linux-kernel - Re: [PATCH v3 13/28] slub: create duplicate cache

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Date:	Wed, 30 May 2012 17:02:35 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, linux-mm@...ck.org,
	kamezawa.hiroyu@...fujitsu.com, Li Zefan <lizefan@...wei.com>,
	Greg Thelen <gthelen@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>, devel@...nvz.org,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH v3 13/28] slub: create duplicate cache

Hello, Glauber.

On Wed, May 30, 2012 at 4:54 PM, Glauber Costa <glommer@...allels.com> wrote:
> On 05/30/2012 05:29 AM, Tejun Heo wrote:
>>
>> The two goals for cgroup controllers that I think are important are
>> proper (no, not crazy perfect but good enough) isolation and an
>> implementation which doesn't impact !cg path in an intrusive manner -
>> if someone who doesn't care about cgroup but knows and wants to work
>> on the subsystem should be able to mostly ignore cgroup support.  If
>> that means overhead for cgroup users, so be it.
>
>
> Well, my code in the slab is totally wrapped in static branches. They only
> come active when the first group is *limited* (not even created: you can
> have a thousand memcg, if none of them are kmem limited, nothing will
> happen).

Great, but I'm not sure why you're trying to emphasize that while my
point was about memory overhead and that it's OK to have some
overheads for cg users. :)

> After that, the cost paid is to find out at which cgroup the process is at.
> I believe that if we had a faster way for this (like for instance: if we had
> a single hierarchy, the scheduler could put this in a percpu variable after
> context switch - or any other method), then the cost of it could be really
> low, even when this is enabled.

Someday, hopefully.

> I will rework this series to try work more towards this goal, but at least
> for now I'll keep duplicating the caches. I still don't believe that a loose
> accounting to the extent Christoph proposed will achieve what we need this
> to achieve.

Yeah, I prefer your per-cg cache approach but do hope that it stays as
far from actual allocator code as possible. Christoph, would it be
acceptable if the cg logic is better separated?

Thanks.

-- 
tejun
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