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

[Jan Kara <jack@suse.cz>]

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.

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

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR