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Subject: Re: [PATCH 2/2] ufs: don't use the fair tag sharings
To:     Bart Van Assche <bvanassche@acm.org>,
        Christoph Hellwig <hch@infradead.org>
Cc:     Ed Tsai <ed.tsai@mediatek.com>, axboe@kernel.dk,
        linux-block@vger.kernel.org, linux-scsi@vger.kernel.org,
        linux-kernel@vger.kernel.org, martin.petersen@oracle.com,
        stanley.chu@mediatek.com, peter.wang@mediatek.com,
        chun-hung.wu@mediatek.com, alice.chao@mediatek.com,
        powen.kao@mediatek.com, naomi.chu@mediatek.com,
        wsd_upstream@mediatek.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20230509065230.32552-1-ed.tsai@mediatek.com>
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 <aa9af9ae-62a4-6469-244c-b5d9106bb044@acm.org>
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 <2740ee82-e35f-1cbf-f5d0-373f94eb14a5@acm.org>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <de3f41a0-b13d-d4f6-765a-19b857bce53e@huaweicloud.com>
Date:   Sat, 13 May 2023 11:09:31 +0800
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Hi,

在 2023/05/13 2:12, Bart Van Assche 写道:
> The fair tag sharing algorithm has a negative impact on all SCSI devices 
> with multiple logical units. This is because logical units are 
> considered active until (request timeout) seconds have elapsed after the 
> logical unit stopped being used (see also the blk_mq_tag_idle() call in 
> blk_mq_timeout_work()). UFS users are hit by this because UFS 3.0 
> devices have a limited queue depth (32) and because power management 
> commands are submitted to a logical unit (WLUN). Hence, it happens often 
> that the block layer "active queue" counter is equal to 2 while only one 
> logical unit is being used actively (a logical unit backed by NAND 
> flash). The performance difference between queue depths 16 and 32 for 
> UFS devices is significant.

We meet similiar problem before, but I think remove tag fair sharing
might cause some problems, because get tag is not fair currently, for
example 2 devices share 32 tag, while device a issue large amount of
io concurrently, and device b only issue one io, in this case, if fair
tag sharing is removed, device b can get bad io latency.

By the way, I tried to propose a way to workaround this by following:

1) disable fair tag sharing untill get tag found no tag is avaiable;
2) enable fair tag sharing again if the disk donesn't faild to get tag
for a period of time;

Can this approch be considered?

Thanks,
Kuai