linux-kernel - Re: [External] Re: [RFC PATCH] blk-throtl: Introduce sync queue for write ios

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Date:   Wed, 14 Dec 2022 12:02:53 +0800
From:   hanjinke <hanjinke.666@...edance.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     josef@...icpanda.com, axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [RFC PATCH] blk-throtl: Introduce sync queue for
 write ios



在 2022/12/13 上午6:40, Tejun Heo 写道:
> On Wed, Dec 07, 2022 at 12:38:26AM +0800, Jinke Han wrote:
>> From: Jinke Han <hanjinke.666@...edance.com>
>>
>> Now we don't distinguish sync write ios from normal buffer write ios
>> in blk-throtl. A bio with REQ_SYNC tagged always mean it will be wait
>> until write completion soon after it submit. So it's reasonable for sync
>> io to complete as soon as possible.
>>
>> In our test, fio writes a 100g file in sequential 4k blocksize in
>> a container with low bps limit configured (wbps=10M). More than 1200
>> ios were throttled in blk-throtl queue and the avarage throtle time
>> of each io is 140s. At the same time, the operation of saving a small
>> file by vim will be blocked amolst 140s. As a fsync will be send by vim,
>> the sync ios of fsync will be blocked by a huge amount of buffer write
>> ios ahead. This is also a priority inversion problem within one cgroup.
>> In the database scene, things got really bad with blk-throtle enabled
>> as fsync is called very often.
>>
>> This patch introduces a independent sync queue for write ios and gives
>> a huge priority to sync write ios. I think it's a nice respond to the
>> semantics of REQ_SYNC. Bios with REQ_META and REQ_PRIO gains the same
>> priority as they are important to fs. This may avoid some potential
>> priority inversion problems.
> 
> I think the idea makes sense but wonder whether the implementation would be
> cleaner / simpler if the sq->queued[] are indexed by SYNC, ASYNC and the
> sync writes are queued in the sync queue together with reads.
> 
> Thanks.
> 
Should we keep the main category of io based READ and WRITE, and within 
READ / WRITE the subcategory were SYNC and ASYNC ? This may give less 
intrusion into existing frameworks.

Thanks.