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Commit e8b45534 authored by Dan Magenheimer's avatar Dan Magenheimer Committed by Greg Kroah-Hartman
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zcache: Set SWIZ_BITS to 8 to reduce tmem bucket lock contention. 



SWIZ_BITS > 8 results in a much larger number of "tmem_obj"
allocations, likely one per page-placed-in-frontswap.  The
tmem_obj is not huge (roughly 100 bytes), but it is large
enough to add a not-insignificant memory overhead to zcache.

The SWIZ_BITS=8  will get roughly the same lock contention
without the space wastage.

The effect of SWIZ_BITS can be thought of as "2^SWIZ_BITS is
the number of unique oids that be generated" (This concept is
limited to frontswap's use of tmem).

Acked-by: default avatarSeth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@suse.de>
parent 9256a478

drivers/staging/zcache/zcache-main.c

+2 −2
Original line number Diff line number Diff line
@@ -1781,9 +1781,9 @@ 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()

 * frontswap_get_page(), but has side-effects. Hence using 8.

 */

#define SWIZ_BITS		27

#define SWIZ_BITS		8

#define SWIZ_MASK		((1 << SWIZ_BITS) - 1)

#define _oswiz(_type, _ind)	((_type << SWIZ_BITS) | (_ind & SWIZ_MASK))

#define iswiz(_ind)		(_ind >> SWIZ_BITS)