linux-kernel - Re: [PATCH 2/2] memcg: Allow hard guarantee mode for low limit reclaim

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Date:	Wed, 11 Jun 2014 16:11:17 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Michel Lespinasse <walken@...gle.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] memcg: Allow hard guarantee mode for low limit
 reclaim

On Wed 11-06-14 08:31:09, Tejun Heo wrote:
> Hello, Michal.
> 
> On Wed, Jun 11, 2014 at 09:57:29AM +0200, Michal Hocko wrote:
> > Is this the kind of symmetry Tejun is asking for and that would make
> > change is Nack position? I am still not sure it satisfies his soft
> 
> Yes, pretty much.  What primarily bothered me was the soft/hard
> guarantees being chosen by a toggle switch while the soft/hard limits
> can be configured separately and combined.

The last consensus at LSF was that there would be a knob which will
distinguish hard/best effort behavior. The weaker semantic has strong
usecases IMHO so I wanted to start with it and add a knob for the hard
guarantee later when explicitly asked for.

Going with min, low, high and hard makes more sense to me of course.

> > guarantee objections from other email.
> 
> I was wondering about the usefulness of "low" itself in isolation and

I think it has more usecases than "min" from simply practical POV. OOM
means a potential service down time and that is a no go. Optimistic
isolation on the other hand adds an advantages of the isolation most of
the time while not getting completely flat on an exception (be it
misconfiguration or a corner case like mentioned during the discussion).

That doesn't mean "min" is not useful. It definitely is, the category
of usecases will be more specific though.

> I still think it'd be less useful than "high", but as there seem to be
> use cases which can be served with that and especially as a part of a
> consistent control scheme, I have no objection.
> 
> "low" definitely requires a notification mechanism tho.

Would vmpressure notification be sufficient? That one is in place for
any memcg which is reclaimed.

Or are you thinking about something more like oom_control?

-- 
Michal Hocko
SUSE Labs
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