linux-kernel - Re: [PATCH 2/4] slqb: Record what node is local to a kmem_cache

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Date:	Sun, 4 Oct 2009 15:06:45 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...e.de>,
	heiko.carstens@...ibm.com, sachinp@...ibm.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Tejun Heo <tj@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 2/4] slqb: Record what node is local to a kmem_cache_cpu

On Thu, Oct 1, 2009 at 2:45 AM, Christoph Lameter
<cl@...ux-foundation.org> wrote:
>> This is essentially the "unqueued" nature of SLUB. It's objective "I have this
>> page here which I'm going to use until I can't use it no more and will depend
>> on the page allocator to sort my stuff out". I have to read up on SLUB up
>> more to see if it's compatible with SLQB or not though. In particular, how
>> does SLUB deal with frees from pages that are not the "current" page? SLQB
>> does not care what page the object belongs to as long as it's node-local
>> as the object is just shoved onto a LIFO for maximum hotness.
>
> Frees are done directly to the target slab page if they are not to the
> current active slab page. No centralized locks. Concurrent frees from
> processors on the same node to multiple other nodes (or different pages
> on the same node) can occur.
>
>> > SLAB deals with it in fallback_alloc(). It scans the nodes in zonelist
>> > order for free objects of the kmem_cache and then picks up from the
>> > nearest node. Ugly but it works. SLQB would have to do something similar
>> > since it also has the per node object bins that SLAB has.
>> >
>>
>> In a real sense, this is what the patch ends up doing. When it fails to
>> get something locally but sees that the local node is memoryless, it
>> will check the remote node lists in zonelist order. I think that's
>> reasonable behaviour but I'm biased because I just want the damn machine
>> to boot again. What do you think? Pekka, Nick?
>
> Look at fallback_alloc() in slab. You can likely copy much of it. It
> considers memory policies and cpuset constraints.

Sorry for the delay. I went ahead and merged Mel's patch to make
things boot on PPC. Fallback policy needs a bit more work as Christoph
says but I'd really love to have Nick's input on this.

Mel, do you have a Kconfig patch laying around somewhere to enable
SLQB on PPC and S390?

                        Pekka
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