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:

quoted

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