Re: question about relative control for sync io using bfq

["yukuai (C)" <yukuai3@huawei.com>]

On 2021/02/09 3:05, Paolo Valente wrote:
> 
> 
>> Il giorno 7 feb 2021, alle ore 13:49, yukuai (C) <yukuai3@huawei.com> ha scritto:
>>
>>
>> On 2021/02/05 15:49, Paolo Valente wrote:
>>>> Il giorno 29 gen 2021, alle ore 09:28, yukuai (C) <yukuai3@huawei.com> ha scritto:
>>>>
>>>> Hi,
>>>>
>>>> Thanks for your response, and my apologize for the delay, my tmie
>>>> is very limited recently.
>>>>
>>> I do know that problem ...
>>>> On 2021/01/22 18:09, Paolo Valente wrote:
>>>>> Hi,
>>>>> this is a core problem, not of BFQ but of any possible solution that
>>>>> has to provide bandwidth isolation with sync I/O.  One of the examples
>>>>
>>>> I'm not sure about this, so I test it with iocost in mq and cfq in sq,
>>>> result shows that they do can provide bandwidth isolation with sync I/O
>>>> without significant performance degradation.
>>> Yep, that means just that, with your specific workload, bandwidth
>>> isolation gets guaranteed without idling.  So that's exactly one of
>>> the workloads for which I'm suggesting my handling of a special case.
>>>>> is the one I made for you in my other email.  At any rate, the problem
>>>>> that you report seems to occur with just one group.  We may think of
>>>>> simply changing my condition
>>>>> bfqd->num_groups_with_pending_reqs > 0
>>>>> to
>>>>> bfqd->num_groups_with_pending_reqs > 1
>>>>
>>>> We aredy tried this, the problem will dispeare if only one group is
>>>> active. And I think this modification is reasonable because
>>>> bandwidth isolation is not necessary in this case.
>>>>
>>> Thanks for your feedback. I'll consider submitting this change.
>>>> However, considering the common case, when more than one
>>>> group is active, and one of the group is issuing sync IO, I think
>>>> we need to find a way to prevent the preformance degradation.
>>> I agree.  What do you think of my suggestion for solving the problem?
>>> Might you help with that?
>>
>> Hi
>>
>> Do you mead the suggestion that you mentioned in another email:
>> "a varied_rq_size flag, similar to the varied_weights flag" ?
>> I'm afraid that's just a circumvention plan, not a solution to the
>> special case.
>>
> 
> I'm a little confused.  Could you explain why you think this is a
> circumvention plan?  Maybe even better, could you describe in detail
> the special case you have in mind?  We could start from there, to think
> of a possible, satisfactory solution.
> 
Hi,

First of all, there are two conditions to trigger the problem in bfq:
a. issuing sync IO concurrently. (I was testing in one cgroup, and
I think multiple cgroups is the same.)
b. not issuing in root cgroup.

The phenomenon is that the performance will degradated to single
process. The reason is that bfq_queue will never expired untill
BUDGET_TIMEOUT since num_groups_with_pending_reqs will always be
nonzero after issuing io to driver, which means that there is only
one request in progress(during D2C) at any given moment.

I was trying to skip the checking of active group if bfq.weight is not
changed, and it's implemented by adding a varible 'check_active_group',
it'll only set to true if bfq.weight is changed.

This approach will work fine is the weight stay unchanged, and will
not be effective if the weight ever changed. This is why I said it's
a circumvention plan.

By the way, while testing with cfq, I found that the current active cfq
queue can be preempted in such special case. I wonder if it's worth
referring to bfq.

Thanks,
Yu Kuai

> 
>> By the way, I'm glad if there is anything I can help, however it'll
>> wait for a few days cause the Spring Festival is coming.
>>
> 
> Ok, Happy Spring Festival then.
> 
> Thanks.
> Paolo
> 
>> Thanks,
>> Yu Kuai
>>
>>> Thanks,
>>> Paolo
>>>>> If this simple solution does solve the problem you report, then I
>>>>> could run my batch of tests to check whether it causes some
>>>>> regression.
>>>>> What do you think?
>>>>> Thanks.
>>>>> Paolo
>>>>
>>>> Thanks
>>>> Yu Kuai
>>>>> .
>>> .
> 
> .
>