linux-kernel - Re: [PATCH RFC 0/5] Virtual Memory Resource Controller for cgroups

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Date:	Fri, 4 Jul 2014 14:16:21 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	cgroups@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Balbir Singh <bsingharora@...il.com>
Subject: Re: [PATCH RFC 0/5] Virtual Memory Resource Controller for cgroups

On Thu 03-07-14 16:48:16, Vladimir Davydov wrote:
> Hi,
> 
> Typically, when a process calls mmap, it isn't given all the memory pages it
> requested immediately. Instead, only its address space is grown, while the
> memory pages will be actually allocated on the first use. If the system fails
> to allocate a page, it will have no choice except invoking the OOM killer,
> which may kill this or any other process. Obviously, it isn't the best way of
> telling the user that the system is unable to handle his request. It would be
> much better to fail mmap with ENOMEM instead.
> 
> That's why Linux has the memory overcommit control feature, which accounts and
> limits VM size that may contribute to mem+swap, i.e. private writable mappings
> and shared memory areas. However, currently it's only available system-wide,
> and there's no way of avoiding OOM in cgroups.
>
> This patch set is an attempt to fill the gap. It implements the resource
> controller for cgroups that accounts and limits address space allocations that
> may contribute to mem+swap.

Well, I am not really sure how helpful is this. Could you be more
specific about real use cases? If the only problem is that memcg OOM can
trigger to easily then I do not think this is the right approach to
handle it. Strict no-overcommit is basically unusable for many
workloads. Especially those which try to do their own memory usage
optimization in a much larger address space.

Once I get from internal things (which will happen soon hopefully) I
will post a series with a new sets of memcg limits. One of them is
high_limit which can be used as a trigger for memcg reclaim. Unlike
hard_limit there won't be any OOM if the reclaim fails at this stage. So
if the high_limit is configured properly the admin will have enough time
to make additional steps before OOM happens.
[...]
-- 
Michal Hocko
SUSE Labs
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