Re: How to find out how many other processes share VM with $PID?

Fengguang Wu
2007-08-29T07:14:44Z

On Tue, Aug 28, 2007 at 09:00:41PM +0100, Denys Vlasenko wrote:
> On Tuesday 28 August 2007 01:10, Fengguang Wu wrote:
> > > > There is a nice LWN article on this issue:
> > > > ELC: How much memory are applications really using?
> > > > http://lwn.net/Articles/230975/
> > > >
> > > > Another helpful patch could be:
> > > > maps: PSS(proportional set size) accounting in smaps
> > > > http://lkml.org/lkml/2007/8/19/23
> > >
> > > Thanks a lot, very useful pages indeed.
> > >
> > > However they still don't explain how I can avoid counting memory
> > > twice for /proc/PID1 and /proc/PID2 when PID2 is a child of PID1,
> > > created with CLONE_VM.
> > >
> > > The example: I allocate 1234k, dirty it, then clone with CLONE_VM.
> > > I will seemingly have two processes, each using 1234k, _privately_
> > > (i.e., pages are not shown as shared in smaps) -
> > > which is technically correct, pages are not shared with other VMs,
> > > but they ARE shared by means of these two processes having the same VM!
> > >
> > > How userspace tools can figure out that these processes have shared VM?
> > >
> > > IOW: do we need "VMsharecount: N" in addition to "Threads: N"
> > > in /proc/PID/status?
> >
> > A full solution would require two parameters, i.e. VmUsers/VmMagic.
> >
> > But please make sure the new lines won't break important tools like
> > ps/top/pmaps/...
>
> Should be safe - tools skip lines they do not recognize.

Except yours ;-)

FYI, I found another tool that depends on status: atop.

> Ok, we have "Threads: N".
>
> I can cook up a patch which adds count of processes
> which share VM with us - it's just atomic_read(&current->mm->mm_users).

Yeah, the code itself would be simple.

> What name do you like?
>
> SharedVmCount: N
> VmUsers: N
> other?

I'd prefer VmUsers: that's the choice of source code.

Fengguang