[Devel] Re: [PATCH v2 04/11] kmem accounting basic infrastructure

[Michal Hocko]

On Wed 15-08-12 12:50:55, Ying Han wrote:
> On Tue, Aug 14, 2012 at 9:21 AM, Michal Hocko <mhocko at suse.cz> wrote:
> > On Thu 09-08-12 17:01:12, Glauber Costa wrote:
> >> This patch adds the basic infrastructure for the accounting of the slab
> >> caches. To control that, the following files are created:
> >>
> >>  * memory.kmem.usage_in_bytes
> >>  * memory.kmem.limit_in_bytes
> >>  * memory.kmem.failcnt
> >>  * memory.kmem.max_usage_in_bytes
> >>
> >> They have the same meaning of their user memory counterparts. They
> >> reflect the state of the "kmem" res_counter.
> >>
> >> The code is not enabled until a limit is set. This can be tested by the
> >> flag "kmem_accounted". This means that after the patch is applied, no
> >> behavioral changes exists for whoever is still using memcg to control
> >> their memory usage.
> >>
> >> We always account to both user and kernel resource_counters. This
> >> effectively means that an independent kernel limit is in place when the
> >> limit is set to a lower value than the user memory. A equal or higher
> >> value means that the user limit will always hit first, meaning that kmem
> >> is effectively unlimited.
> >
> > Well, it contributes to the user limit so it is not unlimited. It just
> > falls under a different limit and it tends to contribute less. This can
> > be quite confusing.  I am still not sure whether we should mix the two
> > things together. If somebody wants to limit the kernel memory he has to
> > touch the other limit anyway.  Do you have a strong reason to mix the
> > user and kernel counters?
> 
> The reason to mix the two together is a compromise of the two use
> cases we've heard by far. In google, we only need one limit which
> limits u & k, and the reclaim kicks in when the total usage hits the
> limit.
> 
> > My impression was that kernel allocation should simply fail while user
> > allocations might reclaim as well. Why should we reclaim just because of
> > the kernel allocation (which is unreclaimable from hard limit reclaim
> > point of view)?
> 
> Some of kernel objects are reclaimable if we have per-memcg shrinker.

Agreed and I think we need that before this is merged as I state in
other email.

> > I also think that the whole thing would get much simpler if those two
> > are split. Anyway if this is really a must then this should be
> > documented here.
> 
> What would be the use case you have in your end?

I do not have any specific unfortunately but I would like to prevent us
from closing other possible. I realize this sounds hand wavy and that is
why I do not want to block this work but I think we should give it some
time before this gets merged.

> --Ying
-- 
Michal Hocko
SUSE Labs