linux-ext4 - Re: [RFC] performance regression with "ext4: Allow parallel DIO reads"

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Date:   Fri, 23 Aug 2019 15:57:02 +0800
From:   Joseph Qi <joseph.qi@...ux.alibaba.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Joseph Qi <jiangqi903@...il.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>,
        Liu Bo <bo.liu@...ux.alibaba.com>
Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO
 reads"

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
-----------------------------------------