linux-kernel - Re: [PATCH] mm, memcg: Optionally disable memcg by default using Kconfig

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Message-ID: <1432055392.12412.34.camel@decadent.org.uk>
Date:	Tue, 19 May 2015 18:09:52 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, cgroups@...r.kernel.org,
	Debian kernel maintainers <debian-kernel@...ts.debian.org>
Subject: Re: [PATCH] mm, memcg: Optionally disable memcg by default using
 Kconfig

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=linux-mm&m=143203206402073&w=2 and it seems Debian
> is disabling memcg controller already so this might be of your interest]
> 
> 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.
> 
> I've read through
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=534964 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=534964#152). This is
> no longer the case since 1306a85aed3e ("mm: embed the memcg pointer
> directly into struct page") merged in 3.19.
> 
> 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.

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

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