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When logging a directory, after copying all directory index items from the
subvolume tree to the log tree, we iterate over the subvolume tree to find
all dir index items that are located in leaves COWed (or created) in the
current transaction. If we keep logging a directory several times during
the same transaction, we end up iterating over the same dir index items
everytime we log the directory, wasting time and adding extra lock
contention on the subvolume tree.
So just keep track of the last logged dir index offset in order to start
the search for that index (+1) the next time the directory is logged, as
dir index values (key offsets) come from a monotonically increasing
counter.
The following test measures the difference before and after this change:
$ cat test.sh
#!/bin/bash
DEV=/dev/nullb0
MNT=/mnt/nullb0
umount $DEV &> /dev/null
mkfs.btrfs -f $DEV
mount -o ssd $DEV $MNT
# Time values in milliseconds.
declare -a fsync_times
# Total number of files added to the test directory.
num_files=1000000
# Fsync directory after every N files are added.
fsync_period=100
mkdir $MNT/testdir
fsync_total_time=0
for ((i = 1; i <= $num_files; i++)); do
echo -n > $MNT/testdir/file_$i
if [ $((i % fsync_period)) -eq 0 ]; then
start=$(date +%s%N)
xfs_io -c "fsync" $MNT/testdir
end=$(date +%s%N)
fsync_total_time=$((fsync_total_time + (end - start)))
fsync_times[i]=$(( (end - start) / 1000000 ))
echo -n -e "Progress $i / $num_files\r"
fi
done
echo -e "\nHistogram of directory fsync duration in ms:\n"
printf '%s\n' "${fsync_times[@]}" | \
perl -MStatistics::Histogram -e '@d = <>; print get_histogram(\@d);'
fsync_total_time=$((fsync_total_time / 1000000))
echo -e "\nTotal time spent in fsync: $fsync_total_time ms\n"
echo
umount $MNT
The test was run on a non-debug kernel (Debian's default kernel config)
against a 15G null block device.
Result before this change:
Histogram of directory fsync duration in ms:
Count: 10000
Range: 3.000 - 362.000; Mean: 34.556; Median: 31.000; Stddev: 25.751
Percentiles: 90th: 71.000; 95th: 77.000; 99th: 81.000
3.000 - 5.278: 1423 #################################
5.278 - 8.854: 1173 ###########################
8.854 - 14.467: 591 ##############
14.467 - 23.277: 1025 #######################
23.277 - 37.105: 1422 #################################
37.105 - 58.809: 2036 ###############################################
58.809 - 92.876: 2316 #####################################################
92.876 - 146.346: 6 |
146.346 - 230.271: 6 |
230.271 - 362.000: 2 |
Total time spent in fsync: 350527 ms
Result after this change:
Histogram of directory fsync duration in ms:
Count: 10000
Range: 3.000 - 1088.000; Mean: 8.704; Median: 8.000; Stddev: 12.576
Percentiles: 90th: 12.000; 95th: 14.000; 99th: 17.000
3.000 - 6.007: 3222 #################################
6.007 - 11.276: 5197 #####################################################
11.276 - 20.506: 1551 ################
20.506 - 36.674: 24 |
36.674 - 201.552: 1 |
201.552 - 353.841: 4 |
353.841 - 1088.000: 1 |
Total time spent in fsync: 92114 ms
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.