From: Amir Goldstein <email@example.com>
To: Dave Chinner <firstname.lastname@example.org>
Cc: "Darrick J . Wong" <email@example.com>,
Christoph Hellwig <firstname.lastname@example.org>,
Matthew Wilcox <email@example.com>,
Subject: Re: [POC][PATCH] xfs: reduce ilock contention on buffered randrw workload
Date: Fri, 5 Apr 2019 17:02:33 +0300 [thread overview]
Message-ID: <CAOQ4uxjQNmxqmtA_VbYW0Su9rKRk2zobJmahcyeaEVOFKVQ5dw@mail.gmail.com> (raw)
On Fri, Apr 5, 2019 at 12:17 AM Dave Chinner <firstname.lastname@example.org> wrote:
> On Thu, Apr 04, 2019 at 07:57:37PM +0300, Amir Goldstein wrote:
> > This patch improves performance of mixed random rw workload
> > on xfs without relaxing the atomic buffered read/write guaranty
> > that xfs has always provided.
> > We achieve that by calling generic_file_read_iter() twice.
> > Once with a discard iterator to warm up page cache before taking
> > the shared ilock and once again under shared ilock.
> This will race with thing like truncate, hole punching, etc that
> serialise IO and invalidate the page cache for data integrity
> reasons under the IOLOCK. These rely on there being no IO to the
> inode in progress at all to work correctly, which this patch
> violates. IOWs, while this is fast, it is not safe and so not a
> viable approach to solving the problem.
This statement leaves me wondering, if ext4 does not takes
i_rwsem on generic_file_read_iter(), how does ext4 (or any other
fs for that matter) guaranty buffered read synchronization with
truncate, hole punching etc?
The answer in ext4 case is i_mmap_sem, which is read locked
in the page fault handler.
And xfs does the same type of synchronization with MMAPLOCK,
so while my patch may not be safe, I cannot follow why from your
explanation, so please explain if I am missing something.
One thing that Darrick mentioned earlier was that IOLOCK is also
used by xfs to synchronization pNFS leases (probably listed under
'etc' in your explanation). I consent that my patch does not look safe
w.r.t pNFS leases, but that can be sorted out with a hammer
or with finer instruments.
> FYI, I'm working on a range lock implementation that should both
> solve the performance issue and the reader starvation issue at the
> same time by allowing concurrent buffered reads and writes to
> different file ranges.
> IO range locks will allow proper exclusion for other extent
> manipulation operations like fallocate and truncate, and eventually
> even allow truncate, hole punch, file extension, etc to run
> concurrently with other non-overlapping IO. They solve more than
> just the performance issue you are seeing....
I'm glad to hear that. IO range locks are definitely a more wholesome
solution to the problem looking forward.
However, I am still interested to continue the discussion on my POC
patch. One reason is that I am guessing it would be much easier for
distros to backport and pick up to solve performance issues.
Even if my patch doesn't get applied upstream nor picked by distros,
I would still like to understand its flaws and limitations. I know...
if I break it, I get to keep the pieces, but the information that you
provide helps me make my risk assessments.
next prev parent reply other threads:[~2019-04-05 14:02 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-04 16:57 [POC][PATCH] xfs: reduce ilock contention on buffered randrw workload Amir Goldstein
2019-04-04 21:17 ` Dave Chinner
2019-04-05 14:02 ` Amir Goldstein [this message]
2019-04-07 23:27 ` Dave Chinner
2019-04-08 9:02 ` Amir Goldstein
2019-04-08 14:11 ` Jan Kara
2019-04-08 17:41 ` Amir Goldstein
2019-04-09 8:26 ` Jan Kara
2022-06-17 14:48 ` Amir Goldstein
2022-06-17 15:11 ` Jan Kara
2022-06-18 8:38 ` Amir Goldstein
2022-06-20 9:11 ` Jan Kara
2022-06-21 7:49 ` Amir Goldstein
2022-06-21 8:59 ` Jan Kara
2022-06-21 12:53 ` Amir Goldstein
2022-06-22 3:23 ` Matthew Wilcox
2022-06-22 9:00 ` Amir Goldstein
2022-06-22 9:34 ` Jan Kara
2022-06-22 16:26 ` Amir Goldstein
2022-09-13 14:40 ` Amir Goldstein
2022-09-14 16:01 ` Darrick J. Wong
2022-09-14 16:29 ` Amir Goldstein
2022-09-14 17:39 ` Darrick J. Wong
2022-09-19 23:09 ` Dave Chinner
2022-09-20 2:24 ` Dave Chinner
2022-09-20 3:08 ` Amir Goldstein
2022-09-21 11:20 ` Amir Goldstein
2019-04-08 11:03 ` Jan Kara
2019-04-22 10:55 ` Boaz Harrosh
2019-04-08 10:33 ` Jan Kara
2019-04-08 16:37 ` Davidlohr Bueso
2019-04-11 1:11 ` Dave Chinner
2019-04-16 12:22 ` Dave Chinner
2019-04-18 3:10 ` Dave Chinner
2019-04-18 18:21 ` Davidlohr Bueso
2019-04-20 23:54 ` Dave Chinner
2019-05-03 4:17 ` Dave Chinner
2019-05-03 5:17 ` Dave Chinner
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