We noticed a huge amount of slab memory consumed on a large ppc64 box:
Slab: 2094336 kB
Almost 2GB. This box is not balanced and some nodes do not have local
memory, causing slub to be very inefficient in its slab usage.
Each time we call kmem_cache_alloc_node slub checks the per cpu slab,
sees it isn't node local, deactivates it and tries to allocate a new
slab. On empty nodes we will allocate a new remote slab and use the
first slot, but as explained above when we get called a second time
we will just deactivate that slab and retry.
As such we end up only using 1 entry in each slab:
slab mem objects
used active
------------------------------------
kmalloc-16384 1404 MB 4.90%
task_struct 668 MB 2.90%
kmalloc-128 193 MB 3.61%
kmalloc-192 152 MB 5.23%
kmalloc-8192 72 MB 23.40%
kmalloc-16 64 MB 7.43%
kmalloc-512 33 MB 22.41%
The patch below checks that a node is not empty before deactivating a
slab and trying to allocate it again. With this patch applied we now
use about 352MB:
Slab: 360192 kB
And our efficiency is much better:
slab mem objects
used active
------------------------------------
kmalloc-16384 92 MB 74.27%
task_struct 23 MB 83.46%
idr_layer_cache 18 MB 100.00%
pgtable-2^12 17 MB 100.00%
kmalloc-65536 15 MB 100.00%
inode_cache 14 MB 100.00%
kmalloc-256 14 MB 97.81%
kmalloc-8192 14 MB 85.71%
Signed-off-by: Anton Blanchard <anton@xxxxxxxxx>
---
Thoughts? It seems like we could hit a similar situation if a machine
is balanced but we run out of memory on a single node.
Index: b/mm/slub.c
===================================================================
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2278,10 +2278,17 @@ redo:
if (unlikely(!node_match(page, node))) {
stat(s, ALLOC_NODE_MISMATCH);
- deactivate_slab(s, page, c->freelist);
- c->page = NULL;
- c->freelist = NULL;
- goto new_slab;
+
+ /*
+ * If the node contains no memory there is no point in trying
+ * to allocate a new node local slab
+ */
+ if (node_spanned_pages(node)) {
+ deactivate_slab(s, page, c->freelist);
+ c->page = NULL;
+ c->freelist = NULL;
+ goto new_slab;
+ }
}
/*
--
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