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Date:   Thu, 20 Aug 2020 17:06:27 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     linux-ext4@...r.kernel.org
Cc:     jack@...e.cz, tytso@....edu, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Ritesh Harjani <riteshh@...ux.ibm.com>
Subject: [RFC 0/1] Optimize ext4 DAX overwrites

In case of dax writes, currently we start a journal txn irrespective of whether
it's an overwrite or not. In case of an overwrite we don't need to start a
jbd2 txn since the blocks are already allocated.
So this patch optimizes away the txn start in case of DAX overwrites.
This could significantly boost performance for multi-threaded random write
(overwrite). Fio script used to collect perf numbers is mentioned below.

Below numbers were calculated on a QEMU setup on ppc64 box with simulated
pmem device.

Performance numbers with different threads - (~10x improvement)
==========================================

vanilla_kernel(kIOPS)
 60 +-+---------------+-------+--------+--------+--------+-------+------+-+   
     |                 +       +        +        +**      +       +        |   
  55 +-+                                          **                     +-+   
     |                                   **       **                       |   
     |                                   **       **                       |   
  50 +-+                                 **       **                     +-+   
     |                                   **       **                       |   
  45 +-+                                 **       **                     +-+   
     |                                   **       **                       |   
     |                                   **       **                       |   
  40 +-+                                 **       **                     +-+   
     |                                   **       **                       |   
  35 +-+                        **       **       **                     +-+   
     |                          **       **       **               **      |   
     |                          **       **       **      **       **      |   
  30 +-+               **       **       **       **      **       **    +-+   
     |                 **      +**      +**      +**      **      +**      |   
  25 +-+---------------**------+**------+**------+**------**------+**----+-+   
                       1       2        4        8       12      16            
                                     Threads                                   
patched_kernel(kIOPS)
  600 +-+--------------+--------+--------+-------+--------+-------+------+-+   
      |                +        +        +       +        +       +**      |   
      |                                                            **      |   
  500 +-+                                                          **    +-+   
      |                                                            **      |   
      |                                                    **      **      |   
  400 +-+                                                  **      **    +-+   
      |                                                    **      **      |   
  300 +-+                                         **       **      **    +-+   
      |                                           **       **      **      |   
      |                                           **       **      **      |   
  200 +-+                                         **       **      **    +-+   
      |                                  **       **       **      **      |   
      |                                  **       **       **      **      |   
  100 +-+                        **      **       **       **      **    +-+   
      |                          **      **       **       **      **      |   
      |                +**      +**      **      +**      +**     +**      |   
    0 +-+--------------+**------+**------**------+**------+**-----+**----+-+   
                       1        2        4       8       12      16            
                                     Threads                                   
fio script
==========
[global]
rw=randwrite
norandommap=1
invalidate=0
bs=4k
numjobs=16 		--> changed this for different thread options
time_based=1
ramp_time=30
runtime=60
group_reporting=1
ioengine=psync
direct=1
size=16G
filename=file1.0.0:file1.0.1:file1.0.2:file1.0.3:file1.0.4:file1.0.5:file1.0.6:file1.0.7:file1.0.8:file1.0.9:file1.0.10:file1.0.11:file1.0.12:file1.0.13:file1.0.14:file1.0.15:file1.0.16:file1.0.17:file1.0.18:file1.0.19:file1.0.20:file1.0.21:file1.0.22:file1.0.23:file1.0.24:file1.0.25:file1.0.26:file1.0.27:file1.0.28:file1.0.29:file1.0.30:file1.0.31
file_service_type=random
nrfiles=32
directory=/mnt/

[name]
directory=/mnt/
direct=1

NOTE:
======
1. Looking at ~10x perf delta, I probed a bit deeper to understand what's causing
this scalability problem. It seems when we are starting a jbd2 txn then slab
alloc code is observing some serious contention around spinlock.

Even though the spinlock contention could be related to some other
issue (looking into it internally). But I could still see the perf improvement
of close to ~2x on QEMU setup on x86 with simulated pmem device with the
patched_kernel v/s vanilla_kernel with same fio workload.

perf report from vanilla_kernel (this is not seen with patched kernel) (ppc64)
=======================================================================

  47.86%  fio              [kernel.vmlinux]            [k] do_raw_spin_lock
             |
             ---do_raw_spin_lock
                |
                |--19.43%--_raw_spin_lock
                |          |
                |           --19.31%--0
                |                     |
                |                     |--9.77%--deactivate_slab.isra.61
                |                     |          ___slab_alloc
                |                     |          __slab_alloc
                |                     |          kmem_cache_alloc
                |                     |          jbd2__journal_start
                |                     |          __ext4_journal_start_sb
<...>

2. Kept this as RFC, since maybe using the ext4_iomap_overwrite_ops,
will be better here. We could check for overwrite in ext4_dax_write_iter(),
like how we do for DIO writes. Thoughts?

3. This problem was reported by Dan Williams at [1]

Links
======
[1]: https://lore.kernel.org/linux-ext4/20190802144304.GP25064@quack2.suse.cz/T/

Ritesh Harjani (1):
  ext4: Optimize ext4 DAX overwrites

 fs/ext4/ext4.h  | 1 +
 fs/ext4/file.c  | 2 +-
 fs/ext4/inode.c | 8 +++++++-
 3 files changed, 9 insertions(+), 2 deletions(-)

-- 
2.25.4

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