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