SLUB: rework slab order determination

In some cases SLUB is creating uselessly slabs that are larger than
slub_max_order. Also the layout of some of the slabs was not satisfactory.

Go to an iterarive approach.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/slub.c b/mm/slub.c
index ae28310..c81f52a 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1577,37 +1577,75 @@ static int slub_nomerge;
  * requested a higher mininum order then we start with that one instead of
  * the smallest order which will fit the object.
  */
-static int calculate_order(int size)
+static inline int slab_order(int size, int min_objects,
+                               int max_order, int fract_leftover)
 {
        int order;
        int rem;
 
-       for (order = max(slub_min_order, fls(size - 1) - PAGE_SHIFT);
-                       order < MAX_ORDER; order++) {
-               unsigned long slab_size = PAGE_SIZE << order;
+       for (order = max(slub_min_order,
+                               fls(min_objects * size - 1) - PAGE_SHIFT);
+                       order <= max_order; order++) {
 
-               if (order < slub_max_order &&
-                               slab_size < slub_min_objects * size)
-                       continue;
+               unsigned long slab_size = PAGE_SIZE << order;
 
-               if (slab_size < size)
+               if (slab_size < min_objects * size)
                        continue;
 
-               if (order >= slub_max_order)
-                       break;
-
                rem = slab_size % size;
 
-               if (rem <= slab_size / 8)
+               if (rem <= slab_size / fract_leftover)
                        break;
 
        }
-       if (order >= MAX_ORDER)
-               return -E2BIG;
 
        return order;
 }
 
+static inline int calculate_order(int size)
+{
+       int order;
+       int min_objects;
+       int fraction;
+
+       /*
+        * Attempt to find best configuration for a slab. This
+        * works by first attempting to generate a layout with
+        * the best configuration and backing off gradually.
+        *
+        * First we reduce the acceptable waste in a slab. Then
+        * we reduce the minimum objects required in a slab.
+        */
+       min_objects = slub_min_objects;
+       while (min_objects > 1) {
+               fraction = 8;
+               while (fraction >= 4) {
+                       order = slab_order(size, min_objects,
+                                               slub_max_order, fraction);
+                       if (order <= slub_max_order)
+                               return order;
+                       fraction /= 2;
+               }
+               min_objects /= 2;
+       }
+
+       /*
+        * We were unable to place multiple objects in a slab. Now
+        * lets see if we can place a single object there.
+        */
+       order = slab_order(size, 1, slub_max_order, 1);
+       if (order <= slub_max_order)
+               return order;
+
+       /*
+        * Doh this slab cannot be placed using slub_max_order.
+        */
+       order = slab_order(size, 1, MAX_ORDER, 1);
+       if (order <= MAX_ORDER)
+               return order;
+       return -ENOSYS;
+}
+
 /*
  * Figure out what the alignment of the objects will be.
  */