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

[Jan Kara <jack@suse.cz>]

On Sat 24-08-19 12:18:40, Dave Chinner wrote:
> On Fri, Aug 23, 2019 at 09:08:53PM +0800, Joseph Qi wrote:
> > 
> > 
> > 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.
> 
> No, you didn't mislead me. IIUC, the shared locking was added to the
> direct IO read path so that it can't run concurrently with
> operations like hole punch that free the blocks the dio read might
> currently be operating on (use after free).
> 
> i.e. the shared locking fixes an actual bug, but the performance
> regression is a result of only partially converting the direct IO
> path to use shared locking. Only half the job was done from a
> performance perspective. Seems to me that the two options here to
> fix the performance regression are to either finish the shared
> locking conversion, or remove the shared locking on read and re-open
> a potential data exposure issue...

We actually had a separate locking mechanism in ext4 code to avoid stale
data exposure during hole punch when unlocked DIO reads were running. But
it was kind of ugly and making things complex. I agree we need to move ext4
DIO path conversion further to avoid taking exclusive lock when we won't
actually need it.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR