Re: Block IO controller hierarchy suppport (Was: Re: [PATCH RFC cgroup/for-3.7] cgroup: mark subsystems with broken hierarchy support and whine if cgroups are nested for them)

[Tejun Heo <tj@kernel.org>]

Hey, Vivek.

(cc'ing Rakesh and Chad who work on iosched in google).

On Thu, Sep 13, 2012 at 10:53:41AM -0400, Vivek Goyal wrote:
> Biggest problem with blkcg CFQ implementation is idling on cgroup. If
> we don't idle on cgroup, then we don't get the service differentiaton
> for most of the workloads and if we do idle then performance starts
> to suck very soon (The moment few cgroups are created). And hierarchy
> will just exacertbate this problem because then one will try to idle
> at each group in hierarchy.
> 
> This problem is something similar to CFQ's idling on sequential queues
> and iopriority. Because we never idled on random IO queue, ioprios never
> worked on random IO queues. And same is true for buffered write queues.
> Similary, if you don't idle on groups, then for most of the workloads,
> service differentiation is not visible. Only the one which are highly
> sequential on nature, one can see service differentiation.
> 
> That's one fundamental problem for which we need to have a good answer
> before we try to do more work on blkcg. Because we can write as much
> code but at the end of the day it might still not be useful because
> of the above mentioned issue I faced.

I talked with Rakesh about this as the modified cfq-iosched used in
google supports proper hierarchy and the feature is heavily depended
upon.  I was told that nesting doesn't really change anything.  The
only thing which matters is the number of active cgroups and whether
they're nested or how deep doesn't matter - IIUC there's no need to
idle for internal nodes if they don't have IOs pending.

He draw me some diagrams which made sense for me and the code
apparently actually works, so there doesn't seem to be any fundamental
issue in implementing hierarchy support in cfq.

Thoughts?

Thanks.

-- 
tejun