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

[Greg Thelen <gthelen@google.com>]

On Mon, Feb 03 2014, Michal Hocko wrote:

> On Thu 30-01-14 16:28:27, Greg Thelen wrote:
>> On Thu, Jan 30 2014, Michal Hocko wrote:
>> 
>> > On Wed 29-01-14 11:08:46, Greg Thelen wrote:
>> > [...]
>> >> The series looks useful.  We (Google) have been using something similar.
>> >> In practice such a low_limit (or memory guarantee), doesn't nest very
>> >> well.
>> >> 
>> >> Example:
>> >>   - parent_memcg: limit 500, low_limit 500, usage 500
>> >>     1 privately charged non-reclaimable page (e.g. mlock, slab)
>> >>   - child_memcg: limit 500, low_limit 500, usage 499
>> >
>> > I am not sure this is a good example. Your setup basically say that no
>> > single page should be reclaimed. I can imagine this might be useful in
>> > some cases and I would like to allow it but it sounds too extreme (e.g.
>> > a load which would start trashing heavily once the reclaim starts and it
>> > makes more sense to start it again rather than crowl - think about some
>> > mathematical simulation which might diverge).
>> 
>> Pages will still be reclaimed the usage_in_bytes is exceeds
>> limit_in_bytes.  I see the low_limit as a way to tell the kernel: don't
>> reclaim my memory due to external pressure, but internal pressure is
>> different.
>
> That sounds strange and very confusing to me. What if the internal
> pressure comes from children memcgs? Lowlimit is intended for protecting
> a group from reclaim and it shouldn't matter whether the reclaim is a
> result of the internal or external pressure.
>
>> >> If a streaming file cache workload (e.g. sha1sum) starts gobbling up
>> >> page cache it will lead to an oom kill instead of reclaiming. 
>> >
>> > Does it make any sense to protect all of such memory although it is
>> > easily reclaimable?
>> 
>> I think protection makes sense in this case.  If I know my workload
>> needs 500 to operate well, then I reserve 500 using low_limit.  My app
>> doesn't want to run with less than its reservation.
>> 
>> >> One could argue that this is working as intended because child_memcg
>> >> was promised 500 but can only get 499.  So child_memcg is oom killed
>> >> rather than being forced to operate below its promised low limit.
>> >> 
>> >> This has led to various internal workarounds like:
>> >> - don't charge any memory to interior tree nodes (e.g. parent_memcg);
>> >>   only charge memory to cgroup leafs.  This gets tricky when dealing
>> >>   with reparented memory inherited to parent from child during cgroup
>> >>   deletion.
>> >
>> > Do those need any protection at all?
>> 
>> Interior tree nodes don't need protection from their children.  But
>> children and interior nodes need protection from siblings and parents.
>
> Why? They contains only reparented pages in the above case. Those would
> be #1 candidate for reclaim in most cases, no?

I think we're on the same page.  My example interior node has reclaimed
pages and is a #1 candidate for reclaim induced from charges against
parent_memcg, but not a candidate for reclaim due to global memory
pressure induced by a sibling of parent_memcg.

>> >> - don't set low_limit on non leafs (e.g. do not set low limit on
>> >>   parent_memcg).  This constrains the cgroup layout a bit.  Some
>> >>   customers want to purchase $MEM and setup their workload with a few
>> >>   child cgroups.  A system daemon hands out $MEM by setting low_limit
>> >>   for top-level containers (e.g. parent_memcg).  Thereafter such
>> >>   customers are able to partition their workload with sub memcg below
>> >>   child_memcg.  Example:
>> >>      parent_memcg
>> >>          \
>> >>           child_memcg
>> >>             /     \
>> >>         server   backup
>> >
>> > I think that the low_limit makes sense where you actually want to
>> > protect something from reclaim. And backup sounds like a bad fit for
>> > that.
>> 
>> The backup job would presumably have a small low_limit, but it may still
>> have a minimum working set required to make useful forward progress.
>> 
>> Example:
>>   parent_memcg
>>       \
>>        child_memcg limit 500, low_limit 500, usage 500
>>          /     \
>>          |   backup   limit 10, low_limit 10, usage 10
>>          |
>>       server limit 490, low_limit 490, usage 490
>> 
>> One could argue that problems appear when
>> server.low_limit+backup.lower_limit=child_memcg.limit.  So the safer
>> configuration is leave some padding:
>>   server.low_limit + backup.low_limit + padding = child_memcg.limit
>> but this just defers the problem.  As memory is reparented into parent,
>> then padding must grow.
>
> Which all sounds like a drawback of internal vs. external pressure
> semantic which you have mentioned above.

Huh?  I probably confused matters with the internal vs external talk
above.  Forgetting about that, I'm happy with the following
configuration assuming low_limit_fallback (ll_fallback) is eventually
available.

   parent_memcg
       \
        child_memcg limit 500, low_limit 500, usage 500, ll_fallback 0
          /     \
          |   backup   limit 10, low_limit 10, usage 10, ll_fallback 1
          |
       server limit 490, low_limit 490, usage 490, ll_fallback 1

>> >>   Thereafter customers often want some weak isolation between server and
>> >>   backup.  To avoid undesired oom kills the server/backup isolation is
>> >>   provided with a softer memory guarantee (e.g. soft_limit).  The soft
>> >>   limit acts like the low_limit until priority becomes desperate.
>> >
>> > Johannes was already suggesting that the low_limit should allow for a
>> > weaker semantic as well. I am not very much inclined to that but I can
>> > leave with a knob which would say oom_on_lowlimit (on by default but
>> > allowed to be set to 0). We would fallback to the full reclaim if
>> > no groups turn out to be reclaimable.
>> 
>> I like the strong semantic of your low_limit at least at level:1 cgroups
>> (direct children of root).  But I have also encountered situations where
>> a strict guarantee is too strict and a mere preference is desirable.
>> Perhaps the best plan is to continue with the proposed strict low_limit
>> and eventually provide an additional mechanism which provides weaker
>> guarantees (e.g. soft_limit or something else if soft_limit cannot be
>> altered).  These two would offer good support for a variety of use
>> cases.
>> 
>> I thinking of something like:
>> 
>> bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
>> 		struct mem_cgroup *root,
>> 		int priority)
>> {
>> 	do {
>> 		if (memcg == root)
>> 			break;
>> 		if (!res_counter_low_limit_excess(&memcg->res))
>> 			return false;
>> 		if ((priority >= DEF_PRIORITY - 2) &&
>> 		    !res_counter_soft_limit_exceed(&memcg->res))
>> 			return false;
>> 	} while ((memcg = parent_mem_cgroup(memcg)));
>> 	return true;
>> }
>
> Mixing soft limit into the picture is more than confusing because it
> has its own meaning now and we shouldn't recycle it until it is dead
> completely.
> Another thing which seems to be more serious is that such a reclaim
> logic would inherently lead to a potential over reclaim because 2
> priority cycles would be wasted with no progress and when we finally
> find somebody then it gets hammered more at lower priority.
>
> What I would like much more is to fallback to ignore low_limit if
> nothing is reclaimable due to low_limit. That would be controlled on a
> memcg level (something like memory.low_limit_fallback).

Sure, but that would require a sweep through the candidate memcg to
confirm that all cgroups are operating below their low limit.  I suppose
we could have an optimization where the number of children above
low_limit is recorded in the parent.  Then reclaim in the parent would
immediately determine if low_limit should be violated (if
memory.low_limit_fallback=1).  But this can be deferred to later
patches.

>> But this soft_limit,priority extension can be added later.
>
> Yes, I would like to have the strong semantic first and then deal with a
> weaker form. Either by a new limit or a flag.

Sounds good.

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