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

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

On 19/8/21 11:34, Theodore Y. Ts'o wrote:
> On Wed, Aug 21, 2019 at 09:04:57AM +0800, Joseph Qi wrote:
>> 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).
> 
> Right, but you were asserting that performance hit was *because* of
> the shared lock.  I'm asking what leading you to have that opinion.
> The fact that parallel dioread reads doesn't necessarily say that it
> was because of that particular shared lock.  It could be due to any
> number of other things.  Have you looked at /proc/lock_stat (enabeld
> via CONFIG_LOCK_STAT) to see where the locking bottlenecks might be?
> 
I've enabled CONFIG_LOCK_STAT and CONFIG_DEBUG_RWSEMS, but doesn't see
any statistics for i_rwsem. Am I missing something?

Thanks,
Joseph