Re: [RFC][PATCH 4/7] RSS accounting hooks over the code

[Nick Piggin]

Eric W. Biederman wrote:
> Herbert Poetzl <herbert@xxxxxxxxxxxx> writes:
>
>
> > On Mon, Mar 12, 2007 at 09:50:08AM -0700, Dave Hansen wrote:
> >
> > > On Mon, 2007-03-12 at 19:23 +0300, Kirill Korotaev wrote:
> > >
> > > > For these you essentially need per-container page->_mapcount counter,
> > > > otherwise you can't detect whether rss group still has the page 
> > > > in question being mapped in its processes' address spaces or not. 
> >
> > > What do you mean by this?  You can always tell whether a process has a
> > > particular page mapped.  Could you explain the issue a bit more.  I'm
> > > not sure I get it.
> >
> > OpenVZ wants to account _shared_ pages in a guest
> > different than separate pages, so that the RSS
> > accounted values reflect the actual used RAM instead
> > of the sum of all processes RSS' pages, which for
> > sure is more relevant to the administrator, but IMHO
> > not so terribly important to justify memory consuming
> > structures and sacrifice performance to get it right
> >
> > YMMV, but maybe we can find a smart solution to the
> > issue too :)
>
>
> I will tell you what I want.
>
> I want a shared page cache that has nothing to do with RSS limits.
>
> I want an RSS limit that once I know I can run a deterministic
> application with a fixed set of inputs in I want to know it will
> always run.
>
> First touch page ownership does not guarantee give me anything useful
> for knowing if I can run my application or not.  Because of page
> sharing my application might run inside the rss limit only because
> I got lucky and happened to share a lot of pages with another running
> application.  If the next I run and it isn't running my application
> will fail.  That is ridiculous.

Let's be practical here, what you're asking is basically impossible.

Unless by deterministic you mean that it never enters the a non
trivial syscall, in which case, you just want to know about maximum
RSS of the process, which we already account).

> I don't want sharing between vservers/VE/containers to affect how many
> pages I can have mapped into my processes at once.

You seem to want total isolation. You could use virtualization?

> Now sharing is sufficiently rare that I'm pretty certain that problems
> come up rarely.  So maybe these problems have not shown up in testing
> yet.  But until I see the proof that actually doing the accounting for
> sharing properly has intolerable overhead.  I want proper accounting
> not this hand waving that is only accurate on the third Tuesday of the
> month.

It is basically handwaving anyway. The only approach I've seen with
a sane (not perfect, but good) way of accounting memory use is this
one. If you care to define "proper", then we could discuss that.

--
SUSE Labs, Novell Inc.
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