Re: [PATCH v5 0/3] mm, proc: Implement /proc/<pid>/totmaps

[Michal Hocko <mhocko@kernel.org>]

On Tue 13-09-16 13:27:39, Sonny Rao wrote:
> On Tue, Sep 13, 2016 at 12:12 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > On Mon 12-09-16 10:28:53, Sonny Rao wrote:
> >> On Mon, Sep 12, 2016 at 10:15 AM, Michal Hocko <mhocko@kernel.org> wrote:
> >> > On Mon 12-09-16 08:31:36, Sonny Rao wrote:
> > [...]
> >> >> but how about the other fields like Swap, Private_Dirty and
> >> >> Private_Shared?
> >> >
> >> > Private_Shared can be pretty confusing as well without the whole context
> >> > as well see my other emails in the original thread (just to remind
> >> > shmem/tmpfs makes all this really confusing).
> >>
> >> But this is exactly the issue -- RSS is can be just as confusing if
> >> you don't know something about the application.
> >
> > I agree that rss can be confusing but we will not make the situation any
> > better if we add yet another confusing metric.
> >
> >> I think the issue is
> >> how common that situation is, and you seem to believe that it's so
> >> uncommon that it's actually better to keep the information more
> >> difficult to get for those of us who know something about our systems.
> >>
> >> That's fine, I guess we just have to disagree here, thanks for look at this.
> >
> > I think you should just step back and think more about what exactly
> > you expect from the counter(s). I believe what you want is an
> > estimate of a freeable memory when the particular process dies or is
> > killed. That would mean resident single mapped private anonymous memory
> > + unlinked single mapped shareable mappings + single mapped swapped out
> > memory. Maybe I've missed something but it should be something along
> > those lines. Definitely something that the current smaps infrastructure
> > doesn't give you, though.
> 
> Yes your description of what we want is pretty good.  Having a
> reasonable lower bound on the estimate is fine, though we probably
> want to break out swapped out memory separately.

Why would you want to separate that?

> Given that smaps
> doesn't provide this in a straightforward way, what do you think is
> the right way to provide this information?

I would be tempted to sneak it into /proc/<pid>/statm because that looks
like a proper place but getting this information is not for free
performance wise so I am not really sure something that relies on this
file would see unexpected stalls. Maybe this could be worked around by
some caching... I would suggest to check who is actually using this file
(top/ps etc...)

If this would be unacceptable then a new file could be considered.
-- 
Michal Hocko
SUSE Labs