[PATCH 0/8] Cleanup and improve sync (v4)

[Jan Kara <jack@suse.cz>]

  Hello,

  this is a fourth iteration of my series improving handling of sync syscall.
Since previous submission I have slightly improved cleaned up iteration loops
so that we don't have to pass void * around. Christoph also asked about why
we do non-blocking ->sync_fs() pass. My answer to it was:

I did also measurements where non-blocking ->sync_fs was removed and I didn't
see any regression with ext3, ext4, xfs, or btrfs. OTOH I can imagine *some*
filesystem can do an equivalent of filemap_fdatawrite() on some metadata for
non-blocking ->sync_fs and filemap_fdatawrite_and_wait() on the blocking one
and if there are more such filesystems on different backing storages the
performance difference can be noticeable (actually, checking the filesystems,
JFS and Ceph seem to be doing something like this). So I that's why I didn't
include the change in the end...

So Christoph, if you think we should get rid of non-blocking ->sync_fs, I can
include the patch but personally I think it has some use. Arguably a cleaner
interface for the users will be something like two methods ->sync_fs_begin
and ->sync_fs_end. Filesystems that don't have much to optimize in ->sync_fs()
would just use one of these functions.

I have run three tests below to verify performance impact of the patch series.
Each test has been run with 1, 2, and 4 filesystems mounted; test with 2
filesystems was run with each filesystem on a different disk, test with 4
filesystems had 2 filesystems on the first disk and 2 filesystems on the second
disk.

Test 1: Run 200 times sync with filesystem mounted to verify overhead of
  sync when there are no data to write.
Test 2: For each filesystem run a process creating 40 KB files, sleep
  for 3 seconds, run sync.
Test 3: For each filesystem run a process creating 20 GB file, sleep for
  5 seconds, run sync.

I have performed 10 runs of each test for xfs, ext3, ext4, and btrfs
filesystems.

Results of test 1
-----------------
Numbers are time it took 200 syncs to complete.
Character in braces is + if the time increased with 2*STDDEV reliability,
- if it decreased with 2*STDDEV reliability, 0 otherwise.
		      BASE		      PATCHED
FS		AVG      STDDEV         AVG      STDDEV
xfs, 1 disks	4.189300 0.051525	2.141300 0.063389 (-)
xfs, 2 disks	4.820600 0.019096	4.611400 0.066322 (-)
xfs, 4 disks	6.518300 1.440362	6.435700 0.510641 (0)
ext4, 1 disks	4.085000 0.011375	1.689500 0.001360 (-)
ext4, 2 disks	4.088100 0.006488	1.705000 0.026359 (-)
ext4, 4 disks	4.107300 0.011934	1.702900 0.001814 (-)
ext3, 1 disks	4.080200 0.009527	1.703400 0.030559 (-)
ext3, 2 disks	4.138300 0.143909	1.694000 0.001414 (-)
ext3, 4 disks	4.107200 0.002482	1.702900 0.007778 (-)
btrfs, 1 disks	11.214600 0.086619	8.737200 0.081076 (-)
btrfs, 2 disks	32.910000 0.162089	30.673400 0.538820 (-)
btrfs, 4 disks	67.987700 1.655654	67.247100 1.971887 (0)

So we see nice improvements almost all over the board.

Results of test 2
-----------------
Numbers are time it took sync to complete.

		    BASE		    PATCHED
FS		AVG      STDDEV         AVG      STDDEV
xfs, 1 disks	0.436000 0.012000	0.506000 0.014283 (+)
xfs, 2 disks	1.105000 0.055543	1.274000 0.244426 (0)
xfs, 4 disks	5.880000 2.997135	4.837000 3.875448 (0)
ext4, 1 disks	0.791000 0.055579	0.853000 0.042438 (0)
ext4, 2 disks	18.232000 13.505638	17.254000 2.000506 (0)
ext4, 4 disks	491.790000 218.565229	696.783000 234.933562 (0)
ext3, 1 disks	15.315000 2.065465	1.900000 0.184662 (-)
ext3, 2 disks	128.524000 18.090519	55.278000 1.530554 (-)
ext3, 4 disks	221.202000 30.090432	232.849000 68.745423 (0)
btrfs, 1 disks	0.452000 0.026000	0.494000 0.023749 (0)
btrfs, 2 disks	5.156000 4.530852	4.083000 1.560519 (0)
btrfs, 4 disks	31.154000 11.220828	36.987000 17.334126 (0)

Except for ext3 which got a nice boost here and XFS which seems to be a tad bit
slower, there are no changes that would stand out of the noise.

Results of test 3
-----------------
Numbers are time it took sync to complete.

		    BASE		    PATCHED
FS		AVG      STDDEV         AVG      STDDEV
xfs, 1 disks	12.083000 0.058660	10.898000 0.285475 (-)
xfs, 2 disks	20.182000 0.549614	14.977000 0.351114 (-)
xfs, 4 disks	35.814000 5.318310	28.452000 3.332281 (0)
ext4, 1 disks	32.956000 5.753789	20.865000 3.892098 (0)
ext4, 2 disks	34.922000 3.051966	27.411000 2.752978 (0)
ext4, 4 disks	44.508000 6.829004	28.360000 2.561437 (0)
ext3, 1 disks	23.475000 1.288885	17.116000 0.319631 (-)
ext3, 2 disks	43.508000 4.998647	41.547000 2.597976 (0)
ext3, 4 disks	92.130000 11.344117	79.362000 9.891208 (0)
btrfs, 1 disks	12.478000 0.394304	12.847000 0.171117 (0)
btrfs, 2 disks	15.030000 0.777817	18.014000 2.011418 (0)
btrfs, 4 disks	32.395000 4.248859	38.411000 3.179939 (0)

Here we see XFS and ext3 had some improvements, ext4 likely as well although
the results are relatively noisy.

								Honza