Re: [PACTH v2 0/3] Implement /proc/<pid>/totmaps

[Sonny Rao]

On Thu, Aug 18, 2016 at 11:01 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Thu 18-08-16 10:47:57, Sonny Rao wrote:
>> On Thu, Aug 18, 2016 at 12:44 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>> > On Wed 17-08-16 11:57:56, Sonny Rao wrote:
> [...]
>> >> 2) User space OOM handling -- we'd rather do a more graceful shutdown
>> >> than let the kernel's OOM killer activate and need to gather this
>> >> information and we'd like to be able to get this information to make
>> >> the decision much faster than 400ms
>> >
>> > Global OOM handling in userspace is really dubious if you ask me. I
>> > understand you want something better than SIGKILL and in fact this is
>> > already possible with memory cgroup controller (btw. memcg will give
>> > you a cheap access to rss, amount of shared, swapped out memory as
>> > well). Anyway if you are getting close to the OOM your system will most
>> > probably be really busy and chances are that also reading your new file
>> > will take much more time. I am also not quite sure how is pss useful for
>> > oom decisions.
>>
>> I mentioned it before, but based on experience RSS just isn't good
>> enough -- there's too much sharing going on in our use case to make
>> the correct decision based on RSS.  If RSS were good enough, simply
>> put, this patch wouldn't exist.
>
> But that doesn't answer my question, I am afraid. So how exactly do you
> use pss for oom decisions?

We use PSS to calculate the memory used by a process among all the
processes in the system, in the case of Chrome this tells us how much
each renderer process (which is roughly tied to a particular "tab" in
Chrome) is using and how much it has swapped out, so we know what the
worst offenders are -- I'm not sure what's unclear about that?

Chrome tends to use a lot of shared memory so we found PSS to be
better than RSS, and I can give you examples of the  RSS and PSS on
real systems to illustrate the magnitude of the difference between
those two numbers if that would be useful.

>
>> So even with memcg I think we'd have the same problem?
>
> memcg will give you instant anon, shared counters for all processes in
> the memcg.
>

We want to be able to get per-process granularity quickly.  I'm not
sure if memcg provides that exactly?

>> > Don't take me wrong, /proc/<pid>/totmaps might be suitable for your
>> > specific usecase but so far I haven't heard any sound argument for it to
>> > be generally usable. It is true that smaps is unnecessarily costly but
>> > at least I can see some room for improvements. A simple patch I've
>> > posted cut the formatting overhead by 7%. Maybe we can do more.
>>
>> It seems like a general problem that if you want these values the
>> existing kernel interface can be very expensive, so it would be
>> generally usable by any application which wants a per process PSS,
>> private data, dirty data or swap value.
>
> yes this is really unfortunate. And if at all possible we should address
> that. Precise values require the expensive rmap walk. We can introduce
> some caching to help that. But so far it seems the biggest overhead is
> to simply format the output and that should be addressed before any new
> proc file is added.
>
>> I mentioned two use cases, but I guess I don't understand the comment
>> about why it's not usable by other use cases.
>
> I might be wrong here but a use of pss is quite limited and I do not
> remember anybody asking for large optimizations in that area. I still do
> not understand your use cases properly so I am quite skeptical about a
> general usefulness of a new file.

How do you know that usage of PSS is quite limited?  I can only say
that we've been using it on Chromium OS for at least four years and
have found it very valuable, and I think I've explained the use cases
in this thread. If you have more specific questions then I can try to
clarify.

>
> --
> Michal Hocko
> SUSE Labs