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Subject[patch 82/86] zcache: Set SWIZ_BITS to 8 to reduce tmem bucket lock contention.
3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dan Magenheimer <dan.magenheimer@oracle.com>

commit e8b4553457e78bcff90f70a31212a40a8fd4f0db upstream.

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: Seth Jennings <sjenning@linux.vnet.ibm.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/staging/zcache/zcache-main.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/staging/zcache/zcache-main.c
+++ b/drivers/staging/zcache/zcache-main.c
@@ -1782,9 +1782,9 @@ 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)



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