On Tue, 17 Jun 2008, Andrea Righi wrote:
> > With this bandwidth controller, a cpu-intensive job which otherwise does
> > not care about its IO
> > performance needs to be pin-point accurate about IO bandwidth required in
> > order to not suffer
> > from cpu-throttling. IMHO, if a cgroup is exceeding its limit for a given
> > resource, the throttling
> > should be done _only_ for that resource.
>
> I understand your point of view. It would be nice if we could just
> "disable" the i/o for a cgroup that exceeds its limit, instead of
> scheduling some sleep()s, so the tasks running in this cgroup would be
> able to continue their non-i/o operations as usual.
>
> However, how to do if the tasks continue to perform i/o ops under this
> condition? we could just cache the i/o in memory and at the same time
> reduce the i/o priority of those tasks' requests, but this would require
> a lot of memory, more space in the page cache, and probably could lead
> to potential OOM conditions. A safer approach IMHO is to force the tasks
> to wait synchronously on each operation that directly or indirectly
> generates i/o. The last one is the solution implemented by this
> bandwidth controller.

What about AIO? Is this approach going to make the task sleep as well?
Would it better to return from aio_write()/_read() with EAGAIN?

Thanks.
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Eric Rannaud wrote:
> On Tue, 17 Jun 2008, Andrea Righi wrote:
>>> With this bandwidth controller, a cpu-intensive job which otherwise does
>>> not care about its IO
>>> performance needs to be pin-point accurate about IO bandwidth required in
>>> order to not suffer
>>> from cpu-throttling. IMHO, if a cgroup is exceeding its limit for a given
>>> resource, the throttling
>>> should be done _only_ for that resource.
>> I understand your point of view. It would be nice if we could just
>> "disable" the i/o for a cgroup that exceeds its limit, instead of
>> scheduling some sleep()s, so the tasks running in this cgroup would be
>> able to continue their non-i/o operations as usual.
>>
>> However, how to do if the tasks continue to perform i/o ops under this
>> condition? we could just cache the i/o in memory and at the same time
>> reduce the i/o priority of those tasks' requests, but this would require
>> a lot of memory, more space in the page cache, and probably could lead
>> to potential OOM conditions. A safer approach IMHO is to force the tasks
>> to wait synchronously on each operation that directly or indirectly
>> generates i/o. The last one is the solution implemented by this
>> bandwidth controller.
>
> What about AIO? Is this approach going to make the task sleep as well?
> Would it better to return from aio_write()/_read() with EAGAIN?

Good point. I should check, but it seems sleeps are incorrectly
performed also for AIO requests. I agree the correct behaviour would be
to return EAGAIN instead, as you suggested. I'll look at it if nobody
comes up with a solution.

Thanks,
-Andrea
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