1. Background
Current slub has three layers:
* cpu_slab
* percpu_partial
* per node partial list
Slub allocator tries to get an object from top to bottom. When it can't
get an object from the upper two layers, it will search the per node
partial list. The is done in get_partial().
The abstraction of get_partial() may looks like this:
get_partial()
get_partial_node()
get_any_partial()
for_each_zone_zonelist()
The idea behind this is: it first try a local node, then try other nodes
if caller doesn't specify a node.
2. Room for Improvement
When we look one step deeper in get_any_partial(), it tries to get a
proper node by for_each_zone_zonelist(), which iterates on the
node_zonelists.
This behavior would introduce some redundant check on the same node.
Because:
* the local node is already checked in get_partial_node()
* one node may have several zones on node_zonelists
3. Solution Proposed in Patch
We could reduce these redundant check by record the last unsuccessful
node and then skip it.
4. Tests & Result
After some tests, the result shows this may improve the system a little,
especially on a machine with only one node.
4.1 Test Description
There are two cases for two system configurations.
Test Cases:
1. counter comparison
2. kernel build test
System Configuration:
1. One node machine with 4G
2. Four node machine with 8G
4.2 Result for Test 1
Test 1: counter comparison
This is a test with hacked kernel to record times function
get_any_partial() is invoked and times the inner loop iterates. By
comparing the ratio of two counters, we get to know how many inner
loops we skipped.
Here is a snip of the test patch.
---
static void *get_any_partial() {
get_partial_count++;
do {
for_each_zone_zonelist() {
get_partial_try_count++;
}
} while();
return NULL;
}
---
The result of (get_partial_count / get_partial_try_count):
+----------+----------------+------------+-------------+
| | Base | Patched | Improvement|
+----------+----------------+------------+-------------+
|One Node | 1:3 | 1:0 | - 100% |
+----------+----------------+------------+-------------+
|Four Nodes| 1:5.8 | 1:2.5 | - 56% |
+----------+----------------+------------+-------------+
4.3 Result for Test 2
Test 2: kernel build
Command used:
> time make -j8 bzImage
Each version/system configuration combination has four round kernel
build tests. Take the average result of real to compare.
+----------+----------------+------------+-------------+
| | Base | Patched | Improvement|
+----------+----------------+------------+-------------+
|One Node | 4m41s | 4m32s | - 4.47% |
+----------+----------------+------------+-------------+
|Four Nodes| 4m45s | 4m39s | - 2.92% |
+----------+----------------+------------+-------------+
Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
---
v3:
* replace nmask with except to reduce potential stack overflow and copy
overhead
* test this in two cases and two system configurations and list the result
v2:
* rewrite the changelog and add a comment based on Andrew's comment
---
mm/slub.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index e3629cd7aff1..3d93a07d86d9 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1873,7 +1873,7 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
* Get a page from somewhere. Search in increasing NUMA distances.
*/
static void *get_any_partial(struct kmem_cache *s, gfp_t flags,
- struct kmem_cache_cpu *c)
+ struct kmem_cache_cpu *c, int except)
{
#ifdef CONFIG_NUMA
struct zonelist *zonelist;
@@ -1911,6 +1911,9 @@ static void *get_any_partial(struct kmem_cache *s, gfp_t flags,
for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
struct kmem_cache_node *n;
+ if (except == zone_to_nid(zone))
+ continue;
+
n = get_node(s, zone_to_nid(zone));
if (n && cpuset_zone_allowed(zone, flags) &&
@@ -1927,6 +1930,13 @@ static void *get_any_partial(struct kmem_cache *s, gfp_t flags,
return object;
}
}
+ /*
+ * Fail to get object from this node, either because
+ * 1. Fails in if check
+ * 2. NULl object returns from get_partial_node()
+ * Skip it next time.
+ */
+ except = zone_to_nid(zone);
}
} while (read_mems_allowed_retry(cpuset_mems_cookie));
#endif
@@ -1951,7 +1961,7 @@ static void *get_partial(struct kmem_cache *s, gfp_t flags, int node,
if (object || node != NUMA_NO_NODE)
return object;
- return get_any_partial(s, flags, c);
+ return get_any_partial(s, flags, c, searchnode);
}
#ifdef CONFIG_PREEMPT
--
2.15.1
