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

[Andrea Righi <arighi@develer.com>]

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.

-Andrea