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SubjectRe: [RFC] blk-cgroup: Allow creation of hierarchical cgroups
* Vivek Goyal <vgoyal@redhat.com> [2010-11-02 18:20:30]:

> o Allow hierarchical cgroup creation for blkio controller
>
> o Currently we disallow it as both the io controller policies (throttling
> as well as proportion bandwidth) do not support hierarhical accounting
> and control. But the flip side is that blkio controller can not be used with
> libvirt as libvirt creates a cgroup hierarchy deeper than 1 level.
>
> <top-level-cgroup-dir>/<controller>/libvirt/qemu/<virtual-machine-groups>
>
> o So this patch will allow creation of cgroup hierarhcy but at the backend
> everything will be treated as flat. So if somebody created a an hierarchy
> like as follows.
>
> root
> / \
> test1 test2
> |
> test3
>
> CFQ and throttling will practically treat all groups at same level.
>
> pivot
> / | \ \
> root test1 test2 test3
>
> o Once we have actual support for hierarchical accounting and control
> then we can introduce another cgroup tunable file "blkio.use_hierarchy"
> which will be 0 by default but if user wants to enforce hierarhical
> control then it can be set to 1. This way there should not be any
> ABI problems down the line.
>
> o The only not so pretty part is introduction of extra file "use_hierarchy"
> down the line. Kame-san had mentioned that hierarhical accounting is
> expensive in memory controller hence they keep it off by default. I
> suspect same will be the case for IO controller also as for each IO
> completion we shall have to account IO through hierarchy up to the root.
> if yes, then it probably is not a very bad idea to introduce this extra
> file so that it will be used only when somebody needs it and some people
> might enable hierarchy only in part of the hierarchy.
>
> o This is how basically memory controller also uses "use_hierarhcy" and
> they also allowed creation of hierarchies when actual backend support
> was not available.
>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>


Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>


--
Three Cheers,
Balbir


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