Re: [discuss][memcg] oom-kill extension

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 29 Oct 2008 09:38:20 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > Under memory resource controller(memcg), oom-killer can be invoked when it
> > reaches limit and no memory can be reclaimed.
> > 
> > In general, not under memcg, oom-kill(or panic) is an only chance to recover
> > the system because there is no available memory. But when oom occurs under
> > memcg, it just reaches limit and it seems we can do something else.
> > 
> > Does anyone have plan to enhance oom-kill ?
> > 
> > What I can think of now is
> >   - add an notifier to user-land.
> >     - receiver of notify should work in another cgroup.
> 
> The discussion at the mini-summit was to notify a FIFO in the cgroup and any
> application can listen in for events.
> 
add FIFO rather than netlink or user mode helper ?

> >     - automatically extend the limit as emergency
> 
> No.. I don't like this
> 
Oh, I should write as
 "automatically extend the limit as emergency via userland daemon
  which receives notify"

> >     - trigger fail-over process.
> 
> I had suggested memrlimits for the ability to fail application allocations, but
> no-one liked the idea. We can still implement overcommit functionality if needed
> and catch failures at allocation time.
> 
Difficult point of memrlimit is that system engineer cannot guarantee 
"your application will do proper fail over process when malloc() returns NULL".

Important applications have emergency-fail-over method via signal(SIGTERM or some..
(if not killed by SIGKILL.)

I wonder adding an "moderate oom kill mode" to memcg and send SIGTERM rather
than SIGKILL may help many? applications.
(But to do fail over, the apps may use more memory....)

> >     - automatically create a precise report of OOM.
> >       - record snapshot of 'ps -elf' and so on of memcg which triggers oom.
> > 
> >   - freeze processes under cgroup.
> >     - maybe freezer cgroup should be mounted at the same time.
> >     - can we add memcg-oom-freezing-point in somewhere we can sleep ?
> >   
> > Is there a chance to add oom_notifier to memcg ? (netlink ?)
> > 
> 
> Yes, we should add the oom-notifier. We already have cgroupstats if you want to
> make use of it.
> 
ok, look into that.

> > But the real problem is that what we can do in the kernel is limited
> > and we need proper userland, anyway ;)
> > 
> 
> Agreed.
> 

Thanks,
-Kame

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