Re: RSS accounting (was: Re: 2.6.19-rc1-mm1)

[Arjan van de Ven <arjan@infradead.org>]

On Wed, 2006-10-11 at 06:07 -0600, Eric W. Biederman wrote:
> Arjan van de Ven <arjan@infradead.org> writes:
> 
> > On Tue, 2006-10-10 at 17:54 -0600, Eric W. Biederman wrote:
> >
> >> For processes shared pages are not special.
> 
> Actually the above is not quite true you can map a shared page twice
> into the same process but in practice it rarely happens.

yeah I'm entirely fine with ignoring that case (or making the person who
does it pay for it :)

> 
> > depends on what question you want to answer with RSS.
> > If the question is "workload working set size" then you are right. If
> > the question is "how much ram does my application cause to be used" the
> > answer is FAR less clear....
> 
> There are two basic concerns.  How do you keep an application from
> going crazy and trashing the rest of your system?  A question on what
> number do you need to implement a resource limit.

yet at the same time if 2 apps mmap a shared file, and app 1 keeps it in
pagecache, it doesn't cause app2 to trash, or rather, it's not like if
app 2 did NOT have the page from that file, the system wouldn't trash.


> > You seem to have an implicit definition on what RSS should mean; but
> > it's implicit. Mind making an explicit definition of what RSS should be
> > in your opinion? I think that's the biggest problem we have right now;
> > several people have different ideas about what it should/could be, and
> > as such we're not talking about the same thing. Lets first agree/specify
> > what it SHOULD mean, and then we can figure out what gets counted for
> > that ;)
> 
> Well I tried to defined it in terms of what you can use it for.
> 
> I would define the resident set size as the total number of bytes
> of physical RAM that a process (or set of processes) is using,
> irrespective of the rest of the system.  
>  
> 
> So I think the counting should be primarily about what is mapped into
> the page tables.  But other things can be added as is appropriate or
> easy.
> 
> The practical effect should be that an application that needs more
> pages than it's specified RSS to avoid thrashing should thrash but
> it shouldn't take the rest of the system with it.


so by your definition, hugepages are part of RSS.

Ken: what is your definition of RSS ?