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Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO
 reads"
To:     Dave Chinner <david@fromorbit.com>, Jan Kara <jack@suse.cz>
Cc:     Joseph Qi <joseph.qi@linux.alibaba.com>,
        Andreas Dilger <adilger@dilger.ca>,
        Theodore Ts'o <tytso@mit.edu>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>,
        Liu Bo <bo.liu@linux.alibaba.com>
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 <6DADA28C-542F-45F6-ADB0-870A81ABED23@dilger.ca>
 <15112e38-94fe-39d6-a8e2-064ff47187d5@linux.alibaba.com>
 <20190728225122.GG7777@dread.disaster.area>
From:   Joseph Qi <jiangqi903@gmail.com>
Message-ID: <960bb915-20cc-26a0-7abc-bfca01aa39c0@gmail.com>
Date:   Tue, 30 Jul 2019 09:34:39 +0800
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On 19/7/29 06:51, Dave Chinner wrote:
> On Fri, Jul 26, 2019 at 09:12:07AM +0800, Joseph Qi wrote:
>>
>>
>> On 19/7/26 05:20, Andreas Dilger wrote:
>>>
>>>> On Jul 23, 2019, at 5:17 AM, Joseph Qi <jiangqi903@gmail.com> wrote:
>>>>
>>>> Hi Ted & Jan,
>>>> Could you please give your valuable comments?
>>>
>>> It seems like the original patches should be reverted?  There is no data
>>
>> From my test result, yes.
>> I've also tested libaio with iodepth 16, it behaves the same. Here is the test
>> data for libaio 4k randrw:
>>
>> -------------------------------------------------------------------------------------------
>> w/ parallel dio reads | READ 78313KB/s, 19578, 1698.70us  | WRITE 78313KB/s, 19578, 4837.60us
>> -------------------------------------------------------------------------------------------
>> w/o parallel dio reads| READ 387774KB/s, 96943, 1009.73us | WRITE 387656KB/s，96914, 308.87us
>> -------------------------------------------------------------------------------------------
>>
>> Since this commit went into upstream long time ago，to be precise, Linux
>> 4.9, I wonder if someone else has also observed this regression, or
>> anything I missed?
> 
> I suspect that the second part of this set of mods that Jan had
> planned to do (on the write side to use shared locking as well)
> did not happen and so the DIO writes are serialising the workload.
> 

Thanks for the inputs, Dave.
Hi Jan, Could you please confirm this?
If so, should we revert this commit at present?

Thanks,
Joseph