On Tue, 13 Mar 2012 09:39:14 -0700
Tejun Heo <tj@xxxxxxxxxx> wrote:
> Hello, KAMEZAWA.
>
> On Tue, Mar 13, 2012 at 03:11:48PM +0900, KAMEZAWA Hiroyuki wrote:
> > The trouble for pre_destroy() is _not_ refcount, Memory cgroup has its own refcnt
> > and use it internally. The problem is 'charges'. It's not related to refcnt.
>
> Hmmm.... yeah, I'm not familiar with memcg internals at all. For
> blkcg, refcnt matters but if it doesn't for memcg, great.
>
> > Cgroup is designed to exists with 'tasks'. But memory may not be related to any
> > task...just related to a cgroup.
> >
> > But ok, pre_destory() & rmdir() is complicated, I agree.
> >
> > Now, we prevent rmdir() if we can't move charges to its parent. If pre_destory()
> > shouldn't fail, I can think of some alternatives.
> >
> > * move all charges to the parent and if it fails...move all charges to
> > root cgroup.
> > (drop_from_memory may not work well in swapless system.)
>
> I think this one is better and this shouldn't fail if hierarchical
> mode is in use, right?
>
Right.
> > I think.. if pre_destory() never fails, we don't need pre_destroy().
>
> For memcg maybe, blkcg still needs it.
>
> > > The last one seems more tricky. On destruction of cgroup, the
> > > charges are transferred to its parent and the parent may not have
> > > enough room for that. Greg told me that this should only be a
> > > problem for !hierarchical case. I think this can be dealt with by
> > > dumping what's left over to root cgroup with a warning message.
> >
> > I don't like warning ;)
>
> I agree this isn't perfect but then again failing rmdir isn't perfect
> either and given that the condition can be wholly avoided in
> hierarchical mode, which should be the default anyway (is there any
> reason to keep flat mode except for backward compatibility?), I don't
> think the trade off is too bad.
>
One reason is 'performance'. You can see performance trouble when you
creates deep tree of memcgs in hierarchy mode. The deeper memcg tree,
the more res_coutners will be shared.
For example, libvirt creates cgroup tree as
/cgroup/memory/libvirt/qemu/GuestXXX/....
/cgroup/memory/libvirt/lxc/GuestXXX/...
No one don't want to count up 4 res_coutner, which is very very heavy,
for handling independent workloads of "Guest".
IIUC, in general, even in the processes are in a tree, in major case
of servers, their workloads are independent.
I think FLAT mode is the dafault. 'heararchical' is a crazy thing which
cannot be managed.
Thanks,
-Kame
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