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Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO
 reads"
To:     "Theodore Y. Ts'o" <tytso@mit.edu>
Cc:     Jan Kara <jack@suse.cz>, Joseph Qi <jiangqi903@gmail.com>,
        Dave Chinner <david@fromorbit.com>,
        Andreas Dilger <adilger@dilger.ca>,
        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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From:   Joseph Qi <joseph.qi@linux.alibaba.com>
Message-ID: <f89131c9-6f84-ac3c-b53c-d3d55887ea89@linux.alibaba.com>
Date:   Wed, 21 Aug 2019 09:04:57 +0800
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Hi Ted，

On 19/8/21 00:08, Theodore Y. Ts'o wrote:
> On Tue, Aug 20, 2019 at 11:00:39AM +0800, Joseph Qi wrote:
>>
>> I've tested parallel dio reads with dioread_nolock, it doesn't have
>> significant performance improvement and still poor compared with reverting
>> parallel dio reads. IMO, this is because with parallel dio reads, it take
>> inode shared lock at the very beginning in ext4_direct_IO_read().
> 
> Why is that a problem?  It's a shared lock, so parallel threads should
> be able to issue reads without getting serialized?
> 
The above just tells the result that even mounting with dioread_nolock,
parallel dio reads still has poor performance than before (w/o parallel
dio reads).

> Are you using sufficiently fast storage devices that you're worried
> about cache line bouncing of the shared lock?  Or do you have some
> other concern, such as some other thread taking an exclusive lock?
> 
The test case is random read/write described in my first mail. And
from my preliminary investigation, shared lock consumes more in such
scenario.

Thanks,
Joseph