During a rename we pin the log to make sure no one commits a log that
reflects an ongoing rename operation, as it might result in a committed
log where it recorded the unlink of the old name without having recorded
the new name. However we are taking the subvolume's log_mutex before
incrementing the log_writers counter, which is not necessary since that
counter is atomic and we only remove the old name from the log and add
the new name to the log after we have incremented log_writers, ensuring
that no one can commit the log after we have removed the old name from
the log and before we added the new name to the log.
By taking the log_mutex lock we are just adding unnecessary contention on
the lock, which can become visible for workloads that mix renames with
fsyncs, writes for files opened with O_SYNC and unlink operations (if the
inode or its parent were fsynced before in the current transaction).
So just remove the lock and unlock of the subvolume's log_mutex at
btrfs_pin_log_trans().
Using dbench, which mixes different types of operations that end up taking
that mutex (fsyncs, renames, unlinks and writes into files opened with
O_SYNC) revealed some small gains. The following script that calls dbench
was used:
#!/bin/bash
DEV=/dev/nvme0n1
MNT=/mnt/btrfs
MOUNT_OPTIONS="-o ssd -o space_cache=v2"
MKFS_OPTIONS="-m single -d single"
THREADS=32
echo "performance" | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
mkfs.btrfs -f $MKFS_OPTIONS $DEV
mount $MOUNT_OPTIONS $DEV $MNT
dbench -s -t 600 -D $MNT $THREADS
umount $MNT
The test was run on bare metal, no virtualization, on a box with 12 cores
(Intel i7-8700), 64Gb of RAM and using a NVMe device, with a kernel
configuration that is the default of typical distributions (debian in this
case), without debug options enabled (kasan, kmemleak, slub debug, debug
of page allocations, lock debugging, etc).
Results before this patch:
Operation Count AvgLat MaxLat
----------------------------------------
NTCreateX 4410848 0.017 738.640
Close 3240222 0.001 0.834
Rename 186850 7.478 1272.476
Unlink 890875 0.128 785.018
Deltree 128 2.846 12.081
Mkdir 64 0.002 0.003
Qpathinfo 3997659 0.009 11.171
Qfileinfo 701307 0.001 0.478
Qfsinfo 733494 0.002 1.103
Sfileinfo 359362 0.004 3.266
Find 1546226 0.041 4.128
WriteX 2202803 7.905 1376.989
ReadX 6917775 0.003 3.887
LockX 14392 0.002 0.043
UnlockX 14392 0.001 0.085
Flush 309225 0.128 1033.936
Throughput 231.555 MB/sec (sync open) 32 clients 32 procs max_latency=1376.993 ms
Results after this patch:
Operation Count AvgLat MaxLat
----------------------------------------
NTCreateX 4603244 0.017 232.776
Close 3381299 0.001 1.041
Rename 194871 7.251 1073.165
Unlink 929730 0.133 119.233
Deltree 128 2.871 10.199
Mkdir 64 0.002 0.004
Qpathinfo 4171343 0.009 11.317
Qfileinfo 731227 0.001 1.635
Qfsinfo 765079 0.002 3.568
Sfileinfo 374881 0.004 1.220
Find 1612964 0.041 4.675
WriteX 2296720 7.569 1178.204
ReadX 7213633 0.003 3.075
LockX 14976 0.002 0.076
UnlockX 14976 0.001 0.061
Flush 322635 0.102 579.505
Throughput 241.4 MB/sec (sync open) 32 clients 32 procs max_latency=1178.207 ms
(+4.3% throughput, -14.4% max latency)
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
5522a27e59