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

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Date:   Thu, 22 Aug 2019 15:40:01 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Joseph Qi <jiangqi903@...il.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>,
        Liu Bo <bo.liu@...ux.alibaba.com>
Subject: Re: [RFC] performance regression with "ext4: Allow parallel DIO
 reads"

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...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com