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

From: Vladimir Davydov
Date: Thu Jul 10 2014 - 12:36:14 EST

Hi Greg,

On Wed, Jul 09, 2014 at 10:04:21AM -0700, Greg Thelen wrote:
> On Wed, Jul 9, 2014 at 9:36 AM, Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> 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?

We saw this happened on our customer's node some time ago. A container
running mongodb and several other services got OOM-kills from time to
time, which made the customer unhappy. Limiting overcommit helped then.

> 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()?

This way we may fault-in lots of pages, evicting someone's working set
along the way, only to get ENOMEM eventually. This doesn't look optimal.
Also, this requires modifications of userspace apps, which isn't always

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