Make backend local tracking of buffer pins memory efficient.

Since the dawn of time (aka Postgres95) multiple pins of the same
buffer by one backend have been optimized not to modify the shared
refcount more than once. This optimization has always used a NBuffer
sized array in each backend keeping track of a backend's pins.

That array (PrivateRefCount) was one of the biggest per-backend memory
allocations, depending on the shared_buffers setting. Besides the
waste of memory it also has proven to be a performance bottleneck when
assertions are enabled as we make sure that there's no remaining pins
left at the end of transactions. Also, on servers with lots of memory
and a correspondingly high shared_buffers setting the amount of random
memory accesses can also lead to poor cpu cache efficiency.

Because of these reasons a backend's buffers pins are now kept track
of in a small statically sized array that overflows into a hash table
when necessary. Benchmarks have shown neutral to positive performance
results with considerably lower memory usage.

Patch by me, review by Robert Haas.

Discussion: 20140321182231.GA17111@alap3.anarazel.de

src/include/storage/bufmgr.h

@@ -55,7 +55,6 @@ extern int	target_prefetch_pages;
 
 /* in buf_init.c */
 extern PGDLLIMPORT char *BufferBlocks;
-extern PGDLLIMPORT int32 *PrivateRefCount;
 
 /* in localbuf.c */
 extern PGDLLIMPORT int NLocBuffer;
@@ -101,24 +100,6 @@ extern PGDLLIMPORT int32 *LocalRefCount;
 	(bufnum) != InvalidBuffer  \
 )
 
-/*
- * BufferIsPinned
- *		True iff the buffer is pinned (also checks for valid buffer number).
- *
- *		NOTE: what we check here is that *this* backend holds a pin on
- *		the buffer.  We do not care whether some other backend does.
- */
-#define BufferIsPinned(bufnum) \
-( \
-	!BufferIsValid(bufnum) ? \
-		false \
-	: \
-		BufferIsLocal(bufnum) ? \
-			(LocalRefCount[-(bufnum) - 1] > 0) \
-		: \
-			(PrivateRefCount[(bufnum) - 1] > 0) \
-)
-
 /*
  * BufferGetBlock
  *		Returns a reference to a disk page image associated with a buffer.