
testing; apparently, the savings from sending signals less frequently
outweighs the benefit of letting the sender know about available buffer
space sooner.

Patch by me, reviewed by Andres Freund and tested by Rafia Sabih.

Discussion: http://postgr.es/m/CA+TgmoYK7RFj6r7KLEfSGtYZCi3zqTRhAz8mcsDbUAjEmLOZ3Q@mail.gmail.com

src/backend/storage/ipc/shm_mq.c

@@ -143,10 +143,11 @@ struct shm_mq_handle
 };
 
 static void shm_mq_detach_internal(shm_mq *mq);
-static shm_mq_result shm_mq_send_bytes(shm_mq_handle *mq, Size nbytes,
+static shm_mq_result shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes,
 				  const void *data, bool nowait, Size *bytes_written);
-static shm_mq_result shm_mq_receive_bytes(shm_mq *mq, Size bytes_needed,
-					 bool nowait, Size *nbytesp, void **datap);
+static shm_mq_result shm_mq_receive_bytes(shm_mq_handle *mqh,
+					 Size bytes_needed, bool nowait, Size *nbytesp,
+					 void **datap);
 static bool shm_mq_counterparty_gone(shm_mq *mq,
 						 BackgroundWorkerHandle *handle);
 static bool shm_mq_wait_internal(shm_mq *mq, PGPROC **ptr,
@@ -582,8 +583,14 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 		mqh->mqh_counterparty_attached = true;
 	}
 
-	/* Consume any zero-copy data from previous receive operation. */
-	if (mqh->mqh_consume_pending > 0)
+	/*
+	 * If we've consumed an amount of data greater than 1/4th of the ring
+	 * size, mark it consumed in shared memory.  We try to avoid doing this
+	 * unnecessarily when only a small amount of data has been consumed,
+	 * because SetLatch() is fairly expensive and we don't want to do it too
+	 * often.
+	 */
+	if (mqh->mqh_consume_pending > mq->mq_ring_size / 4)
 	{
 		shm_mq_inc_bytes_read(mq, mqh->mqh_consume_pending);
 		mqh->mqh_consume_pending = 0;
@@ -594,7 +601,7 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 	{
 		/* Try to receive the message length word. */
 		Assert(mqh->mqh_partial_bytes < sizeof(Size));
-		res = shm_mq_receive_bytes(mq, sizeof(Size) - mqh->mqh_partial_bytes,
+		res = shm_mq_receive_bytes(mqh, sizeof(Size) - mqh->mqh_partial_bytes,
 								   nowait, &rb, &rawdata);
 		if (res != SHM_MQ_SUCCESS)
 			return res;
@@ -614,13 +621,7 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 			needed = MAXALIGN(sizeof(Size)) + MAXALIGN(nbytes);
 			if (rb >= needed)
 			{
-				/*
-				 * Technically, we could consume the message length
-				 * information at this point, but the extra write to shared
-				 * memory wouldn't be free and in most cases we would reap no
-				 * benefit.
-				 */
-				mqh->mqh_consume_pending = needed;
+				mqh->mqh_consume_pending += needed;
 				*nbytesp = nbytes;
 				*datap = ((char *) rawdata) + MAXALIGN(sizeof(Size));
 				return SHM_MQ_SUCCESS;
@@ -632,7 +633,7 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 			 */
 			mqh->mqh_expected_bytes = nbytes;
 			mqh->mqh_length_word_complete = true;
-			shm_mq_inc_bytes_read(mq, MAXALIGN(sizeof(Size)));
+			mqh->mqh_consume_pending += MAXALIGN(sizeof(Size));
 			rb -= MAXALIGN(sizeof(Size));
 		}
 		else
@@ -651,7 +652,7 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 			}
 			Assert(mqh->mqh_buflen >= sizeof(Size));
 
-			/* Copy and consume partial length word. */
+			/* Copy partial length word; remember to consume it. */
 			if (mqh->mqh_partial_bytes + rb > sizeof(Size))
 				lengthbytes = sizeof(Size) - mqh->mqh_partial_bytes;
 			else
@@ -659,7 +660,7 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 			memcpy(&mqh->mqh_buffer[mqh->mqh_partial_bytes], rawdata,
 				   lengthbytes);
 			mqh->mqh_partial_bytes += lengthbytes;
-			shm_mq_inc_bytes_read(mq, MAXALIGN(lengthbytes));
+			mqh->mqh_consume_pending += MAXALIGN(lengthbytes);
 			rb -= lengthbytes;
 
 			/* If we now have the whole word, we're ready to read payload. */
@@ -681,13 +682,13 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 		 * we need not copy the data and can return a pointer directly into
 		 * shared memory.
 		 */
-		res = shm_mq_receive_bytes(mq, nbytes, nowait, &rb, &rawdata);
+		res = shm_mq_receive_bytes(mqh, nbytes, nowait, &rb, &rawdata);
 		if (res != SHM_MQ_SUCCESS)
 			return res;
 		if (rb >= nbytes)
 		{
 			mqh->mqh_length_word_complete = false;
-			mqh->mqh_consume_pending = MAXALIGN(nbytes);
+			mqh->mqh_consume_pending += MAXALIGN(nbytes);
 			*nbytesp = nbytes;
 			*datap = rawdata;
 			return SHM_MQ_SUCCESS;
@@ -727,13 +728,13 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 		mqh->mqh_partial_bytes += rb;
 
 		/*
-		 * Update count of bytes read, with alignment padding.  Note that this
-		 * will never actually insert any padding except at the end of a
-		 * message, because the buffer size is a multiple of MAXIMUM_ALIGNOF,
-		 * and each read and write is as well.
+		 * Update count of bytes that can be consumed, accounting for
+		 * alignment padding.  Note that this will never actually insert any
+		 * padding except at the end of a message, because the buffer size is
+		 * a multiple of MAXIMUM_ALIGNOF, and each read and write is as well.
 		 */
 		Assert(mqh->mqh_partial_bytes == nbytes || rb == MAXALIGN(rb));
-		shm_mq_inc_bytes_read(mq, MAXALIGN(rb));
+		mqh->mqh_consume_pending += MAXALIGN(rb);
 
 		/* If we got all the data, exit the loop. */
 		if (mqh->mqh_partial_bytes >= nbytes)
@@ -741,7 +742,7 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 
 		/* Wait for some more data. */
 		still_needed = nbytes - mqh->mqh_partial_bytes;
-		res = shm_mq_receive_bytes(mq, still_needed, nowait, &rb, &rawdata);
+		res = shm_mq_receive_bytes(mqh, still_needed, nowait, &rb, &rawdata);
 		if (res != SHM_MQ_SUCCESS)
 			return res;
 		if (rb > still_needed)
@@ -1007,9 +1008,10 @@ shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes, const void *data,
  * is SHM_MQ_SUCCESS.
  */
 static shm_mq_result
-shm_mq_receive_bytes(shm_mq *mq, Size bytes_needed, bool nowait,
+shm_mq_receive_bytes(shm_mq_handle *mqh, Size bytes_needed, bool nowait,
 					 Size *nbytesp, void **datap)
 {
+	shm_mq	   *mq = mqh->mqh_queue;
 	Size		ringsize = mq->mq_ring_size;
 	uint64		used;
 	uint64		written;
@@ -1021,7 +1023,13 @@ shm_mq_receive_bytes(shm_mq *mq, Size bytes_needed, bool nowait,
 
 		/* Get bytes written, so we can compute what's available to read. */
 		written = pg_atomic_read_u64(&mq->mq_bytes_written);
-		read = pg_atomic_read_u64(&mq->mq_bytes_read);
+
+		/*
+		 * Get bytes read.  Include bytes we could consume but have not yet
+		 * consumed.
+		 */
+		read = pg_atomic_read_u64(&mq->mq_bytes_read) +
+			mqh->mqh_consume_pending;
 		used = written - read;
 		Assert(used <= ringsize);
 		offset = read % (uint64) ringsize;
@@ -1052,6 +1060,16 @@ shm_mq_receive_bytes(shm_mq *mq, Size bytes_needed, bool nowait,
 		if (mq->mq_detached)
 			return SHM_MQ_DETACHED;
 
+		/*
+		 * We didn't get enough data to satisfy the request, so mark any data
+		 * previously-consumed as read to make more buffer space.
+		 */
+		if (mqh->mqh_consume_pending > 0)
+		{
+			shm_mq_inc_bytes_read(mq, mqh->mqh_consume_pending);
+			mqh->mqh_consume_pending = 0;
+		}
+
 		/* Skip manipulation of our latch if nowait = true. */
 		if (nowait)
 			return SHM_MQ_WOULD_BLOCK;