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

Wanpeng Li
2014-01-07T09:53:08Z

On Tue, Jan 07, 2014 at 04:41:36PM +0900, Joonsoo Kim wrote:
>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.
>>
>> 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;
>> + }
>> }
>>
>> /*
>
>Hello,
>
>I think that we need more efforts to solve unbalanced node problem.
>
>With this patch, even if node of current cpu slab is not favorable to
>unbalanced node, allocation would proceed and we would get the unintended memory.
>
>And there is one more problem. Even if we have some partial slabs on
>compatible node, we would allocate new slab, because get_partial() cannot handle
>this unbalance node case.
>
>To fix this correctly, how about following patch?
>
>Thanks.
>
>------------->8--------------------
>diff --git a/mm/slub.c b/mm/slub.c
>index c3eb3d3..a1f6dfa 100644
>--- a/mm/slub.c
>+++ b/mm/slub.c
>@@ -1672,7 +1672,19 @@ static void *get_partial(struct kmem_cache *s, gfp_t flags, int node,
> {
> void *object;
> int searchnode = (node == NUMA_NO_NODE) ? numa_node_id() : node;
>+ struct zonelist *zonelist;
>+ struct zoneref *z;
>+ struct zone *zone;
>+ enum zone_type high_zoneidx = gfp_zone(flags);
>
>+ if (!node_present_pages(searchnode)) {
>+ zonelist = node_zonelist(searchnode, flags);
>+ for_each_zone_zonelist(zone, z, zonelist, high_zoneidx) {
>+ searchnode = zone_to_nid(zone);
>+ if (node_present_pages(searchnode))
>+ break;
>+ }
>+ }

Why change searchnode instead of depending on fallback zones/nodes in
get_any_partial() to allocate partial slabs?

Regards,
Wanpeng Li

> object = get_partial_node(s, get_node(s, searchnode), c, flags);
> if (object || node != NUMA_NO_NODE)
> return object;
>
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