Re: [RFC] [PATCH 0/2] memcg: per cgroup dirty limit

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Feb 23, 2010 at 10:55:55AM +0100, Andrea Righi wrote:
> On Mon, Feb 22, 2010 at 01:29:34PM -0500, Vivek Goyal wrote:
> > > I would't like to add many different interfaces to do the same thing.
> > > I'd prefer to choose just one interface and always use it. We just have
> > > to define which is the best one. IMHO dirty_bytes is more generic. If
> > > we want to define the limit as a % we can always do that in userspace.
> > > 
> > 
> > dirty_ratio is easy to configure. One system wide default value works for
> > all the newly created cgroups. For dirty_bytes, you shall have to
> > configure each and individual cgroup with a specific value depneding on
> > what is the upper limit of memory for that cgroup.
> 
> OK.
> 
> > 
> > Secondly, memory cgroup kind of partitions global memory resource per
> > cgroup. So if as long as we have global dirty ratio knobs, it makes sense
> > to have per cgroup dirty ratio knob also. 
> > 
> > But I guess we can introduce that later and use gloabl dirty ratio for
> > all the memory cgroups (instead of each cgroup having a separate dirty
> > ratio). The only thing is that we need to enforce this dirty ratio on the
> > cgroup and if I am reading the code correctly, your modifications of
> > calculating available_memory() per cgroup should take care of that.
> 
> At the moment (with dirty_bytes) if the cgroup has dirty_bytes == 0, it
> simply uses the system wide available_memory(), ignoring the memory
> upper limit for that cgroup and fallbacks to the current behaviour.
> 
> With dirty_ratio, should we change the code to *always* apply this
> percentage to the cgroup memory upper limit, and automatically set it
> equal to the global dirty_ratio by default when the cgroup is created?
> mmmh... I vote yes.

Yes inheriting global dirty ratio as per cgroup ratio makes sense. Only
thing different here is that we apply dirty_ratio on cgroup memory limit
and not actual available memory. So if one configures cgroup with total
limit higher than available memory, we could end up lot more dirty pages
in system.

But one can then argue that it is wrong system configuration and admin should
take care of that while configuring the system. :-)

So, I would vote, yes.

Thanks
Vivek

> 
> -Andrea