Every time we log an inode we lookup in the fs/subvol tree for xattrs and
if we have any, log them into the log tree. However it is very common to
have inodes without any xattrs, so doing the search wastes times, but more
importantly it adds contention on the fs/subvol tree locks, either making
the logging code block and wait for tree locks or making the logging code
making other concurrent operations block and wait.
The most typical use cases where xattrs are used are when capabilities or
ACLs are defined for an inode, or when SELinux is enabled.
This change makes the logging code detect when an inode does not have
xattrs and skip the xattrs search the next time the inode is logged,
unless the inode is evicted and loaded again or a xattr is added to the
inode. Therefore skipping the search for xattrs on inodes that don't ever
have xattrs and are fsynced with some frequency.
The following script that calls dbench was used to measure the impact of
this change on a VM with 8 CPUs, 16Gb of ram, using a raw NVMe device
directly (no intermediary filesystem on the host) and using a non-debug
kernel (default configuration on Debian distributions):
$ cat test.sh
#!/bin/bash
DEV=/dev/sdk
MNT=/mnt/sdk
MOUNT_OPTIONS="-o ssd"
mkfs.btrfs -f -m single -d single $DEV
mount $MOUNT_OPTIONS $DEV $MNT
dbench -D $MNT -t 200 40
umount $MNT
The results before this change:
Operation Count AvgLat MaxLat
----------------------------------------
NTCreateX 5761605 0.172 312.057
Close 4232452 0.002 10.927
Rename 243937 1.406 277.344
Unlink 1163456 0.631 298.402
Deltree 160 11.581 221.107
Mkdir 80 0.003 0.005
Qpathinfo 5221410 0.065 122.309
Qfileinfo 915432 0.001 3.333
Qfsinfo 957555 0.003 3.992
Sfileinfo 469244 0.023 20.494
Find 2018865 0.448 123.659
WriteX 2874851 0.049 118.529
ReadX 9030579 0.004 21.654
LockX 18754 0.003 4.423
UnlockX 18754 0.002 0.331
Flush 403792 10.944 359.494
Throughput 908.444 MB/sec 40 clients 40 procs max_latency=359.500 ms
The results after this change:
Operation Count AvgLat MaxLat
----------------------------------------
NTCreateX 6442521 0.159 230.693
Close 4732357 0.002 10.972
Rename 272809 1.293 227.398
Unlink 1301059 0.563 218.500
Deltree 160 7.796 54.887
Mkdir 80 0.008 0.478
Qpathinfo 5839452 0.047 124.330
Qfileinfo 1023199 0.001 4.996
Qfsinfo 1070760 0.003 5.709
Sfileinfo 524790 0.033 21.765
Find 2257658 0.314 125.611
WriteX 3211520 0.040 232.135
ReadX 10098969 0.004 25.340
LockX 20974 0.003 1.569
UnlockX 20974 0.002 3.475
Flush 451553 10.287 331.037
Throughput 1011.77 MB/sec 40 clients 40 procs max_latency=331.045 ms
+10.8% throughput, -8.2% max latency
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
f2f121ab50