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authorFilipe Manana <fdmanana@suse.com>2020-08-11 12:43:58 +0100
committerDavid Sterba <dsterba@suse.com>2020-10-07 12:06:56 +0200
commit487781796d302266aff993bee17d4e1ddd73445b (patch)
tree12a24df5b7ccabcaa7f92961817434c40a3802a5 /fs/btrfs/ordered-data.h
parent75b463d2b47aef96fe1dc3e0237629963034764b (diff)
btrfs: make fast fsyncs wait only for writeback
Currently regardless of a full or a fast fsync we always wait for ordered extents to complete, and then start logging the inode after that. However for fast fsyncs we can just wait for the writeback to complete, we don't need to wait for the ordered extents to complete since we use the list of modified extents maps to figure out which extents we must log and we can get their checksums directly from the ordered extents that are still in flight, otherwise look them up from the checksums tree. Until commit b5e6c3e170b770 ("btrfs: always wait on ordered extents at fsync time"), for fast fsyncs, we used to start logging without even waiting for the writeback to complete first, we would wait for it to complete after logging, while holding a transaction open, which lead to performance issues when using cgroups and probably for other cases too, as wait for IO while holding a transaction handle should be avoided as much as possible. After that, for fast fsyncs, we started to wait for ordered extents to complete before starting to log, which adds some latency to fsyncs and we even got at least one report about a performance drop which bisected to that particular change: https://lore.kernel.org/linux-btrfs/20181109215148.GF23260@techsingularity.net/ This change makes fast fsyncs only wait for writeback to finish before starting to log the inode, instead of waiting for both the writeback to finish and for the ordered extents to complete. This brings back part of the logic we had that extracts checksums from in flight ordered extents, which are not yet in the checksums tree, and making sure transaction commits wait for the completion of ordered extents previously logged (by far most of the time they have already completed by the time a transaction commit starts, resulting in no wait at all), to avoid any data loss if an ordered extent completes after the transaction used to log an inode is committed, followed by a power failure. When there are no other tasks accessing the checksums and the subvolume btrees, the ordered extent completion is pretty fast, typically taking 100 to 200 microseconds only in my observations. However when there are other tasks accessing these btrees, ordered extent completion can take a lot more time due to lock contention on nodes and leaves of these btrees. I've seen cases over 2 milliseconds, which starts to be significant. In particular when we do have concurrent fsyncs against different files there is a lot of contention on the checksums btree, since we have many tasks writing the checksums into the btree and other tasks that already started the logging phase are doing lookups for checksums in the btree. This change also turns all ranged fsyncs into full ranged fsyncs, which is something we already did when not using the NO_HOLES features or when doing a full fsync. This is to guarantee we never miss checksums due to writeback having been triggered only for a part of an extent, and we end up logging the full extent but only checksums for the written range, which results in missing checksums after log replay. Allowing ranged fsyncs to operate again only in the original range, when using the NO_HOLES feature and doing a fast fsync is doable but requires some non trivial changes to the writeback path, which can always be worked on later if needed, but I don't think they are a very common use case. Several tests were performed using fio for different numbers of concurrent jobs, each writing and fsyncing its own file, for both sequential and random file writes. The tests were run on bare metal, no virtualization, on a box with 12 cores (Intel i7-8700), 64Gb of RAM and 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). The following script that calls fio was used: $ cat test-fsync.sh #!/bin/bash DEV=/dev/nvme0n1 MNT=/mnt/btrfs MOUNT_OPTIONS="-o ssd -o space_cache=v2" MKFS_OPTIONS="-d single -m single" if [ $# -ne 5 ]; then echo "Use $0 NUM_JOBS FILE_SIZE FSYNC_FREQ BLOCK_SIZE [write|randwrite]" exit 1 fi NUM_JOBS=$1 FILE_SIZE=$2 FSYNC_FREQ=$3 BLOCK_SIZE=$4 WRITE_MODE=$5 if [ "$WRITE_MODE" != "write" ] && [ "$WRITE_MODE" != "randwrite" ]; then echo "Invalid WRITE_MODE, must be 'write' or 'randwrite'" exit 1 fi cat <<EOF > /tmp/fio-job.ini [writers] rw=$WRITE_MODE fsync=$FSYNC_FREQ fallocate=none group_reporting=1 direct=0 bs=$BLOCK_SIZE ioengine=sync size=$FILE_SIZE directory=$MNT numjobs=$NUM_JOBS EOF echo "performance" | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor echo echo "Using config:" echo cat /tmp/fio-job.ini echo umount $MNT &> /dev/null mkfs.btrfs -f $MKFS_OPTIONS $DEV mount $MOUNT_OPTIONS $DEV $MNT fio /tmp/fio-job.ini umount $MNT The results were the following: ************************* *** sequential writes *** ************************* ==== 1 job, 8GiB file, fsync frequency 1, block size 64KiB ==== Before patch: WRITE: bw=36.6MiB/s (38.4MB/s), 36.6MiB/s-36.6MiB/s (38.4MB/s-38.4MB/s), io=8192MiB (8590MB), run=223689-223689msec After patch: WRITE: bw=40.2MiB/s (42.1MB/s), 40.2MiB/s-40.2MiB/s (42.1MB/s-42.1MB/s), io=8192MiB (8590MB), run=203980-203980msec (+9.8%, -8.8% runtime) ==== 2 jobs, 4GiB files, fsync frequency 1, block size 64KiB ==== Before patch: WRITE: bw=35.8MiB/s (37.5MB/s), 35.8MiB/s-35.8MiB/s (37.5MB/s-37.5MB/s), io=8192MiB (8590MB), run=228950-228950msec After patch: WRITE: bw=43.5MiB/s (45.6MB/s), 43.5MiB/s-43.5MiB/s (45.6MB/s-45.6MB/s), io=8192MiB (8590MB), run=188272-188272msec (+21.5% throughput, -17.8% runtime) ==== 4 jobs, 2GiB files, fsync frequency 1, block size 64KiB ==== Before patch: WRITE: bw=50.1MiB/s (52.6MB/s), 50.1MiB/s-50.1MiB/s (52.6MB/s-52.6MB/s), io=8192MiB (8590MB), run=163446-163446msec After patch: WRITE: bw=64.5MiB/s (67.6MB/s), 64.5MiB/s-64.5MiB/s (67.6MB/s-67.6MB/s), io=8192MiB (8590MB), run=126987-126987msec (+28.7% throughput, -22.3% runtime) ==== 8 jobs, 1GiB files, fsync frequency 1, block size 64KiB ==== Before patch: WRITE: bw=64.0MiB/s (68.1MB/s), 64.0MiB/s-64.0MiB/s (68.1MB/s-68.1MB/s), io=8192MiB (8590MB), run=126075-126075msec After patch: WRITE: bw=86.8MiB/s (91.0MB/s), 86.8MiB/s-86.8MiB/s (91.0MB/s-91.0MB/s), io=8192MiB (8590MB), run=94358-94358msec (+35.6% throughput, -25.2% runtime) ==== 16 jobs, 512MiB files, fsync frequency 1, block size 64KiB ==== Before patch: WRITE: bw=79.8MiB/s (83.6MB/s), 79.8MiB/s-79.8MiB/s (83.6MB/s-83.6MB/s), io=8192MiB (8590MB), run=102694-102694msec After patch: WRITE: bw=107MiB/s (112MB/s), 107MiB/s-107MiB/s (112MB/s-112MB/s), io=8192MiB (8590MB), run=76446-76446msec (+34.1% throughput, -25.6% runtime) ==== 32 jobs, 512MiB files, fsync frequency 1, block size 64KiB ==== Before patch: WRITE: bw=93.2MiB/s (97.7MB/s), 93.2MiB/s-93.2MiB/s (97.7MB/s-97.7MB/s), io=16.0GiB (17.2GB), run=175836-175836msec After patch: WRITE: bw=111MiB/s (117MB/s), 111MiB/s-111MiB/s (117MB/s-117MB/s), io=16.0GiB (17.2GB), run=147001-147001msec (+19.1% throughput, -16.4% runtime) ==== 64 jobs, 512MiB files, fsync frequency 1, block size 64KiB ==== Before patch: WRITE: bw=108MiB/s (114MB/s), 108MiB/s-108MiB/s (114MB/s-114MB/s), io=32.0GiB (34.4GB), run=302656-302656msec After patch: WRITE: bw=133MiB/s (140MB/s), 133MiB/s-133MiB/s (140MB/s-140MB/s), io=32.0GiB (34.4GB), run=246003-246003msec (+23.1% throughput, -18.7% runtime) ************************ *** random writes *** ************************ ==== 1 job, 8GiB file, fsync frequency 16, block size 4KiB ==== Before patch: WRITE: bw=11.5MiB/s (12.0MB/s), 11.5MiB/s-11.5MiB/s (12.0MB/s-12.0MB/s), io=8192MiB (8590MB), run=714281-714281msec After patch: WRITE: bw=11.6MiB/s (12.2MB/s), 11.6MiB/s-11.6MiB/s (12.2MB/s-12.2MB/s), io=8192MiB (8590MB), run=705959-705959msec (+0.9% throughput, -1.7% runtime) ==== 2 jobs, 4GiB files, fsync frequency 16, block size 4KiB ==== Before patch: WRITE: bw=12.8MiB/s (13.5MB/s), 12.8MiB/s-12.8MiB/s (13.5MB/s-13.5MB/s), io=8192MiB (8590MB), run=638101-638101msec After patch: WRITE: bw=13.1MiB/s (13.7MB/s), 13.1MiB/s-13.1MiB/s (13.7MB/s-13.7MB/s), io=8192MiB (8590MB), run=625374-625374msec (+2.3% throughput, -2.0% runtime) ==== 4 jobs, 2GiB files, fsync frequency 16, block size 4KiB ==== Before patch: WRITE: bw=15.4MiB/s (16.2MB/s), 15.4MiB/s-15.4MiB/s (16.2MB/s-16.2MB/s), io=8192MiB (8590MB), run=531146-531146msec After patch: WRITE: bw=17.8MiB/s (18.7MB/s), 17.8MiB/s-17.8MiB/s (18.7MB/s-18.7MB/s), io=8192MiB (8590MB), run=460431-460431msec (+15.6% throughput, -13.3% runtime) ==== 8 jobs, 1GiB files, fsync frequency 16, block size 4KiB ==== Before patch: WRITE: bw=19.9MiB/s (20.8MB/s), 19.9MiB/s-19.9MiB/s (20.8MB/s-20.8MB/s), io=8192MiB (8590MB), run=412664-412664msec After patch: WRITE: bw=22.2MiB/s (23.3MB/s), 22.2MiB/s-22.2MiB/s (23.3MB/s-23.3MB/s), io=8192MiB (8590MB), run=368589-368589msec (+11.6% throughput, -10.7% runtime) ==== 16 jobs, 512MiB files, fsync frequency 16, block size 4KiB ==== Before patch: WRITE: bw=29.3MiB/s (30.7MB/s), 29.3MiB/s-29.3MiB/s (30.7MB/s-30.7MB/s), io=8192MiB (8590MB), run=279924-279924msec After patch: WRITE: bw=30.4MiB/s (31.9MB/s), 30.4MiB/s-30.4MiB/s (31.9MB/s-31.9MB/s), io=8192MiB (8590MB), run=269258-269258msec (+3.8% throughput, -3.8% runtime) ==== 32 jobs, 512MiB files, fsync frequency 16, block size 4KiB ==== Before patch: WRITE: bw=36.9MiB/s (38.7MB/s), 36.9MiB/s-36.9MiB/s (38.7MB/s-38.7MB/s), io=16.0GiB (17.2GB), run=443581-443581msec After patch: WRITE: bw=41.6MiB/s (43.6MB/s), 41.6MiB/s-41.6MiB/s (43.6MB/s-43.6MB/s), io=16.0GiB (17.2GB), run=394114-394114msec (+12.7% throughput, -11.2% runtime) ==== 64 jobs, 512MiB files, fsync frequency 16, block size 4KiB ==== Before patch: WRITE: bw=45.9MiB/s (48.1MB/s), 45.9MiB/s-45.9MiB/s (48.1MB/s-48.1MB/s), io=32.0GiB (34.4GB), run=714614-714614msec After patch: WRITE: bw=48.8MiB/s (51.1MB/s), 48.8MiB/s-48.8MiB/s (51.1MB/s-51.1MB/s), io=32.0GiB (34.4GB), run=672087-672087msec (+6.3% throughput, -6.0% runtime) Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
Diffstat (limited to 'fs/btrfs/ordered-data.h')
-rw-r--r--fs/btrfs/ordered-data.h11
1 files changed, 11 insertions, 0 deletions
diff --git a/fs/btrfs/ordered-data.h b/fs/btrfs/ordered-data.h
index d61ea9c880a3..644258a7dfb1 100644
--- a/fs/btrfs/ordered-data.h
+++ b/fs/btrfs/ordered-data.h
@@ -56,6 +56,12 @@ enum {
BTRFS_ORDERED_TRUNCATED,
/* Regular IO for COW */
BTRFS_ORDERED_REGULAR,
+ /* Used during fsync to track already logged extents */
+ BTRFS_ORDERED_LOGGED,
+ /* We have already logged all the csums of the ordered extent */
+ BTRFS_ORDERED_LOGGED_CSUM,
+ /* We wait for this extent to complete in the current transaction */
+ BTRFS_ORDERED_PENDING,
};
struct btrfs_ordered_extent {
@@ -104,6 +110,9 @@ struct btrfs_ordered_extent {
/* list of checksums for insertion when the extent io is done */
struct list_head list;
+ /* used for fast fsyncs */
+ struct list_head log_list;
+
/* used to wait for the BTRFS_ORDERED_COMPLETE bit */
wait_queue_head_t wait;
@@ -174,6 +183,8 @@ struct btrfs_ordered_extent *btrfs_lookup_ordered_range(
struct btrfs_inode *inode,
u64 file_offset,
u64 len);
+void btrfs_get_ordered_extents_for_logging(struct btrfs_inode *inode,
+ struct list_head *list);
int btrfs_find_ordered_sum(struct inode *inode, u64 offset, u64 disk_bytenr,
u8 *sum, int len);
u64 btrfs_wait_ordered_extents(struct btrfs_root *root, u64 nr,