linux-kernel - Re: [PATCH -next RFC 2/6] block: refactor to split bio thoroughly

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Date:   Wed, 30 Mar 2022 09:54:03 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@...radead.org>
CC:     <andriy.shevchenko@...ux.intel.com>, <john.garry@...wei.com>,
        <ming.lei@...hat.com>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>
Subject: Re: [PATCH -next RFC 2/6] block: refactor to split bio thoroughly

On 2022/03/29 22:41, Jens Axboe wrote:
> On 3/29/22 8:40 AM, Christoph Hellwig wrote:
>> On Tue, Mar 29, 2022 at 08:35:29AM -0600, Jens Axboe wrote:
>>>> But more importantly why does your use case even have splits that get
>>>> submitted together?  Is this a case of Linus' stupidly low default
>>>> max_sectors when the hardware supports more, or is the hardware limited
>>>> to a low number of sectors per request?  Or do we hit another reason
>>>> for the split?
>>>
>>> See the posted use case, it's running 512kb ios on a 128kb device.
Hi,

The problem was first found during kernel upgrade(v3.10 to v4.18), and
we maintain a series of io performance test suites, and one of the test
is fio random rw with large bs. In the environment, the 'max_sectors_kb'
is 256kb, and fio bs is 1m.
>>
>> That is an awfully low limit these days.  I'm really not sure we should
>> optimize the block layer for that.
> 
> That's exactly what my replies have been saying. I don't think this is
> a relevant thing to optimize for.

If the use case that large ios get submitted together is not a common
issue（probably not since it's been a long time without complaining),
I agree that we should not optimize the block layer for that.

Thanks,
Kuai
> 
> Fixing fairness for wakeups seems useful, however.
>