[PATCHv3 0/1] Optimize ext4 file overwrites - perf improvement

[Ritesh Harjani]

Hello,

v2 -> v3
1. Switched to suggested approach from Jan to make the approach general
for all file writes rather than only for DAX.
(So as of now both DAX & DIO should benefit from this as both uses the same
iomap path. Although note that I only tested performance improvement for DAX)

Gave a run on xfstests with -g quick,dax and didn't observe any new
issues with this patch.

In case of file 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 file (DAX/DIO) overwrites.
This could significantly boost performance for multi-threaded writes
specially random writes (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 (fsdax) device. 

Didn't observe any new failures with this patch in xfstests "-g quick,dax"

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

vanilla_kernel(kIOPS) (randomwrite)
 60 +-+------+-------+--------+--------+--------+-------+------+-+   
     |        +       +        +        +**      +       +        |   
  55 +-+                                 **                     +-+   
     |                          **       **                       |   
     |                          **       **                       |   
  50 +-+                        **       **                     +-+   
     |                          **       **                       |   
  45 +-+                        **       **                     +-+   
     |                          **       **                       |   
     |                          **       **                       |   
  40 +-+                        **       **                     +-+   
     |                          **       **                       |   
  35 +-+               **       **       **                     +-+   
     |                 **       **       **               **      |   
     |                 **       **       **      **       **      |   
  30 +-+      **       **       **       **      **       **    +-+   
     |        **      +**      +**      +**      **      +**      |   
  25 +-+------**------+**------+**------+**------**------+**----+-+   
              1       2        4        8       12      16            
                                     Threads                                   
patched_kernel(kIOPS) (randomwrite)
  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.

I think that the spinlock contention problem in slab alloc path could be optimized
on PPC in general, will look into it seperately. 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. This problem was reported by Dan Williams at [1]

Links
======
[1]: https://lore.kernel.org/linux-ext4/20190802144304.GP25064@xxxxxxxxxxxxxx/T/
[v2]: https://lkml.org/lkml/2020/8/22/123

Ritesh Harjani (1):
  ext4: Optimize file overwrites

 fs/ext4/inode.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

-- 
2.26.2