linux-kernel - Re: [PATCH v5 14/14] Add documentation about the kmem controller

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Date:	Tue, 16 Oct 2012 23:02:46 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	<linux-mm@...ck.org>, <cgroups@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	<kamezawa.hiroyu@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...nel.org>, <devel@...nvz.org>,
	<linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH v5 14/14] Add documentation about the kmem controller

On 10/16/2012 10:25 PM, Christoph Lameter wrote:
> On Tue, 16 Oct 2012, Glauber Costa wrote:
> 
>>
>> + memory.kmem.limit_in_bytes      # set/show hard limit for kernel memory
>> + memory.kmem.usage_in_bytes      # show current kernel memory allocation
>> + memory.kmem.failcnt             # show the number of kernel memory usage hits limits
>> + memory.kmem.max_usage_in_bytes  # show max kernel memory usage recorded
> 
> Does it actually make sense to limit kernel memory? 

Yes.

> The user generally has
> no idea how much kernel memory a process is using and kernel changes can
> change the memory footprint. Given the fuzzy accounting in the kernel a
> large cache refill (if someone configures the slab batch count to be
> really big f.e.) can account a lot of memory to the wrong cgroup. The
> allocation could fail.
> 

It heavily depends on the type of the user. The user may not know how
much kernel memory precisely will be used, but he/she usually knows
quite well that it shouldn't be all cgroups together shouldn't use more
than available in the system.

IOW: It is usually safe to overcommit user memory, but not kernel
memory. This is absolutely crucial in any high-density container host,
and we've been doing this in OpenVZ for ages (in an uglier form than this)

> Limiting the total memory use of a process (U+K) would make more sense I
> guess. Only U is probably sufficient? In what way would a limitation on
> kernel memory in use be good?
> 

The kmem counter is also fed into the u counter. If the limit value of
"u" is equal or greater than "k", this is actually what you are doing.

For a lot of application yes, only U is sufficient. This is the default,
btw, since "k" is only even accounted if you set the limit.

All those use cases are detailed a bit below in this file.

A limitation of kernel memory use would be good, for example, to prevent
abuse from non-trusted containers in a high density, shared, container
environment.

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