linux-kernel - Re: cgroup, balance RT bandwidth

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Date:	Tue, 10 Mar 2009 15:26:38 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rolando Martins <rolando.martins@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: cgroup, balance RT bandwidth

On Tue, 2009-03-10 at 11:49 +0000, Rolando Martins wrote:
> Just to confirm, cpuset.sched_load_balance doesn't work with RT, right?

It should. It should split the RT balance domain just the same.

> You cannot have tasks for sub-domain 2 to utilize bandwidth of
> sub-domain 3, right?

If you disabled load-balancing on your root domain (1 below) then
indeed, tasks from 2 will not be able to consume bandwidth from tasks in
3.

The available bandwidth is related to the number of cpus in the balance
domain.

> 
>                               __1__
>                              /        \
>                             2         3
>                       (50% rt)  (50% rt )
> 
> For my application domain it would be interesting to have
> rt_runtime_ns as a min. of allocated rt and not a max.

> Ex. If an application of domain 2 needs to go up to 100% and domain 3
> is idle, then it would be cool to let it utilize the full bandwidth.

> (we also could have a hard upper limit in each sub-domain, like
> hard_up=0.8, i.e. even if we could get 100%, we will only utilize
> 80%); in other words, rt having the same cpu bandwidth management behavior
> as the "best-effort" tasks.
> 
> Could this be done?

Possibly, but since RT scheduling is all about determinism, I see no use
in adding something best-effort -- that simply defeats the purpose.

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