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From:   Andreas Dilger <adilger@dilger.ca>
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Subject: Re: [PATCH 0/3] Revert parallel dio reads
Date:   Thu, 29 Aug 2019 13:06:22 -0600
In-Reply-To: <20190829105858.GA22939@quack2.suse.cz>
Cc:     Dave Chinner <david@fromorbit.com>,
        Joseph Qi <joseph.qi@linux.alibaba.com>,
        Theodore Ts'o <tytso@mit.edu>, linux-ext4@vger.kernel.org
To:     Jan Kara <jack@suse.cz>
References: <1566871552-60946-1-git-send-email-joseph.qi@linux.alibaba.com>
 <20190827115118.GY7777@dread.disaster.area>
 <20190829105858.GA22939@quack2.suse.cz>
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On Aug 29, 2019, at 4:58 AM, Jan Kara <jack@suse.cz> wrote:
>=20
> On Tue 27-08-19 21:51:18, Dave Chinner wrote:
>> On Tue, Aug 27, 2019 at 10:05:49AM +0800, Joseph Qi wrote:
>>> This patch set is trying to revert parallel dio reads feature at =
present
>>> since it causes significant performance regression in mixed random
>>> read/write scenario.
>>>=20
>>> Joseph Qi (3):
>>>  Revert "ext4: remove EXT4_STATE_DIOREAD_LOCK flag"
>>>  Revert "ext4: fix off-by-one error when writing back pages before =
dio
>>>    read"
>>>  Revert "ext4: Allow parallel DIO reads"
>>>=20
>>> fs/ext4/ext4.h        | 17 +++++++++++++++++
>>> fs/ext4/extents.c     | 19 ++++++++++++++-----
>>> fs/ext4/inode.c       | 47 =
+++++++++++++++++++++++++++++++----------------
>>> fs/ext4/ioctl.c       |  4 ++++
>>> fs/ext4/move_extent.c |  4 ++++
>>> fs/ext4/super.c       | 12 +++++++-----
>>> 6 files changed, 77 insertions(+), 26 deletions(-)
>>=20
>> Before doing this, you might want to have a chat and co-ordinate
>> with the folks that are currently trying to port the ext4 direct IO
>> code to use the iomap infrastructure:
>>=20
>> =
https://lore.kernel.org/linux-ext4/20190827095221.GA1568@poseidon.bobrowsk=
i.net/T/#t
>>=20
>> That is going to need the shared locking on read and will work just
>> fine with shared locking on write, too (it's the code that XFS uses
>> for direct IO). So it might be best here if you work towards shared
>> locking on the write side rather than just revert the shared locking
>> on the read side....
>=20
> Yeah, after converting ext4 DIO path to iomap infrastructure, using =
shared
> inode lock for all aligned non-extending DIO writes will be easy so =
I'd
> prefer if we didn't have to redo the iomap conversion patches due to =
these
> reverts.

But if the next kernel is LTS and the iomap implementation isn't in the
current merge window (very unlikely) then we're stuck with this =
performance
hit for LTS.  It is also unlikely that LTS will take the revert patches =
if
they have not been landed to master.

Cheers, Andreas


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