Btrfs: improve jitter performance of the sequential buffered write
The performance was slowed down sometimes when we ran sysbench to measure the performance of the sequential buffered write by 2 or more threads. It was because the write order of the test threads might be confused by the task scheduler, and the coming write would be beyond the end of the file, in this case, we need insert dummy file extents and create a hole for the area we skip. But in order to avoid the ongoing ordered extents which are in the area, we need wait for them. Unfortunately, the current code doesn't check if there are ordered extents in the area or not, try to find and flush the dirty pages directly, but in fact, there is no dirty page in that area, this step of the current code is unnecessary, and just wastes time. Sometimes, it would increase the contention of some locks, and makes the performance slow down suddenly. So we remove the ordered extent flush function before the check, and flush the dirty pages and wait for the ordered extents only when we find them. According to my test, we got 1-2 times of the performance regression when we ran the test by 10 times before applying this patch. After applying this patch, the regression went away. Test Environment: CPU: 1CPU * 4Cores Memory: 6GB Partition: 20GB Test Command: # sysbench --test=fileio --file-total-size=16G --file-test-mode=seqwr \ > --num-threads=512 --file-block-size=16384 --max-time=60 --max-requests=0 run Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com> Signed-off-by: NJosef Bacik <jbacik@fusionio.com> Signed-off-by: NChris Mason <chris.mason@fusionio.com>
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