[PATCH] mm: page_alloc: increase size of per-cpu-pages

Increase the page allocator's per-cpu magazines from 1/4MB to 1/2MB.

Over 100+ runs for a workload, the difference in mean is about 2%.  The best
results for both are almost same.  Though the max variation in results with
1/2MB is only 2.2%, whereas with 1/4MB it is 12%.

Signed-off-by: Rohit Seth <rohit.seth@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 94c864eac9c48e26577d46f5e68cb5b9b3bcba61..f799217dc2f3e950f53d66fa88b1338be7f71007 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1721,29 +1721,29 @@ static int __devinit zone_batchsize(struct zone *zone)
 
        /*
         * The per-cpu-pages pools are set to around 1000th of the
-        * size of the zone.  But no more than 1/4 of a meg - there's
-        * no point in going beyond the size of L2 cache.
+        * size of the zone.  But no more than 1/2 of a meg.
         *
         * OK, so we don't know how big the cache is.  So guess.
         */
        batch = zone->present_pages / 1024;
-       if (batch * PAGE_SIZE > 256 * 1024)
-               batch = (256 * 1024) / PAGE_SIZE;
+       if (batch * PAGE_SIZE > 512 * 1024)
+               batch = (512 * 1024) / PAGE_SIZE;
        batch /= 4;             /* We effectively *= 4 below */
        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.
+        * We will be trying to allcoate bigger chunks of contiguous
+        * memory of the order of fls(batch).  This should result in
+        * better cache coloring.
         *
-        * 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.
+        * A sanity check also to ensure that batch is still in limits.
         */
-       batch = (1 << fls(batch + batch/2)) - 1;
+       batch = (1 << fls(batch + batch/2));
+
+       if (fls(batch) >= (PAGE_SHIFT + MAX_ORDER - 2))
+               batch = PAGE_SHIFT + ((MAX_ORDER - 1 - PAGE_SHIFT)/2);
+
        return batch;
 }