linux-kernel - Re: [PATCH 2/2] ufs: don't use the fair tag sharings

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Message-ID: <aa9af9ae-62a4-6469-244c-b5d9106bb044@acm.org>
Date:   Thu, 11 May 2023 08:38:04 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Christoph Hellwig <hch@...radead.org>,
        Ed Tsai <ed.tsai@...iatek.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        martin.petersen@...cle.com, stanley.chu@...iatek.com,
        peter.wang@...iatek.com, chun-hung.wu@...iatek.com,
        alice.chao@...iatek.com, powen.kao@...iatek.com,
        naomi.chu@...iatek.com, wsd_upstream@...iatek.com
Subject: Re: [PATCH 2/2] ufs: don't use the fair tag sharings

On 5/11/23 08:34, Christoph Hellwig wrote:
> On Tue, May 09, 2023 at 02:52:30PM +0800, Ed Tsai wrote:
>> The tags allocation is limited by the fair sharing algorithm. It hurts
>> the performance for UFS devices, because the queue depth of general I/O
>> is reduced by half once the UFS send a control command.
> 
> But it is there for a reason.  You completely fail to explain why you
> think your change is safe, and also why you did not try to even explain
> where the overhead is and how else you tried to mitigate it.

Hi Christoph,

For which devices is the fair sharing algorithm useful? As far as I know 
the legacy block layer did not have an equivalent of the fair sharing 
algorithm and I'm not aware of any complaints about the legacy block 
layer regarding to fairness. This is why I proposed in January to remove 
the fair sharing code entirely. See also 
https://lore.kernel.org/linux-block/20230103195337.158625-1-bvanassche@acm.org/.

Thanks,

Bart.