linux-kernel - Re: [PATCH v3 06/16] memcg: infrastructure to match an allocation to the right cache

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Message-ID: <20120924175619.GD7694@google.com>
Date:	Mon, 24 Sep 2012 10:56:19 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	kamezawa.hiroyu@...fujitsu.com, devel@...nvz.org,
	linux-mm@...ck.org, Suleiman Souhlal <suleiman@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v3 06/16] memcg: infrastructure to match an allocation
 to the right cache

Hello, Glauber.

On Mon, Sep 24, 2012 at 12:46:35PM +0400, Glauber Costa wrote:
> >> +#ifdef CONFIG_MEMCG_KMEM
> >> +	/* Slab accounting */
> >> +	struct kmem_cache *slabs[MAX_KMEM_CACHE_TYPES];
> >> +#endif
> > 
> > Bah, 400 entry array in struct mem_cgroup.  Can't we do something a
> > bit more flexible?
> > 
> 
> I guess. I still would like it to be an array, so we can easily access
> its fields. There are two ways around this:
> 
> 1) Do like the events mechanism and allocate this in a separate
> structure. Add a pointer chase in the access, and I don't think it helps
> much because it gets allocated anyway. But we could at least
> defer it to the time when we limit the cache.

Start at some reasonable size and then double it as usage grows?  How
many kmem_caches do we typically end up using?

> >> +	if (memcg->slabs[idx] == NULL) {
> >> +		memcg_create_cache_enqueue(memcg, cachep);
> > 
> > Do we want to wait for the work item if @gfp allows?
> > 
> 
> I tried this once, and it got complicated enough that I deemed as "not
> worth it". I honestly don't remember much of the details now, it was one
> of the first things I tried, and a bunch of time has passed. If you
> think it is absolutely worth it, I can try it again. But at the very
> best, I view this as an optimization.

I don't know.  It seems like a logical thing to try and depends on how
complex it gets.  I don't think it's a must.  The whole thing is
somewhat opportunistic after all.

Thanks.

-- 
tejun
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