Re: question about relative control for sync io using bfq

On 2021/02/05 15:49, Paolo Valente wrote:

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Il giorno 29 gen 2021, alle ore 09:28, yukuai (C) [off-list ref] ha scritto:

Hi,

Thanks for your response, and my apologize for the delay, my tmie
is very limited recently.

I do know that problem ...

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On 2021/01/22 18:09, Paolo Valente wrote:

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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.

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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.

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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.

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.

Thanks,
Yu Kuai

Thanks,
Paolo

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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

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