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

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Message-ID: <20121016185542.GA5423@cathedrallabs.org>
Date:	Tue, 16 Oct 2012 14:55:42 -0400
From:	Aristeu Rozanski <aris@...vo.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Glauber Costa <glommer@...allels.com>, 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 Tue, Oct 16, 2012 at 06:25:06PM +0000, 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? 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.
> 
> 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?

It's about preventing abuses caused by bugs or malicious use and avoiding
groups stepping on each others' toes. You're saying that letting a group
to allocate 32GB of paged memory is the same as 32GB of kernel memory?

I don't belive sysadmins will keep a tight limit for kernel memory but rather
a safety limit in case something goes wrong. usage_in_bytes will provide
data to get the limits better adjusted.

The innacuracy of the kmem accounting is (AFAIK) a cost tradeoff.

--
Aristeu
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