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

[Sonny Rao]

On Thu, Aug 18, 2016 at 2:05 PM, Robert Foss <robert.foss@xxxxxxxxxxxxx> wrote:
>
>
> On 2016-08-18 02:01 PM, Michal Hocko 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?
>>
>>> 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.
>
>
> Is it technically feasible to add instant pss support to memcg?
>
> @Sonny Rao: Would using cgroups be acceptable for chromiumos?

It's possible, though I think we'd end up putting each renderer in
it's own cgroup to get the PSS stat, so it seems a bit like overkill.
I think memcg also has some overhead that we'd need to quantify but I
could be mistaken about this.

>
>
>>
>>>> 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.
>>
>