Message-ID: <1432055392.12412.34.camel@decadent.org.uk>
Subject: Re: [PATCH] mm, memcg: Optionally disable memcg by default using
 Kconfig
From: Ben Hutchings <ben@decadent.org.uk>
To: Michal Hocko <mhocko@suse.cz>
Cc: Mel Gorman <mgorman@suse.de>, Johannes Weiner <hannes@cmpxchg.org>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, Tejun Heo <tj@kernel.org>,
        cgroups@vger.kernel.org,
        Debian kernel maintainers <debian-kernel@lists.debian.org>
Date: Tue, 19 May 2015 18:09:52 +0100
In-Reply-To: <20150519151541.GJ6203@dhcp22.suse.cz>
References: <20150519104057.GC2462@suse.de>
	 <20150519141807.GA9788@cmpxchg.org> <20150519144345.GF2462@suse.de>
	 <20150519151541.GJ6203@dhcp22.suse.cz>
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On Tue, 2015-05-19 at 17:15 +0200, Michal Hocko wrote:
> [Let's CC Ben here - the email thread has started here:
> http://marc.info/?l=3Dlinux-mm&m=3D143203206402073&w=3D2 and it seems Deb=
ian
> is disabling memcg controller already so this might be of your interest]
>=20
> On Tue 19-05-15 15:43:45, Mel Gorman wrote:
> [...]
> > After I wrote the patch, I spotted that Debian apparently already
> > does something like this and by coincidence they matched the
> > parameter name and values. See the memory controller instructions on
> > https://wiki.debian.org/LXC#Prepare_the_host . So in this case at least
> > upstream would match something that at least one distro in the field
> > already uses.
>=20
> I've read through
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=3D534964 and it seems
> that the primary motivation for the runtime disabling was the _memory_
> overhead of the struct page_cgroup
> (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=3D534964#152). This is
> no longer the case since 1306a85aed3e ("mm: embed the memcg pointer
> directly into struct page") merged in 3.19.
>=20
> I can see some point in disabling the memcg due to runtime overhead.

I was also concerned about runtime overhead.

> There will always be some, albeit hard to notice. If an user really need
> this to happen there is a command line option for that. The question is
> who would do CONFIG_MEMCG && !MEMCG_DEFAULT_ENABLED.  Do you expect any
> distributions go that way?
> Ben, would you welcome such a change upstream or is there a reason to
> change the Debian kernel runtime default now that the memory overhead is
> mostly gone (for 3.19+ kernels of course)?

I have been meaning to reevaluate this as I know the overhead has been
reduced.  Given Mel's benchmark results, I favour keeping it disabled by
default in Debian.  So I would welcome this change.

Ben.

--=20
Ben Hutchings
I'm not a reverse psychological virus.  Please don't copy me into your sig.

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