MDEV-15053 fixup: Reduce contention in buf_page_get_low()

svr_n_page_hash_locks: Increase from 16 to 64. Before MDEV-15058,
we used to have the buf_pool.page_hash partitioned to each instance.

rw_lock_lock_word_decr(): Sleep a little in the spinloop.

rw_lock_s_lock_low(): Correct a comment. The function does perform
spinning.

This improves scalability in read-only workloads on a 32-CPU system
when the number of concurrent connections exceeds the CPU core count.

Thanks to Axel Schwenke for running benchmarks.

storage/innobase/handler/ha_innodb.cc

@@ -19488,7 +19488,7 @@ static MYSQL_SYSVAR_ULONG(buffer_pool_chunk_size, srv_buf_pool_chunk_unit,
 static MYSQL_SYSVAR_ULONG(page_hash_locks, srv_n_page_hash_locks,
   PLUGIN_VAR_OPCMDARG | PLUGIN_VAR_READONLY,
   "Number of rw_locks protecting buffer pool page_hash. Rounded up to the next power of 2",
-  NULL, NULL, 16, 1, MAX_PAGE_HASH_LOCKS, 0);
+  NULL, NULL, 64, 1, MAX_PAGE_HASH_LOCKS, 0);
 
 static MYSQL_SYSVAR_ULONG(doublewrite_batch_size, srv_doublewrite_batch_size,
   PLUGIN_VAR_OPCMDARG | PLUGIN_VAR_READONLY,

storage/innobase/include/sync0rw.h

@@ -320,8 +320,7 @@ rw_lock_validate(
 	const rw_lock_t*	lock);	/*!< in: rw-lock */
 #endif /* UNIV_DEBUG */
 /******************************************************************//**
-Low-level function which tries to lock an rw-lock in s-mode. Performs no
-spinning.
+Low-level function which tries to lock an rw-lock in s-mode.
 @return TRUE if success */
 UNIV_INLINE
 ibool

storage/innobase/include/sync0rw.ic

@@ -220,14 +220,13 @@ rw_lock_lock_word_decr(
 
 			return(true);
 		}
-		ut_delay(1);
+		(void) LF_BACKOFF();
 	}
 	return(false);
 }
 
 /******************************************************************//**
-Low-level function which tries to lock an rw-lock in s-mode. Performs no
-spinning.
+Low-level function which tries to lock an rw-lock in s-mode.
 @return TRUE if success */
 UNIV_INLINE
 ibool

storage/innobase/srv/srv0srv.cc

@@ -209,7 +209,7 @@ const ulint	srv_buf_pool_def_size	= 128 * 1024 * 1024;
 ulong	srv_buf_pool_chunk_unit;
 /** innodb_page_hash_locks (a debug-only parameter);
 number of locks to protect buf_pool.page_hash */
-ulong	srv_n_page_hash_locks = 16;
+ulong	srv_n_page_hash_locks = 64;
 /** innodb_lru_scan_depth; number of blocks scanned in LRU flush batch */
 ulong	srv_LRU_scan_depth;
 /** innodb_flush_neighbors; whether or not to flush neighbors of a block */