bufmgr: Remove freelist, always use clock-sweep

This set of changes removes the list of available buffers and instead simply
uses the clock-sweep algorithm to find and return an available buffer.  This
also removes the have_free_buffer() function and simply caps the
pg_autoprewarm process to at most NBuffers.

While on the surface this appears to be removing an optimization it is in fact
eliminating code that induces overhead in the form of synchronization that is
problematic for multi-core systems.

The main reason for removing the freelist, however, is not the moderate
improvement in scalability, but that having the freelist would require
dedicated complexity in several upcoming patches. As we have not been able to
find a case benefiting from the freelist...

Author: Greg Burd <greg@burd.me>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/70C6A5B5-2A20-4D0B-BC73-EB09DD62D61C@getmailspring.com

1 files changed, 3 insertions, 26 deletions

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 9fc906a4a40..fe470de63f2 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -2094,12 +2094,6 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 		 */
 		UnpinBuffer(victim_buf_hdr);
 
-		/*
-		 * The victim buffer we acquired previously is clean and unused, let
-		 * it be found again quickly
-		 */
-		StrategyFreeBuffer(victim_buf_hdr);
-
 		/* remaining code should match code at top of routine */
 
 		existing_buf_hdr = GetBufferDescriptor(existing_buf_id);
@@ -2158,8 +2152,7 @@ BufferAlloc(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 }
 
 /*
- * InvalidateBuffer -- mark a shared buffer invalid and return it to the
- * freelist.
+ * InvalidateBuffer -- mark a shared buffer invalid.
  *
  * The buffer header spinlock must be held at entry.  We drop it before
  * returning.  (This is sane because the caller must have locked the
@@ -2257,11 +2250,6 @@ retry:
 	 * Done with mapping lock.
 	 */
 	LWLockRelease(oldPartitionLock);
-
-	/*
-	 * Insert the buffer at the head of the list of free buffers.
-	 */
-	StrategyFreeBuffer(buf);
 }
 
 /*
@@ -2679,11 +2667,6 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 		{
 			BufferDesc *buf_hdr = GetBufferDescriptor(buffers[i] - 1);
 
-			/*
-			 * The victim buffer we acquired previously is clean and unused,
-			 * let it be found again quickly
-			 */
-			StrategyFreeBuffer(buf_hdr);
 			UnpinBuffer(buf_hdr);
 		}
 
@@ -2756,12 +2739,6 @@ ExtendBufferedRelShared(BufferManagerRelation bmr,
 			valid = PinBuffer(existing_hdr, strategy);
 
 			LWLockRelease(partition_lock);
-
-			/*
-			 * The victim buffer we acquired previously is clean and unused,
-			 * let it be found again quickly
-			 */
-			StrategyFreeBuffer(victim_buf_hdr);
 			UnpinBuffer(victim_buf_hdr);
 
 			buffers[i] = BufferDescriptorGetBuffer(existing_hdr);
@@ -3658,8 +3635,8 @@ BgBufferSync(WritebackContext *wb_context)
 	uint32		new_recent_alloc;
 
 	/*
-	 * Find out where the freelist clock-sweep currently is, and how many
-	 * buffer allocations have happened since our last call.
+	 * Find out where the clock-sweep currently is, and how many buffer
+	 * allocations have happened since our last call.
 	 */
 	strategy_buf_id = StrategySyncStart(&strategy_passes, &recent_alloc);