Re: [PATCH v5 2/2] mm/memcontrol.c: Reduce reclaim retries in mem_cgroup_resize_limit()

[Shakeel Butt <shakeelb@google.com>]

On Fri, Jan 19, 2018 at 7:11 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Fri 19-01-18 06:49:29, Shakeel Butt wrote:
>> On Fri, Jan 19, 2018 at 5:35 AM, Michal Hocko <mhocko@kernel.org> wrote:
>> > On Fri 19-01-18 16:25:44, Andrey Ryabinin wrote:
>> >> Currently mem_cgroup_resize_limit() retries to set limit after reclaiming
>> >> 32 pages. It makes more sense to reclaim needed amount of pages right away.
>> >>
>> >> This works noticeably faster, especially if 'usage - limit' big.
>> >> E.g. bringing down limit from 4G to 50M:
>> >>
>> >> Before:
>> >>  # perf stat echo 50M > memory.limit_in_bytes
>> >>
>> >>      Performance counter stats for 'echo 50M':
>> >>
>> >>             386.582382      task-clock (msec)         #    0.835 CPUs utilized
>> >>                  2,502      context-switches          #    0.006 M/sec
>> >>
>> >>            0.463244382 seconds time elapsed
>> >>
>> >> After:
>> >>  # perf stat echo 50M > memory.limit_in_bytes
>> >>
>> >>      Performance counter stats for 'echo 50M':
>> >>
>> >>             169.403906      task-clock (msec)         #    0.849 CPUs utilized
>> >>                     14      context-switches          #    0.083 K/sec
>> >>
>> >>            0.199536900 seconds time elapsed
>> >
>> > But I am not going ack this one. As already stated this has a risk
>> > of over-reclaim if there a lot of charges are freed along with this
>> > shrinking. This is more of a theoretical concern so I am _not_ going to
>>
>> If you don't mind, can you explain why over-reclaim is a concern at
>> all? The only side effect of over reclaim I can think of is the job
>> might suffer a bit over (more swapins & pageins). Shouldn't this be
>> within the expectation of the user decreasing the limits?
>
> It is not a disaster. But it is an unexpected side effect of the
> implementation. If you have limit 1GB and want to reduce it 500MB
> then it would be quite surprising to land at 200M just because somebody
> was freeing 300MB in parallel. Is this likely? Probably not but the more
> is the limit touched and the larger are the differences the more likely
> it is. Keep retrying in the smaller amounts and you will not see the
> above happening.
>
> And to be honest, I do not really see why keeping retrying from
> mem_cgroup_resize_limit should be so much faster than keep retrying from
> the direct reclaim path. We are doing SWAP_CLUSTER_MAX batches anyway.
> mem_cgroup_resize_limit loop adds _some_ overhead but I am not really
> sure why it should be that large.
>

Thanks for the explanation. Another query, we do not call
drain_all_stock() in mem_cgroup_resize_limit() but memory_max_write()
does call drain_all_stock(). Was this intentional or missed
accidentally?