[Devel] Re: [patch 1/4] io controller: documentation

[KAMEZAWA Hiroyuki]

On Thu, 06 Nov 2008 10:30:23 -0500
vgoyal at redhat.com wrote:
> +ISSUES
> +======
> +- IO controller can buffer the bios if suffcient tokens were not available
> +  at the time of bio submission. Once the tokens are available, these bios
> +  are dispatched to elevator/lower layers in first come first serve manner.
> +  And this has potential to break CFQ where a RT tasks should be able to
> +  dispatch the bio first or a high priority task should be able to release
> +  more bio as compared to low priority task in same cgroup.
> +
> +  Not sure how to fix it. May be we need to maintain another rb-tree and
> +  keep track of RT tasks and tasks priorities and dispatch accordingly. This
> +  is equivalent of duplicating lots of CFQ logic and not sure how would it
> +  impact AS behaviour.
> 
Why you don't isolate RT tasks into other cgroup ?
   /cgroup/bio-cgroup/group_for_usual/...usual tasks.
                     /group_for_RT/ ...RT tasks. you can use high-speed path.

How about adding RT flag to bio-cgroup and skip buffering at bio-cgroup if RT
flag is set ? I think handling an usual process and RT process in "a" cgroup
just makes the code complex.

Looking into a cpu-scheduler, which is the first module handling RT, it has
some tweaks to handle RT in the system.
 - special RT scheduler.
 - isolated RT domain
 - maximum execution time allowed to RT
 ....

Maybe handling RT in following way is usual way...(if we do something in this layer)

  - Allow RT-bio-cgroup to skip limit check.
  - But RT-bio-cgroup calculates io-throuput, execution time, statistics...
  - When RT tasks in RT-bio-cgroup does excessive I/O which starves the whole system
    too long, raise safeguard-limitter. and tell users Warning or kill it.

Hmm ?

Thanks,
-Kame

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