linux-kernel - Re: [PATCH v3 04/13] kmem accounting basic infrastructure

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Message-ID: <20121005134723.GD27757@dhcp22.suse.cz>
Date:	Fri, 5 Oct 2012 15:47:23 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Glauber Costa <glommer@...allels.com>,
	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>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v3 04/13] kmem accounting basic infrastructure

On Thu 04-10-12 07:43:16, Tejun Heo wrote:
[...]
> > That is right but I think that the current discussion shows that a mixed
> > (kmem disabled and kmem enabled hierarchies) workloads are far from
> > being theoretical and a global knob is just too coarse. I am afraid we
> 
> I'm not sure there's much evidence in this thread.  The strongest upto
> this point seems to be performance overhead / difficulty of general
> enough implementation.  As for "trusted" workload, what are the
> inherent benefits of trusting if you don't have to?

One advantage is that you do _not have_ to consider kernel memory
allocations (which are inherently bound to the kernel version) so the
sizing is much easier and version independent. If you set a limit to XY
because you know what you are doing you certainly do not want to regress
(e.g. because of unnecessary reclaim) just because a certain kernel
allocation got bigger, right?

> > will see "we want that per hierarchy" requests shortly and that would
> > just add a new confusion where global knob would complicate it
> > considerably (do we really want on/off/per_hierarchy global knob?).
> 
> Hmmm?  The global knob is just the same per_hierarchy knob at the
> root.  It's hierarchical after all.

When you said global knob I imagined mount or boot option. If you want
to have yet another memory.enable_kmem then IMHO it is much easier to
use set-accounted semantic (which is hierarchical as well).

> Anyways, as long as the "we silently ignore what happened before being
> enabled" is gone, I won't fight this anymore.  It isn't broken after
> all.  

OK, it is good that we settled this.

> But, please think about making things simpler in general, cgroup
> is riddled with mis-designed complexities and memcg seems to be
> leading the charge at times.

Yes this is an evolution and it seems that we are slowly getting there.

> 
> Thanks.

-- 
Michal Hocko
SUSE Labs
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