Re: [PATCH] mm-slab: allocate kmem_cache with __GFP_REPEAT

Konstantin Khlebnikov
2011-07-20T13:50:38Z

Pekka Enberg wrote:
> On Wed, 20 Jul 2011, Konstantin Khlebnikov wrote:
>>> The changelog isn't that convincing, really. This is kmem_cache_create()
>>> so I'm surprised we'd ever get NULL here in practice. Does this fix some
>>> problem you're seeing? If this is really an issue, I'd blame the page
>>> allocator as GFP_KERNEL should just work.
>>
>> nf_conntrack creates separate slab-cache for each net-namespace,
>> this patch of course not eliminates the chance of failure, but makes it more
>> acceptable.
>
> I'm still surprised you are seeing failures. mm/slab.c hasn't changed
> significantly in a long time. Why hasn't anyone reported this before? I'd
> still be inclined to shift the blame to the page allocator... Mel,
> Christoph?
>
> On Wed, 20 Jul 2011, Konstantin Khlebnikov wrote:
>> struct kmem_size for slub is more compact, it uses pecpu-pointers instead of
>> dumb NR_CPUS-size array.
>> probably better to fix this side...
>
> So how big is 'struct kmem_cache' for your configuration anyway? Fixing
> the per-cpu data structures would be nice but I'm guessing it'll be
> slightly painful for mm/slab.c.

With NR_CPUS=4096 and MAX_NUMNODES=512 its over 9k!
so it require order-4 page, meanwhile PAGE_ALLOC_COSTLY_ORDER is 3

>
> Pekka