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

[Joseph Qi <joseph.qi@linux.alibaba.com>]

On 19/8/23 18:16, Dave Chinner wrote:
> On Fri, Aug 23, 2019 at 03:57:02PM +0800, Joseph Qi wrote:
>> 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.
> 
> Ok, so the problem is not shared locking scalability ('cause that's
> what XFS does and it scaled fine), the problem is almost certainly
> that ext4 is using exclusive locking during writes...
> 

Agree. Maybe I've misled you in my previous mails.I meant shared lock makes worse in case of mixed random read/write, since
we would always take inode lock during write.
And it also conflicts with dioread_nolock. It won't take any inode lock
before with dioread_nolock during read, but now it always takes a shared
lock.

Thanks,
Joseph