Hello, James.

On Sun, Sep 30, 2012 at 09:56:28AM +0100, James Bottomley wrote:

The beancounter approach originally used by OpenVZ does exactly this.
There are two specific problems, though, firstly you can't count
references in generic code, so now you have to extend the cgroup
tentacles into every object, an invasiveness which people didn't really
like.

Yeah, it will need some hooks.  For dentry and inode, I think it would
be pretty well isolated tho.  Wasn't it?

Secondly split accounting causes oddities too, like your total
kernel memory usage can appear to go down even though you do nothing
just because someone else added a share.  Worse, if someone drops the
reference, your usage can go up, even though you did nothing, and push
you over your limit, at which point action gets taken against the
container.  This leads to nasty system unpredictability (The whole point
of cgroup isolation is supposed to be preventing resource usage in one
cgroup from affecting that in another).

In a sense, the fluctuating amount is the actual resource burden the
cgroup is putting on the system, so maybe it just needs to be handled
better or maybe we should charge fixed amount per refcnt?  I don't
know.

We discussed this pretty heavily at the Containers Mini Summit in Santa
Rosa.  The emergent consensus was that no-one really likes first use
accounting, but it does solve all the problems and it has the fewest
unexpected side effects.

But that's like fitting the problem to the mechanism.  Maybe that is
the best which can be done, but the side effect there is way-off
accounting under pretty common workload, which sounds pretty nasty to
me.

Thanks.

-- 
tejun