Re: [patch 0/8] mm: memcg naturalization -rc2

[Ying Han <yinghan@google.com>]

On Thu, Jun 9, 2011 at 4:31 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> On Thu, Jun 09, 2011 at 03:30:27PM -0700, Ying Han wrote:
>> On Thu, Jun 9, 2011 at 11:36 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>> > On Thu, Jun 09, 2011 at 10:36:47AM -0700, Ying Han wrote:
>> >> On Thu, Jun 9, 2011 at 1:35 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>> >> > On Wed, Jun 08, 2011 at 08:52:03PM -0700, Ying Han wrote:
>> >> >> On Wed, Jun 8, 2011 at 8:32 AM, Johannes Weiner <hannes@cmpxchg.org> wrote:
>> >> >> > I guess it would make much more sense to evaluate if reclaiming from
>> >> >> > memcgs while there are others exceeding their soft limit is even a
>> >> >> > problem.  Otherwise this discussion is pretty pointless.
>> >> >>
>> >> >> AFAIK it is a problem since it changes the spec of kernel API
>> >> >> memory.soft_limit_in_bytes. That value is set per-memcg which all the
>> >> >> pages allocated above that are best effort and targeted to reclaim
>> >> >> prior to others.
>> >> >
>> >> > That's not really true.  Quoting the documentation:
>> >> >
>> >> >    When the system detects memory contention or low memory, control groups
>> >> >    are pushed back to their soft limits. If the soft limit of each control
>> >> >    group is very high, they are pushed back as much as possible to make
>> >> >    sure that one control group does not starve the others of memory.
>> >> >
>> >> > I am language lawyering here, but I don't think it says it won't touch
>> >> > other memcgs at all while there are memcgs exceeding their soft limit.
>> >>
>> >> Well... :) I would say that the documentation of soft_limit needs lots
>> >> of work especially after lots of discussions we have after the LSF.
>> >>
>> >> The RFC i sent after our discussion has the following documentation,
>> >> and I only cut & paste the content relevant to our conversation here:
>> >>
>> >> What is "soft_limit"?
>> >> The "soft_limit was introduced in memcg to support over-committing the
>> >> memory resource on the host. Each cgroup can be configured with
>> >> "hard_limit", where it will be throttled or OOM killed by going over
>> >> the limit. However, the allocation can go above the "soft_limit" as
>> >> long as there is no memory contention. The "soft_limit" is the kernel
>> >> mechanism for re-distributing spare memory resource among cgroups.
>> >>
>> >> What we have now?
>> >> The current implementation of softlimit is based on per-zone RB tree,
>> >> where only the cgroup exceeds the soft_limit the most being selected
>> >> for reclaim.
>> >>
>> >> It makes less sense to only reclaim from one cgroup rather than
>> >> reclaiming all cgroups based on calculated propotion. This is required
>> >> for fairness.
>> >>
>> >> Proposed design:
>> >> round-robin across the cgroups where they have memory allocated on the
>> >> zone and also exceed the softlimit configured.
>> >>
>> >> there was a question on how to do zone balancing w/o global LRU. This
>> >> could be solved by building another cgroup list per-zone, where we
>> >> also link cgroups under their soft_limit. We won't scan the list
>> >> unless the first list being exhausted and
>> >> the free pages is still under the high_wmark.
>> >>
>> >> Since the per-zone memcg list design is being replaced by your
>> >> patchset, some of the details doesn't apply. But the concept still
>> >> remains where we would like to scan some memcgs first (above
>> >> soft_limit) .
>> >
>> > I think the most important thing we wanted was to round-robin scan all
>> > soft limit excessors instead of just the biggest one.  I understood
>> > this is the biggest fault with soft limits right now.
>> >
>> > We came up with maintaining a list of excessors, rather than a tree,
>> > and from this particular implementation followed naturally that this
>> > list is scanned BEFORE we look at other memcgs at all.
>> >
>> > This is a nice to have, but it was never the primary problem with the
>> > soft limit implementation, as far as I understood.
>> >
>> >> > It would be a lie about the current code in the first place, which
>> >> > does soft limit reclaim and then regular reclaim, no matter the
>> >> > outcome of the soft limit reclaim cycle.  It will go for the soft
>> >> > limit first, but after an allocation under pressure the VM is likely
>> >> > to have reclaimed from other memcgs as well.
>> >> >
>> >> > I saw your patch to fix that and break out of reclaim if soft limit
>> >> > reclaim did enough.  But this fix is not much newer than my changes.
>> >>
>> >> My soft_limit patch was developed in parallel with your patchset, and
>> >> most of that wouldn't apply here.
>> >> Is that what you are referring to?
>> >
>> > No, I meant that the current behaviour is old and we are only changing
>> > it only now, so we are not really breaking backward compatibility.
>> >
>> >> > The second part of this is:
>> >> >
>> >> >    Please note that soft limits is a best effort feature, it comes with
>> >> >    no guarantees, but it does its best to make sure that when memory is
>> >> >    heavily contended for, memory is allocated based on the soft limit
>> >> >    hints/setup. Currently soft limit based reclaim is setup such that
>> >> >    it gets invoked from balance_pgdat (kswapd).
>> >>
>> >> We had patch merged which add the soft_limit reclaim also in the global ttfp.
>> >>
>> >> memcg-add-the-soft_limit-reclaim-in-global-direct-reclaim.patch
>> >>
>> >> > It's not the pages-over-soft-limit that are best effort.  It says that
>> >> > it tries its best to take soft limits into account while reclaiming.
>> >> Hmm. Both cases are true. The best effort pages I referring to means
>> >> "the page above the soft_limit are targeted to reclaim first under
>> >> memory contention"
>> >
>> > I really don't know where you are taking this from.  That is neither
>> > documented anywhere, nor is it the current behaviour.
>>
>> I got the email from andrew on may 27 and you were on the cc-ed :)
>> Anyway, i just forwarded you that one.
>
> I wasn't asking about this patch at all...  This is the conversation:
>
> Me:
>
>> >> > It's not the pages-over-soft-limit that are best effort.  It says that
>> >> > it tries its best to take soft limits into account while reclaiming.
>
> You:
>
>> >> Hmm. Both cases are true. The best effort pages I referring to means
>> >> "the page above the soft_limit are targeted to reclaim first under
>> >> memory contention"
>
> Me:
>
>> > I really don't know where you are taking this from.  That is neither
>> > documented anywhere, nor is it the current behaviour.
>
> And this is still my question.
>
> Current: scan up to all pages of the biggest soft limit offender, then
> reclaim from random memcgs (because of the global LRU).
agree.

>
> After my patch: scan all memcgs according to their size, with double
> the pressure on those over their soft limit.
agree.
>
> Please tell me exactly how my patch regresses existing behaviour, a
> user interface, a documented feature, etc.
>

Ok, thank you for clarifying it. Now i understand what's the confusion here.

I agree that your patch doesn't regress from what we have now
currently. What i referred earlier was the improvement from the
current design. So we were comparing to two targets.

Please go ahead with your patch, and I don't have problem with that
now. I will propose the soft_limit reclaim improvement as separate
thread.

Thanks

--Ying

> If you have an even better idea, please propose it.
>

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