linux-kernel - Re: [PATCH] mm-slab: allocate kmem_cache with __GFP

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Date:	Wed, 20 Jul 2011 17:50:29 +0400
From:	Konstantin Khlebnikov <khlebnikov@...allels.com>
To:	Pekka Enberg <penberg@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Matt Mackall <mpm@...enic.com>,
	"mgorman@...e.de" <mgorman@...e.de>
Subject: Re: [PATCH] mm-slab: allocate kmem_cache with __GFP_REPEAT

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

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