Date:   Mon, 26 Sep 2022 16:22:42 +0200
From:   Jan Kara <jack@...e.cz>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
        paolo.valente@...aro.org, axboe@...nel.dk,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v3 3/5] block, bfq: don't disable wbt if
 CONFIG_BFQ_GROUP_IOSCHED is disabled

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 :)

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

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