When removing an extent map at try_release_extent_mapping(), called through
the page release callback (btrfs_releasepage()), we always set the full
sync flag on the inode, which forces the next fsync to use a slower code
path.
This hurts performance for workloads that dirty an amount of data that
exceeds or is very close to the system's RAM memory and do frequent fsync
operations (like database servers can for example). In particular if there
are concurrent fsyncs against different files, by falling back to a full
fsync we do a lot more checksum lookups in the checksums btree, as we do
it for all the extents created in the current transaction, instead of only
the new ones since the last fsync. These checksums lookups not only take
some time but, more importantly, they also cause contention on the
checksums btree locks due to the concurrency with checksum insertions in
the btree by ordered extents from other inodes.
We actually don't need to set the full sync flag on the inode, because we
only remove extent maps that are in the list of modified extents if they
were created in a past transaction, in which case an fsync skips them as
it's pointless to log them. So stop setting the full fsync flag on the
inode whenever we remove an extent map.
This patch is part of a patchset that consists of 3 patches, which have
the following subjects:
1/3 btrfs: fix race between page release and a fast fsync
2/3 btrfs: release old extent maps during page release
3/3 btrfs: do not set the full sync flag on the inode during page release
Performance tests were ran against a branch (misc-next) containing the
whole patchset. The test exercises a workload where there are multiple
processes writing to files and fsyncing them (each writing and fsyncing
its own file), and in total the amount of data dirtied ranges from 2x to
4x the system's RAM memory (16GiB), so that the page release callback is
invoked frequently.
The following script, using fio, was used to perform the tests:
$ cat test-fsync.sh
#!/bin/bash
DEV=/dev/sdk
MNT=/mnt/sdk
MOUNT_OPTIONS="-o ssd"
MKFS_OPTIONS="-d single -m single"
if [ $# -ne 3 ]; then
echo "Use $0 NUM_JOBS FILE_SIZE FSYNC_FREQ"
exit 1
fi
NUM_JOBS=$1
FILE_SIZE=$2
FSYNC_FREQ=$3
cat <<EOF > /tmp/fio-job.ini
[writers]
rw=write
fsync=$FSYNC_FREQ
fallocate=none
group_reporting=1
direct=0
bs=64k
ioengine=sync
size=$FILE_SIZE
directory=$MNT
numjobs=$NUM_JOBS
thread
EOF
echo "Using config:"
echo
cat /tmp/fio-job.ini
echo
mkfs.btrfs -f $MKFS_OPTIONS $DEV &> /dev/null
mount $MOUNT_OPTIONS $DEV $MNT
fio /tmp/fio-job.ini
umount $MNT
The tests were performed for different numbers of jobs, file sizes and
fsync frequency. A qemu VM using kvm was used, with 8 cores (the host has
12 cores, with cpu governance set to performance mode on all cores), 16GiB
of ram (the host has 64GiB) and using a NVMe device directly (without an
intermediary filesystem in the host). While running the tests, the host
was not used for anything else, to avoid disturbing the tests.
The obtained results were the following, and the last line printed by
fio is pasted (includes aggregated throughput and test run time).
*****************************************************
**** 1 job, 32GiB file, fsync frequency 1 ****
*****************************************************
Before patchset:
WRITE: bw=29.1MiB/s (30.5MB/s), 29.1MiB/s-29.1MiB/s (30.5MB/s-30.5MB/s), io=32.0GiB (34.4GB), run=1127557-1127557msec
After patchset:
WRITE: bw=29.3MiB/s (30.7MB/s), 29.3MiB/s-29.3MiB/s (30.7MB/s-30.7MB/s), io=32.0GiB (34.4GB), run=1119042-1119042msec
(+0.7% throughput, -0.8% run time)
*****************************************************
**** 2 jobs, 16GiB files, fsync frequency 1 ****
*****************************************************
Before patchset:
WRITE: bw=33.5MiB/s (35.1MB/s), 33.5MiB/s-33.5MiB/s (35.1MB/s-35.1MB/s), io=32.0GiB (34.4GB), run=979000-979000msec
After patchset:
WRITE: bw=39.9MiB/s (41.8MB/s), 39.9MiB/s-39.9MiB/s (41.8MB/s-41.8MB/s), io=32.0GiB (34.4GB), run=821283-821283msec
(+19.1% throughput, -16.1% runtime)
*****************************************************
**** 4 jobs, 8GiB files, fsync frequency 1 ****
*****************************************************
Before patchset:
WRITE: bw=52.1MiB/s (54.6MB/s), 52.1MiB/s-52.1MiB/s (54.6MB/s-54.6MB/s), io=32.0GiB (34.4GB), run=629130-629130msec
After patchset:
WRITE: bw=71.8MiB/s (75.3MB/s), 71.8MiB/s-71.8MiB/s (75.3MB/s-75.3MB/s), io=32.0GiB (34.4GB), run=456357-456357msec
(+37.8% throughput, -27.5% runtime)
*****************************************************
**** 8 jobs, 4GiB files, fsync frequency 1 ****
*****************************************************
Before patchset:
WRITE: bw=76.1MiB/s (79.8MB/s), 76.1MiB/s-76.1MiB/s (79.8MB/s-79.8MB/s), io=32.0GiB (34.4GB), run=430708-430708msec
After patchset:
WRITE: bw=133MiB/s (140MB/s), 133MiB/s-133MiB/s (140MB/s-140MB/s), io=32.0GiB (34.4GB), run=245458-245458msec
(+74.7% throughput, -43.0% run time)
*****************************************************
**** 16 jobs, 2GiB files, fsync frequency 1 ****
*****************************************************
Before patchset:
WRITE: bw=74.7MiB/s (78.3MB/s), 74.7MiB/s-74.7MiB/s (78.3MB/s-78.3MB/s), io=32.0GiB (34.4GB), run=438625-438625msec
After patchset:
WRITE: bw=184MiB/s (193MB/s), 184MiB/s-184MiB/s (193MB/s-193MB/s), io=32.0GiB (34.4GB), run=177864-177864msec
(+146.3% throughput, -59.5% run time)
*****************************************************
**** 32 jobs, 2GiB files, fsync frequency 1 ****
*****************************************************
Before patchset:
WRITE: bw=72.6MiB/s (76.1MB/s), 72.6MiB/s-72.6MiB/s (76.1MB/s-76.1MB/s), io=64.0GiB (68.7GB), run=902615-902615msec
After patchset:
WRITE: bw=227MiB/s (238MB/s), 227MiB/s-227MiB/s (238MB/s-238MB/s), io=64.0GiB (68.7GB), run=288936-288936msec
(+212.7% throughput, -68.0% run time)
*****************************************************
**** 64 jobs, 1GiB files, fsync frequency 1 ****
*****************************************************
Before patchset:
WRITE: bw=98.8MiB/s (104MB/s), 98.8MiB/s-98.8MiB/s (104MB/s-104MB/s), io=64.0GiB (68.7GB), run=663126-663126msec
After patchset:
WRITE: bw=294MiB/s (308MB/s), 294MiB/s-294MiB/s (308MB/s-308MB/s), io=64.0GiB (68.7GB), run=222940-222940msec
(+197.6% throughput, -66.4% run time)
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.
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