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Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO
 reads"
To:     Dave Chinner <david@fromorbit.com>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
        Joseph Qi <jiangqi903@gmail.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>
References: <6DADA28C-542F-45F6-ADB0-870A81ABED23@dilger.ca>
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 <20190815151336.GO14313@quack2.suse.cz>
 <075fd06f-b0b4-4122-81c6-e49200d5bd17@linux.alibaba.com>
 <20190816145719.GA3041@quack2.suse.cz>
 <a8ddec64-d87c-ae7a-9b02-2f24da2396e9@linux.alibaba.com>
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 <f89131c9-6f84-ac3c-b53c-d3d55887ea89@linux.alibaba.com>
 <20190822054001.GT7777@dread.disaster.area>
From:   Joseph Qi <joseph.qi@linux.alibaba.com>
Message-ID: <f0eb766f-3c04-2a53-1669-4088e09d8f73@linux.alibaba.com>
Date:   Fri, 23 Aug 2019 15:57:02 +0800
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Hi Dave,

On 19/8/22 13:40, Dave Chinner wrote:
> On Wed, Aug 21, 2019 at 09:04:57AM +0800, Joseph Qi wrote:
>> 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
> 
> Regardless of dioread_nolock, ext4_direct_IO_read() is taking
> inode_lock_shared() across the direct IO call.  And writes in ext4
> _always_ take the inode_lock() in ext4_file_write_iter(), even
> though it gets dropped quite early when overwrite && dioread_nolock
> is set.  But just taking the lock exclusively in write fro a short
> while is enough to kill all shared locking concurrency...
> 
>> from my preliminary investigation, shared lock consumes more in such
>> scenario.
> 
> If the write lock is also shared, then there should not be a
> scalability issue. The shared dio locking is only half-done in ext4,
> so perhaps comparing your workload against XFS would be an
> informative exercise... 

I've done the same test workload on xfs, it behaves the same as ext4
after reverting parallel dio reads and mounting with dioread_lock.
Here is the test result:
psync, randrw, direct=1, numofjobs=8

4k:
-----------------------------------------
ext4 | READ 123450KB/s | WRITE 123368KB/s
-----------------------------------------
xfs  | READ 123848KB/s | WRITE 123761KB/s
-----------------------------------------

16k:
-----------------------------------------
ext4 | READ 222477KB/s | WRITE 222322KB/s
-----------------------------------------
xfs  | READ 223261KB/s | WRITE 223106KB/s
-----------------------------------------

64k:
-----------------------------------------
ext4 | READ 427406KB/s | WRITE 426197KB/s
-----------------------------------------
xfs  | READ 403697KB/s | WRITE 402555KB/s
-----------------------------------------

512k:
-----------------------------------------
ext4 | READ 618752KB/s | WRITE 619054KB/s
-----------------------------------------
xfs  | READ 614954KB/s | WRITE 615254KB/s
-----------------------------------------

1M:
-----------------------------------------
ext4 | READ 615011KB/s | WRITE 612255KB/s
-----------------------------------------
xfs  | READ 624087KB/s | WRITE 621290KB/s
-----------------------------------------