On Mon, Sep 19, 2016 at 09:51:13PM +0200, Michal Hocko wrote: > [not sure why the CC list was trimmed - do no do that please unless you > have a strong reason for that - if this was not intentional please > restpre it]
Ah, sorry, pressed the wrong key. > On Mon 19-09-16 21:40:01, Jann Horn wrote: > > On Mon, Sep 19, 2016 at 09:32:38PM +0200, Michal Hocko wrote: > > > On Mon 19-09-16 11:16:31, Robert Foss wrote: > > > > On 2016-09-14 05:12 AM, Michal Hocko wrote: > > > > > On Tue 13-09-16 13:27:39, Sonny Rao wrote: > > > [...] > > > > > > 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...) > > > > > > > > What would this caching look like? Can any information be re-used > > > > between > > > > vma walks? > > > > > > yes basically return the same value if called within HZ or something > > > similar. But that assumes that statm latency really matters and it is > > > called often enough. > > > > That sounds horrible. If some application decides that they want to check > > statm directly after some action or so (like after program startup), this is > > going to give them a very bad time. That probably doesn't happen > > often - but still. > > > > I can already imagine some developer going "yeah, that usleep()... that's > > because the kernel API returns stale information for a couple milliseconds > > after we do something *shrug*". > > > > What are you trying to optimize for? Ten users on the same machine, each of > > which is running "top" because it looks so great? > > Please try to read what I wrote again. I didn't say this would be > needed. The idea was that _if_ /proc/<pid>/statm is used very _often_ > than some caching might help to reduce the overhead. Especially when you > consider that the information is not precise anyway. It can change > anytime while you are doing the address space walk.
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