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

[Yu Kuai]

Hi, Jan

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.

Thanks,
Kuai

> So I'm kind of wondering why you'd like to use blk-wbt and BFQ together...
>
> 								Honza