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Sender: Tejun Heo <htejun@gmail.com>
Date:   Tue, 10 Jan 2023 12:21:07 -1000
From:   Tejun Heo <tj@kernel.org>
To:     hanjinke <hanjinke.666@bytedance.com>
Cc:     josef@toxicpanda.com, axboe@kernel.dk, cgroups@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        yinxin.x@bytedance.com
Subject: Re: [External] Re: [PATCH v4] blk-throtl: Introduce sync and async
 queues for blk-throtl
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On Tue, Jan 10, 2023 at 11:59:09PM +0800, hanjinke wrote:
> 
> 
> 在 2023/1/10 上午4:40, Tejun Heo 写道:
> > On Sat, Jan 07, 2023 at 09:07:38PM +0800, Jinke Han wrote:
> > > + * Assumed that there were only bios queued in ASYNC queue and the SYNC
> > > + * queue was empty. The ASYNC bio was selected to dispatch and the
> > > + * disp_sync_cnt was set to 0 after each dispatching. If a ASYNC bio
> > > + * can't be dispatched because of overlimit in current slice, the process
> > > + * of dispatch should give up and the spin lock of the request queue
> > > + * may be released. A new SYNC bio may be queued in the SYNC queue then.
> > > + * When it's time to dispatch this tg, the SYNC bio was selected and pop
> > > + * to dispatch as the disp_sync_cnt is 0 and the SYNC queue is no-empty.
> > > + * If the dispatched bio is smaller than the waiting bio, the bandwidth
> > > + * may be hard to satisfied as the slice may be trimed after each dispatch.
> > 
> > I still can't make a good sense of this scenario. Can you give concrete
> > example scenarios with IOs and why it would matter?
> > 
> > Thanks.
> > 
> 
> Assumed that there are many buffer write bios queued in ASYNC queue and the
> SYNC queue is empty. The buffer write bios are all 1M in size and the bps
> limit is 1M/s. The throtl_slice is 100ms.
> 
> Assumed that the start/end_slice is [jiffies1, jiffies1+100] and
> bytes_disp[w] = 0. The next ASYNC bio can't dispatch because of overlimit
> within this slice. The wait time is 900ms and the slice will be extended to
> [jiffies1, jiffies1 + 1000] in tg_may_dispatch.
> 
> During the waiting of the ASYNC bio, a SYNC 4k bio be queued in SYNC queue.
> After 900ms, it's time to dispatch the ASYNC io, but the SYNC 4k bio be
> selected to be dispatched. Now the slice is [jiffies1, jiffies1+1000] and
> the byte_disp[w] = 4k. The slice may be extended to
> [jiffies1, jiffies1+1100]. In tg_dispatch_one_bio, the slice will be trimed
> to [jiffies1+1000, jiffies1+1100], the byte_disp[w] will set 0.
> 
> After the 4k SYNC bio be dispatched, the WAITING ASYNC bio still cann't
> be dispatched because of overlimit within this slice.
> 
> The same thing may happen DISPACH_SYNC_FACTOR times if alway there is a SYNC
> bio be queued in the SYNC queue when the ASYNC bio is waiting.
> 
> This means that in nearly 5s, we have dispathed 4 4k SYNC bios and a 1m
> ASYNC bio.
> 
> In our test, with 100M/s bps limit setted, the bps only reach to ~80m/s
> when a fio generate buffer write ios and fsync continuous generated by
> dbench in same cgroup.

That makes sense to me. Thanks for the explanation. It'd probably a good
idea to expand the explanation in the comment tho.

Thanks.

-- 
tejun