linux-kernel - Re: [PATCH v2] memcg: first step towards hierarchical controller

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Message-ID: <20120903164148.GS29217@decadent.org.uk>
Date:	Mon, 3 Sep 2012 17:41:48 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org, Dave Jones <davej@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Kay Sievers <kay.sievers@...y.org>,
	Michal Hocko <mhocko@...e.cz>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2] memcg: first step towards hierarchical controller

On Mon, Sep 03, 2012 at 07:46:51PM +0400, Glauber Costa wrote:
> Here is a new attempt to lay down a path that will allow us to deprecate
> the non-hierarchical mode of operation from memcg.  Unlike what I posted
> before, I am making this behavior conditional on a Kconfig option.
> Vanilla users will see no change in behavior unless they don't
> explicitly set this option to on.

There are too many negatives in this sentence - it is not only
unclear, but appears to be incorrect.  I think you should delete
'don't'.

[...]
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -726,6 +726,24 @@ config MEMCG_SWAP
>  	  if boot option "swapaccount=0" is set, swap will not be accounted.
>  	  Now, memory usage of swap_cgroup is 2 bytes per entry. If swap page
>  	  size is 4096bytes, 512k per 1Gbytes of swap.
> +
> +config MEMCG_HIERARCHY_DEFAULT
> +	bool "Hierarchical memcg"
> +	depends on MEMCG
> +	default n
> +	help
> +	  The memory controller has two modes of accounting: hierarchical and
> +	  flat. Hierarchical accounting will charge pages all the way towards a
> +	  group's parent while flat hierarchy will threat all groups as children

typo: 'threat' should be 'treat'

> +	  of the root memcg, regardless of their positioning in the tree.
> +
> +	  Use of flat hierarchies is highly discouraged, but has been the
> +	  default for performance reasons for quite some time. Setting this flag
> +	  to on will make hierarchical accounting the default. It is still
> +	  possible to set it back to flat by writing 0 to the file
> +	  memory.use_hierarchy, albeit discouraged. Distributors are encouraged
> +	  to set this option.
[...]

I don't think that 'default n' is effective encouragement!

Ben.

-- 
Ben Hutchings
We get into the habit of living before acquiring the habit of thinking.
                                                              - Albert Camus
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