bufmgr: Use consistent naming of the clock-sweep algorithm

Minor edits to comments only.

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

src/backend/storage/buffer/README

@@ -211,9 +211,9 @@ Buffer Ring Replacement Strategy
 When running a query that needs to access a large number of pages just once,
 such as VACUUM or a large sequential scan, a different strategy is used.
 A page that has been touched only by such a scan is unlikely to be needed
-again soon, so instead of running the normal clock sweep algorithm and
+again soon, so instead of running the normal clock-sweep algorithm and
 blowing out the entire buffer cache, a small ring of buffers is allocated
-using the normal clock sweep algorithm and those buffers are reused for the
+using the normal clock-sweep algorithm and those buffers are reused for the
 whole scan.  This also implies that much of the write traffic caused by such
 a statement will be done by the backend itself and not pushed off onto other
 processes.

src/backend/storage/buffer/bufmgr.c

@@ -3608,7 +3608,7 @@ BufferSync(int flags)
  * This is called periodically by the background writer process.
  *
  * Returns true if it's appropriate for the bgwriter process to go into
- * low-power hibernation mode.  (This happens if the strategy clock sweep
+ * low-power hibernation mode.  (This happens if the strategy clock-sweep
  * has been "lapped" and no buffer allocations have occurred recently,
  * or if the bgwriter has been effectively disabled by setting
  * bgwriter_lru_maxpages to 0.)
@@ -3658,7 +3658,7 @@ BgBufferSync(WritebackContext *wb_context)
 	uint32		new_recent_alloc;
 
 	/*
-	 * Find out where the freelist clock sweep currently is, and how many
+	 * Find out where the freelist clock-sweep currently is, and how many
 	 * buffer allocations have happened since our last call.
 	 */
 	strategy_buf_id = StrategySyncStart(&strategy_passes, &recent_alloc);
@@ -3679,8 +3679,8 @@ BgBufferSync(WritebackContext *wb_context)
 
 	/*
 	 * Compute strategy_delta = how many buffers have been scanned by the
-	 * clock sweep since last time.  If first time through, assume none. Then
-	 * see if we are still ahead of the clock sweep, and if so, how many
+	 * clock-sweep since last time.  If first time through, assume none. Then
+	 * see if we are still ahead of the clock-sweep, and if so, how many
 	 * buffers we could scan before we'd catch up with it and "lap" it. Note:
 	 * weird-looking coding of xxx_passes comparisons are to avoid bogus
 	 * behavior when the passes counts wrap around.

src/backend/storage/buffer/freelist.c

@@ -33,7 +33,7 @@ typedef struct
 	slock_t		buffer_strategy_lock;
 
 	/*
-	 * Clock sweep hand: index of next buffer to consider grabbing. Note that
+	 * clock-sweep hand: index of next buffer to consider grabbing. Note that
 	 * this isn't a concrete buffer - we only ever increase the value. So, to
 	 * get an actual buffer, it needs to be used modulo NBuffers.
 	 */
@@ -51,7 +51,7 @@ typedef struct
 	 * Statistics.  These counters should be wide enough that they can't
 	 * overflow during a single bgwriter cycle.
 	 */
-	uint32		completePasses; /* Complete cycles of the clock sweep */
+	uint32		completePasses; /* Complete cycles of the clock-sweep */
 	pg_atomic_uint32 numBufferAllocs;	/* Buffers allocated since last reset */
 
 	/*
@@ -311,7 +311,7 @@ StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state, bool *from_r
 		}
 	}
 
-	/* Nothing on the freelist, so run the "clock sweep" algorithm */
+	/* Nothing on the freelist, so run the "clock-sweep" algorithm */
 	trycounter = NBuffers;
 	for (;;)
 	{
@@ -511,7 +511,7 @@ StrategyInitialize(bool init)
 		StrategyControl->firstFreeBuffer = 0;
 		StrategyControl->lastFreeBuffer = NBuffers - 1;
 
-		/* Initialize the clock sweep pointer */
+		/* Initialize the clock-sweep pointer */
 		pg_atomic_init_u32(&StrategyControl->nextVictimBuffer, 0);
 
 		/* Clear statistics */
@@ -759,7 +759,7 @@ GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state)
 	 *
 	 * If usage_count is 0 or 1 then the buffer is fair game (we expect 1,
 	 * since our own previous usage of the ring element would have left it
-	 * there, but it might've been decremented by clock sweep since then). A
+	 * there, but it might've been decremented by clock-sweep since then). A
 	 * higher usage_count indicates someone else has touched the buffer, so we
 	 * shouldn't re-use it.
 	 */

src/backend/storage/buffer/localbuf.c

@@ -229,7 +229,7 @@ GetLocalVictimBuffer(void)
 	ResourceOwnerEnlarge(CurrentResourceOwner);
 
 	/*
-	 * Need to get a new buffer.  We use a clock sweep algorithm (essentially
+	 * Need to get a new buffer.  We use a clock-sweep algorithm (essentially
 	 * the same as what freelist.c does now...)
 	 */
 	trycounter = NLocBuffer;

src/include/storage/buf_internals.h

@@ -80,8 +80,8 @@ StaticAssertDecl(BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS + BUF_FLAG_BITS == 32,
  * The maximum allowed value of usage_count represents a tradeoff between
  * accuracy and speed of the clock-sweep buffer management algorithm.  A
  * large value (comparable to NBuffers) would approximate LRU semantics.
- * But it can take as many as BM_MAX_USAGE_COUNT+1 complete cycles of
- * clock sweeps to find a free buffer, so in practice we don't want the
+ * But it can take as many as BM_MAX_USAGE_COUNT+1 complete cycles of the
+ * clock-sweep hand to find a free buffer, so in practice we don't want the
  * value to be very large.
  */
 #define BM_MAX_USAGE_COUNT	5