Re: [PATCH v3 3/5] block, bfq: don't disable wbt if CONFIG_BFQ_GROUP_IOSCHED is disabled

[Yu Kuai]

Hi,

在 2022/09/28 0:14, Paolo Valente 写道:
> > Il giorno 27 set 2022, alle ore 03:02, Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> ha scritto:
> >
> > Hi, Jan
> >
> > 在 2022/09/26 22:22, Jan Kara 写道:
> > > Hi Kuai!
> > > On Mon 26-09-22 21:00:48, Yu Kuai wrote:
> > > > 在 2022/09/23 19:03, Jan Kara 写道:
> > > > > Hi Kuai!
> > > > >
> > > > > On Fri 23-09-22 18:23:03, Yu Kuai wrote:
> > > > > > 在 2022/09/23 18:06, Jan Kara 写道:
> > > > > > > On Fri 23-09-22 17:50:49, Yu Kuai wrote:
> > > > > > > > Hi, Christoph
> > > > > > > >
> > > > > > > > 在 2022/09/23 16:56, Christoph Hellwig 写道:
> > > > > > > > > On Thu, Sep 22, 2022 at 07:35:56PM +0800, Yu Kuai wrote:
> > > > > > > > > > wbt and bfq should work just fine if CONFIG_BFQ_GROUP_IOSCHED is disabled.
> > > > > > > > >
> > > > > > > > > Umm, wouldn't this be something decided at runtime, that is not
> > > > > > > > > if CONFIG_BFQ_GROUP_IOSCHED is enable/disable in the kernel build
> > > > > > > > > if the hierarchical cgroup based scheduling is actually used for a
> > > > > > > > > given device?
> > > > > > > > > .
> > > > > > > >
> > > > > > > > That's a good point,
> > > > > > > >
> > > > > > > > Before this patch wbt is simply disabled if elevator is bfq.
> > > > > > > >
> > > > > > > > With this patch, if elevator is bfq while bfq doesn't throttle
> > > > > > > > any IO yet, wbt still is disabled unnecessarily.
> > > > > > >
> > > > > > > It is not really disabled unnecessarily. Have you actually tested the
> > > > > > > performance of the combination? I did once and the results were just
> > > > > > > horrible (which is I made BFQ just disable wbt by default). The problem is
> > > > > > > that blk-wbt assumes certain model of underlying storage stack and hardware
> > > > > > > behavior and BFQ just does not fit in that model. For example BFQ wants to
> > > > > > > see as many requests as possible so that it can heavily reorder them,
> > > > > > > estimate think times of applications, etc. On the other hand blk-wbt
> > > > > > > assumes that if request latency gets higher, it means there is too much IO
> > > > > > > going on and we need to allow less of "lower priority" IO types to be
> > > > > > > submitted. These two go directly against one another and I was easily
> > > > > > > observing blk-wbt spiraling down to allowing only very small number of
> > > > > > > requests submitted while BFQ was idling waiting for more IO from the
> > > > > > > process that was currently scheduled.
> > > > > >
> > > > > > Thanks for your explanation, I understand that bfq and wbt should not
> > > > > > work together.
> > > > > >
> > > > > > However, I wonder if CONFIG_BFQ_GROUP_IOSCHED is disabled, or service
> > > > > > guarantee is not needed, does the above phenomenon still exist? I find
> > > > > > it hard to understand... Perhaps I need to do some test.
> > > > >
> > > > > Well, BFQ implements for example idling on sync IO queues which is one of
> > > > > the features that upsets blk-wbt. That does not depend on
> > > > > CONFIG_BFQ_GROUP_IOSCHED in any way. Also generally the idea that BFQ
> > > > > assigns storage *time slots* to different processes and IO from other
> > > > > processes is just queued at those times increases IO completion
> > > > > latency (for IOs of processes that are not currently scheduled) and this
> > > > > tends to confuse blk-wbt.
> > > > Hi, Jan
> > > >
> > > > Just to be curious, have you ever think about or tested wbt with
> > > > io-cost? And even more, how bfq work with io-cost?
> > > >
> > > > I haven't tested yet, but it seems to me some of them can work well
> > > > together.
> > > No, I didn't test these combinations. I actually expect there would be
> > > troubles in both cases under high IO load but you can try :)
> >
> > Just realize I made a clerical error, I actually want to saied that
> > *can't* work well together.
>
> You are right, they can't work together, conceptually. Their logics would simply keep conflicting, and none of the two would make ti to control IO as desired.

Yes, I just run some simple tests, test result is very bad...

Perhaps we can do something like bfq does to disable wbt.

Thanks,
Kuai
> Thanks,
> Paolo
>
> > I'll try to have a test the combinations.
> >
> > Thanks,
> > Kuai
> > > 								Honza
>
> .