zcache: Set SWIZ_BITS to 8 to reduce tmem bucket lock contention.

commit e8b45534 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

@@ -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)