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

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Date:	Wed, 9 Jul 2014 10:04:21 -0700
From:	Greg Thelen <gthelen@...gle.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Tim Hockin <thockin@...kin.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Cgroups <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>,
	Michal Hocko <mhocko@...e.cz>, 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 Wed, Jul 9, 2014 at 9:36 AM, Vladimir Davydov <vdavydov@...allels.com> wrote:
> Hi Tim,
>
> On Wed, Jul 09, 2014 at 08:08:07AM -0700, Tim Hockin wrote:
>> How is this different from RLIMIT_AS?  You specifically mentioned it
>> earlier but you don't explain how this is different.
>
> The main difference is that RLIMIT_AS is per process while this
> controller is per cgroup. RLIMIT_AS doesn't allow us to limit VSIZE for
> a group of unrelated or cooperating through shmem processes.
>
> Also RLIMIT_AS accounts for total VM usage (including file mappings),
> while this only charges private writable and shared mappings, whose
> faulted-in pages always occupy mem+swap and therefore cannot be just
> synced and dropped like file pages. In other words, this controller
> works exactly as the global overcommit control.
>
>> From my perspective, this is pointless.  There's plenty of perfectly
>> correct software that mmaps files without concern for VSIZE, because
>> they never fault most of those pages in.
>
> But there's also software that correctly handles ENOMEM returned by
> mmap. For example, mongodb keeps growing its buffers until mmap fails.
> Therefore, if there's no overcommit control, it will be OOM-killed
> sooner or later, which may be pretty annoying. And we did have customers
> complaining about that.

Is mongodb's buffer growth causing the oom kills?

If yes, I wonder if apps, like mongodb, that want ENOMEM should (1)
use MAP_POPULATE and (2) we change vm_map_pgoff() to propagate
mm_populate() ENOMEM failures back to mmap()?

>> From my observations it is not generally possible to predict an
>> average VSIZE limit that would satisfy your concerns *and* not kill
>> lots of valid apps.
>
> Yes, it's difficult. Actually, we can only guess. Nevertheless, we
> predict and set the VSIZE limit system-wide by default.
>
>> It sounds like what you want is to limit or even disable swap usage.
>
> I want to avoid OOM kill if it's possible to return ENOMEM. OOM can be
> painful. It can kill lots of innocent processes. Of course, the user can
> protect some processes by setting oom_score_adj, but this is difficult
> and requires time and expertise, so an average user won't do that.
>
>> Given your example, your hypothetical user would probably be better of
>> getting an OOM kill early so she can fix her job spec to request more
>> memory.
>
> In my example the user won't get OOM kill *early*...
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