[PATCH] mm: pcp use non powers of 2 for batch size

Jack Steiner reported this to have fixed his problem (bad colouring):
"The patches fix both problems that I found - bad
 coloring & excessive pages in pagesets."

In most workloads this is not likely to be such a pronounced problem,
however it should help corner cases.  And avoiding powers of 2 in these
types of memory operations is always a good idea.

Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c73dbbc1cd8f..08e8627361a0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1671,6 +1671,18 @@ static void __init free_area_init_core(struct pglist_data *pgdat,
                 if (batch < 1)
                         batch = 1;
 
+                /*
+                 * Clamp the batch to a 2^n - 1 value. Having a power
+                 * of 2 value was found to be more likely to have
+                 * suboptimal cache aliasing properties in some cases.
+                 *
+                 * For example if 2 tasks are alternately allocating
+                 * batches of pages, one task can end up with a lot
+                 * of pages of one half of the possible page colors
+                 * and the other with pages of the other colors.
+                 */
+                batch = (1 << fls(batch + batch/2)) - 1;
+
                 for (cpu = 0; cpu < NR_CPUS; cpu++) {
                         struct per_cpu_pages *pcp;