Re: [PATCH] staging, android: remove lowmemory killer from the tree

[Rom Lemarchand <romlem@google.com>]

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+surenb

On Fri, Feb 24, 2017 at 10:38 AM, Tim Murray <timmurray@google.com> wrote:

> Hi all, I've recently been looking at lowmemorykiller, userspace lmkd, and
> memory cgroups on Android.
>
> First of all, no, an Android device will probably not function without a
> kernel or userspace version of lowmemorykiller. Android userspace expects
> that if there are many apps running in the background on a machine and the
> foreground app allocates additional memory, something on the system will
> kill background apps to free up more memory. If this doesn't happen, I
> expect that at the very least you'll see page cache thrashing, and you'll
> probably see the OOM killer run regularly, which has a tendency to cause
> Android userspace to restart. To the best of my knowledge, no device has
> shipped with a userspace lmkd.
>
> Second, yes, the current design and implementation of lowmemorykiller are
> unsatisfactory. I now have some concrete evidence that the design of
> lowmemorykiller is directly responsible for some very negative user-visible
> behaviors (particularly the triggers for when to kill), so I'm currently
> working on an overhaul to the Android memory model that would use mem
> cgroups and userspace lmkd to make smarter decisions about reclaim vs
> killing. Yes, this means that we would move to vmpressure (which will
> require improvements to vmpressure). I can't give a firm ETA for this, as
> it's still in the prototype phase, but the initial results are promising.
>
> On Fri, Feb 24, 2017 at 1:34 AM, Michal Hocko <mhocko@kernel.org> wrote:
>
>> On Thu 23-02-17 21:36:00, Martijn Coenen wrote:
>> > On Thu, Feb 23, 2017 at 9:24 PM, John Stultz <john.stultz@linaro.org>
>> wrote:
>> [...]
>> > > This is reportedly because while the mempressure notifiers provide a
>> > > the signal to userspace, the work the deamon then has to do to look up
>> > > per process memory usage, in order to figure out who is best to kill
>> > > at that point was too costly and resulted in poor device performance.
>> >
>> > In particular, mempressure requires memory cgroups to function, and we
>> > saw performance regressions due to the accounting done in mem cgroups.
>> > At the time we didn't have enough time left to solve this before the
>> > release, and we reverted back to kernel lmkd.
>>
>> I would be more than interested to hear details. We used to have some
>> visible charge path performance footprint but this should be gone now.
>>
>> [...]
>> > > It would be great however to get a discussion going here on what the
>> > > ulmkd needs from the kernel in order to efficiently determine who best
>> > > to kill, and how we might best implement that.
>> >
>> > The two main issues I think we need to address are:
>> > 1) Getting the right granularity of events from the kernel; I once
>> > tried to submit a patch upstream to address this:
>> > https://lkml.org/lkml/2016/2/24/582
>>
>> Not only that, the implementation of tht vmpressure needs some serious
>> rethinking as well. The current one can hit critical events
>> unexpectedly. The calculation also doesn't consider slab reclaim
>> sensibly.
>>
>> > 2) Find out where exactly the memory cgroup overhead is coming from,
>> > and how to reduce it or work around it to acceptable levels for
>> > Android. This was also on 3.10, and maybe this has long been fixed or
>> > improved in more recent kernel versions.
>>
>> 3e32cb2e0a12 ("mm: memcontrol: lockless page counters") has improved
>> situation a lot as all the charging is lockless since then (3.19).
>> --
>> Michal Hocko
>> SUSE Labs
>>
>
>

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