[PATCHv5,0/3] Fix inode_lock sequence to scale performance of DIO mixed R/W workload

Message

Ritesh Harjani Dec. 12, 2019, 5:55 a.m. UTC

Hello, 

Please find v5 version with some minor comments addressed (from Jan).
Also added Tested-by & Reviewed-by.


Changes from RFCv3 => PATCHv4
1. Dropped patch-2 which introduced ext4_ilock/iunlock API based on discussion.
Now the lock/unlock decision is open coded in ext4_dio_write_iter() &
ext4_dio_write_checks().

2. Addressed review comments from Jan on last 2 patches.

Please note that apart from all the conditions mentioned in patch-3 there is
still an existing race. It is between ext4_page_mkwrite & DIO read in parallel.
This is discussed in detail at [7].
Since that race exist even before this patch series and is not caused
due to this patch series, I plan to address that after these patches are merged.
It is to ensure proper testing/review and to not club too many things in one go.

These patches are tested again with "xfstests -g auto". There were no new
failures except the known one.


Background (copied from previous with some edits)
=================================================

These are ilock patches which helps improve the current inode lock scalabiliy
problem in ext4 DIO mixed read/write workload case. The problem was first
reported by Joseph [1]. This should help improve mixed read/write workload
cases for databases which use directIO.

These patches are based upon upstream discussion with Jan Kara & Joseph [2].

The problem really is that in case of DIO overwrites, we start with
a exclusive lock and then downgrade it later to shared lock. This causes a
scalability problem in case of mixed DIO read/write workload case. 
i.e. if we have any ongoing DIO reads and then comes a DIO writes,
(since writes starts with excl. inode lock) then it has to wait until the
shared lock is released (which only happens when DIO read is completed). 
Same is true for vice versa as well.
The same can be easily observed with perf-tools trace analysis [3].

This patch series (Patch-3) helps fix that situation even without
dioread_nolock mount opt. This is inline with the discussions with Jan [4].
More details about this are mentioned in commit msg of patch 2 & 3.

These patches are based on the top of Ted's ext4 master tree.

Patch description
=================
Patch-1: Fixes ext4_dax_read/write inode locking sequence for IOCB_NOWAIT

Patch-2: Starts with shared inode lock in case of DIO instead of exclusive inode
lock. This patchset helps fix the reported scalablity problem. But this fixes it
only for dioread_nolock mount option.

Patch-3: In this we get away with dioread_nolock mount option condition
to check for shared locking. This patch commit msg describe in
detail about why we don't need excl. lock even without dioread_nolock.

Git tree
========
https://github.com/riteshharjani/linux/tree/ext4-ilock-PATCHv4

Testing
=======
Completed xfstests -g auto with default mkfs & mount opts.
No new failures except the known one without these patches.


Performance results
===================
Collected some performance numbers for DIO sync mixed random read/write
workload w.r.t number of threads (ext4) to check for scalability.
The performance historgram shown below is the percentage change in
performance by using this ilock patchset as compared to vanilla kernel.


FIO command:
fio -name=DIO-mixed-randrw -filename=./testfile -direct=1 -iodepth=1 -thread \
-rw=randrw -ioengine=psync -bs=$bs -size=10G -numjobs=$thread \
-group_reporting=1 -runtime=120

Used fioperf tool [5] for collecting this performance scores.

Below shows the performance benefit hist with this ilock patchset in (%)
w.r.t vanilla kernel for mixed randrw workload (for 4K block size).
Notice, the percentage benefit increases with increasing number of
threads. So this patchset help achieve good scalability in the mentioned
workload. Also this gives upto ~140% perf improvement in 24 threads mixed randrw
workload with 4K burst size.
The performance difference can be even higher with high speed storage
devices, since bw speeds without the patch seems to flatten due to lock
contention problem in case of multiple threads.
[Absolute perf delta can be seen at [6]]


		Performance benefit (%) data randrw (read)-4K
		    (default mount options)
  160 +-+------+-------+--------+--------+-------+--------+-------+------+-+
      |        +       +        +        +       +        +       +        |
  140 +-+ 							   **    +-+
      |                                                    **      **      |
  120 +-+                                         **       **      **    +-+
      |                                           **       **      **      |
  100 +-+                                **       **       **      **    +-+
      |                                  **       **       **      **      |
   80 +-+                                **       **       **      **    +-+
      |                                  **       **       **      **      |
      |                          **      **       **       **      **      |
   60 +-+                        **      **       **       **      **    +-+
      |                          **      **       **       **      **      |
   40 +-+                        **      **       **       **      **    +-+
      |                 **       **      **       **       **      **      |
   20 +-+               **       **      **       **       **      **    +-+
      |                 **       **      **       **       **      **      |
    0 +-+       **      **       **      **       **       **      **    +-+
      |        +       +        +        +       +        +       +        |
  -20 +-+------+-------+--------+--------+-------+--------+-------+------+-+
               1       2        4        8      12       16      24
	       		Threads


		Performance benefit (%) data randrw (write)-4K
		     (default mount options)
  160 +-+------+-------+--------+--------+-------+--------+-------+------+-+
      |        +       +        +        +       +        +       +        |
  140 +-+ 							   **    +-+
      |                                                    **      **      |
  120 +-+                                         **       **      **    +-+
      |                                           **       **      **      |
  100 +-+                                **       **       **      **    +-+
      |                                  **       **       **      **      |
   80 +-+                                **       **       **      **    +-+
      |                                  **       **       **      **      |
      |                          **      **       **       **      **      |
   60 +-+                        **      **       **       **      **    +-+
      |                          **      **       **       **      **      |
   40 +-+                        **      **       **       **      **    +-+
      |                 **       **      **       **       **      **      |
   20 +-+               **       **      **       **       **      **    +-+
      |                 **       **      **       **       **      **      |
    0 +-+       **      **       **      **       **       **      **    +-+
      |        +       +        +        +       +        +       +        |
  -20 +-+------+-------+--------+--------+-------+--------+-------+------+-+
               1       2        4        8      12       16      24
			Threads

Previous version
================
v4: https://marc.info/?l=linux-ext4&m=157552839624723&w=2
v3: https://www.spinics.net/lists/linux-ext4/msg68649.html
v2: https://www.spinics.net/lists/kernel/msg3262531.html
v1: https://patchwork.ozlabs.org/cover/1163286/

References
==========
[1]: https://lore.kernel.org/linux-ext4/[e-mail]/
[2]: https://lore.kernel.org/linux-ext4/[e-mail]/
[3]: https://raw.githubusercontent.com/riteshharjani/LinuxStudy/master/ext4/perf.report
[4]: https://patchwork.ozlabs.org/cover/1163286/
[5]: https://github.com/riteshharjani/fioperf
[6]: https://raw.githubusercontent.com/riteshharjani/LinuxStudy/master/ext4/diff_ilock_v3_default_dio_randrw_4K.txt
[7]: https://www.spinics.net/lists/linux-ext4/msg68659.html

-ritesh


Ritesh Harjani (3):
  ext4: fix ext4_dax_read/write inode locking sequence for IOCB_NOWAIT
  ext4: Start with shared i_rwsem in case of DIO instead of exclusive
  ext4: Move to shared i_rwsem even without dioread_nolock mount opt

 fs/ext4/file.c | 198 ++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 145 insertions(+), 53 deletions(-)

Comments

Theodore Ts'o Dec. 26, 2019, 3:06 p.m. UTC | #1

Thanks, I've applied this to the ext4 git tree.

	     	     	     	  - Ted