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

[Johannes Weiner]

On Thu, Jun 05, 2014 at 04:51:09PM +0200, Michal Hocko wrote:
> On Wed 04-06-14 17:45:53, Johannes Weiner wrote:
> > On Wed, Jun 04, 2014 at 12:18:59PM -0700, Hugh Dickins wrote:
> > > On Wed, 4 Jun 2014, Johannes Weiner wrote:
> > > > On Wed, Jun 04, 2014 at 04:46:58PM +0200, Michal Hocko wrote:
> > > > > 
> > > > > In the other email I have suggested to add a knob with the configurable
> > > > > default. Would you be OK with that?
> > > > 
> > > > No, I want to agree on whether we need that fallback code or not.  I'm
> > > > not interested in merging code that you can't convince anybody else is
> > > > needed.
> > > 
> > > I for one would welcome such a knob as Michal is proposing.
> > 
> > Now we have a tie :-)
> > 
> > > I thought it was long ago agreed that the low limit was going to fallback
> > > when it couldn't be satisfied.  But you seem implacably opposed to that
> > > as default, and I can well believe that Google is so accustomed to OOMing
> > > that it is more comfortable with OOMing as the default.  Okay.  But I
> > > would expect there to be many who want the attempt towards isolation that
> > > low limit offers, without a collapse to OOM at the first misjudgement.
> > 
> > At the same time, I only see users like Google pushing the limits of
> > the machine to a point where guarantees cover north of 90% of memory.
> 
> I can think of in-memory database loads which would use the reclaim
> protection which is quite high as well (say 80% of available memory).
> Those would definitely like to see ephemeral reclaim rather than OOM.

The OOM wouldn't apply to the database workload, but to other stuff in
the system.

> > I would expect more casual users to work with much smaller guarantees,
> > and a good chunk of slack on top - otherwise they already had better
> > be set up for the occasional OOM.  Is this an unreasonable assumption
> > to make?
> > 
> > I'm not opposed to this feature per se, but I'm really opposed to
> > merging it for the partial hard bindings argument
> 
> This was just an example that even setup which is not overcomiting the
> limit might be caught in an unreclaimable position. Sure we can mitigate
> those issues to some point and that would be surely welcome.
> 
> The more important part, however, is that not all usecases really
> _require_ hard guarantee. 

It's not about whether hard guarantees are necessary, it's about
getting away without additional fallback semantics.  The default
position is still always simpler semantics, so we are looking for
reasons for the fallback here, not the other way around.

> They are asking for a reasonable memory isolation which they
> currently do not have. Having a risk of OOM would be a no-go for
> them so the feature wouldn't be useful for them.

Let's not go back to Handwaving Land on this, please.  What does
"reasonable memory isolation" mean?

It really boils down to the interaction with other workloads: do we
want other workloads to reclaim our guaranteed memory or OOM?  If you
prefer reclaiming workingset memory over OOM, why can't you set the
low limit more conservatively in the first place?

> I have repeatedly said that I can see also some use for the hard
> guarantee. Mainly to support overcommit on the limit. I didn't hear
> about those usecases yet but it seems that at least Google would like to
> have really hard guarantees.

Everybody who wants to charge for guaranteed memory can not afford to
have other workloads break isolation at will.
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