Re: [PATCH] slub: Don't throw away partial remote slabs if there is no local memory

Wanpeng Li
2014-01-07T10:28:58Z

On Tue, Jan 07, 2014 at 01:21:00PM +1100, Anton Blanchard wrote:
>
>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.
>

Deactive cpu slab cache doesn't always mean free the slab cache to buddy system,
maybe the slab cache will be putback to the remote node's partial list if there
are objects still in used in this unbalance situation. In this case, the slub slow
path can freeze the partial slab in remote node again. So why the slab cache is
fragmented as below?

Regards,
Wanpeng Li

>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@samba.org>
>---
>
>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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