Re: [PATCH v2 0/4] memcg: Low-limit reclaim

[Johannes Weiner]

On Wed, May 28, 2014 at 02:10:23PM +0200, Michal Hocko wrote:
> Hi Andrew, Johannes,
> 
> On Mon 28-04-14 14:26:41, Michal Hocko wrote:
> > This patchset introduces such low limit that is functionally similar
> > to a minimum guarantee. Memcgs which are under their lowlimit are not
> > considered eligible for the reclaim (both global and hardlimit) unless
> > all groups under the reclaimed hierarchy are below the low limit when
> > all of them are considered eligible.
> > 
> > The previous version of the patchset posted as a RFC
> > (http://marc.info/?l=linux-mm&m=138677140628677&w=2) suggested a
> > hard guarantee without any fallback. More discussions led me to
> > reconsidering the default behavior and come up a more relaxed one. The
> > hard requirement can be added later based on a use case which really
> > requires. It would be controlled by memory.reclaim_flags knob which
> > would specify whether to OOM or fallback (default) when all groups are
> > bellow low limit.
> 
> It seems that we are not in a full agreement about the default behavior
> yet. Johannes seems to be more for hard guarantee while I would like to
> see the weaker approach first and move to the stronger model later.
> Johannes, is this absolutely no-go for you? Do you think it is seriously
> handicapping the semantic of the new knob?

Well we certainly can't start OOMing where we previously didn't,
that's called a regression and automatically limits our options.

Any unexpected OOMs will be much more acceptable from a new feature
than from configuration that previously "worked" and then stopped.

> My main motivation for the weaker model is that it is hard to see all
> the corner case right now and once we hit them I would like to see a
> graceful fallback rather than fatal action like OOM killer. Besides that
> the usaceses I am mostly interested in are OK with fallback when the
> alternative would be OOM killer. I also feel that introducing a knob
> with a weaker semantic which can be made stronger later is a sensible
> way to go.

We can't make it stronger, but we can make it weaker.  Stronger is the
simpler definition, it's simpler code, your usecases are fine with it,
Greg and I prefer it too.  I don't even know what we are arguing about
here.

Patch applies on top of mmots.

---