Commit 3d65c85f authored Oct 04, 2011 by Seth Jennings Committed by Greg Kroah-Hartman Oct 12, 2011

staging: zcache: reduce tmem bucket lock contention



tmem uses hash buckets each with their own rbtree and lock to
quickly lookup tmem objects.  tmem has TMEM_HASH_BUCKETS (256)
buckets per pool.  However, because of the way the tmem_oid is
generated for frontswap pages, only 16 unique tmem_oids are being
generated, resulting in only 16 of the 256 buckets being used.
This cause high lock contention for the per bucket locks.

This patch changes SWIZ_BITS to include more bits of the offset.
The result is that all 256 hash buckets are potentially used resulting in a
95% drop in hash bucket lock contention.

Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
Acked-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

parent fc2d6e57

drivers/staging/zcache/zcache-main.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -1804,8 +1804,10 @@ static int zcache_frontswap_poolid = -1;
		/*
		* Swizzling increases objects per swaptype, increasing tmem concurrency
		* for heavy swaploads. Later, larger nr_cpus -> larger SWIZ_BITS
		* Setting SWIZ_BITS to 27 basically reconstructs the swap entry from
		* frontswap_get_page()
		*/
		#define SWIZ_BITS 4
		#define SWIZ_BITS 27
		#define SWIZ_MASK ((1 << SWIZ_BITS) - 1)
		#define _oswiz(_type, _ind) ((_type << SWIZ_BITS) \| (_ind & SWIZ_MASK))
		#define iswiz(_ind) (_ind >> SWIZ_BITS)