Specialize checkpointer sort functions.

When sorting a potentially large number of dirty buffers, the
checkpointer can benefit from a faster sort routine.  One reported
improvement on a large buffer pool system was 1.4s -> 0.6s.
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGJ2-eaDqAum5bxhpMNhvuJmRDZxB_Tow0n-gse%2BHG0Yig%40mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -488,8 +488,8 @@ static void FindAndDropRelFileNodeBuffers(RelFileNode rnode,
 static void AtProcExit_Buffers(int code, Datum arg);
 static void CheckForBufferLeaks(void);
 static int	rnode_comparator(const void *p1, const void *p2);
-static int	buffertag_comparator(const void *p1, const void *p2);
-static int	ckpt_buforder_comparator(const void *pa, const void *pb);
+static inline int buffertag_comparator(const BufferTag *a, const BufferTag *b);
+static inline int ckpt_buforder_comparator(const CkptSortItem *a, const CkptSortItem *b);
 static int	ts_ckpt_progress_comparator(Datum a, Datum b, void *arg);
 
 
@@ -1831,6 +1831,13 @@ UnpinBuffer(BufferDesc *buf, bool fixOwner)
 	}
 }
 
+#define ST_SORT sort_checkpoint_bufferids
+#define ST_ELEMENT_TYPE CkptSortItem
+#define ST_COMPARE(a, b) ckpt_buforder_comparator(a, b)
+#define ST_SCOPE static
+#define ST_DEFINE
+#include <lib/sort_template.h>
+
 /*
  * BufferSync -- Write out all dirty buffers in the pool.
  *
@@ -1931,8 +1938,7 @@ BufferSync(int flags)
 	 * end up writing to the tablespaces one-by-one; possibly overloading the
 	 * underlying system.
 	 */
-	qsort(CkptBufferIds, num_to_scan, sizeof(CkptSortItem),
-		  ckpt_buforder_comparator);
+	sort_checkpoint_bufferids(CkptBufferIds, num_to_scan);
 
 	num_spaces = 0;
 
@@ -4567,11 +4573,9 @@ WaitBufHdrUnlocked(BufferDesc *buf)
 /*
  * BufferTag comparator.
  */
-static int
-buffertag_comparator(const void *a, const void *b)
+static inline int
+buffertag_comparator(const BufferTag *ba, const BufferTag *bb)
 {
-	const BufferTag *ba = (const BufferTag *) a;
-	const BufferTag *bb = (const BufferTag *) b;
 	int			ret;
 
 	ret = rnode_comparator(&ba->rnode, &bb->rnode);
@@ -4598,12 +4602,9 @@ buffertag_comparator(const void *a, const void *b)
  * It is important that tablespaces are compared first, the logic balancing
  * writes between tablespaces relies on it.
  */
-static int
-ckpt_buforder_comparator(const void *pa, const void *pb)
+static inline int
+ckpt_buforder_comparator(const CkptSortItem *a, const CkptSortItem *b)
 {
-	const CkptSortItem *a = (const CkptSortItem *) pa;
-	const CkptSortItem *b = (const CkptSortItem *) pb;
-
 	/* compare tablespace */
 	if (a->tsId < b->tsId)
 		return -1;
@@ -4694,6 +4695,13 @@ ScheduleBufferTagForWriteback(WritebackContext *context, BufferTag *tag)
 		IssuePendingWritebacks(context);
 }
 
+#define ST_SORT sort_pending_writebacks
+#define ST_ELEMENT_TYPE PendingWriteback
+#define ST_COMPARE(a, b) buffertag_comparator(&a->tag, &b->tag)
+#define ST_SCOPE static
+#define ST_DEFINE
+#include <lib/sort_template.h>
+
 /*
  * Issue all pending writeback requests, previously scheduled with
  * ScheduleBufferTagForWriteback, to the OS.
@@ -4713,8 +4721,7 @@ IssuePendingWritebacks(WritebackContext *context)
 	 * Executing the writes in-order can make them a lot faster, and allows to
 	 * merge writeback requests to consecutive blocks into larger writebacks.
 	 */
-	qsort(&context->pending_writebacks, context->nr_pending,
-		  sizeof(PendingWriteback), buffertag_comparator);
+	sort_pending_writebacks(context->pending_writebacks, context->nr_pending);
 
 	/*
 	 * Coalesce neighbouring writes, but nothing else. For that we iterate