btrfs: stop sleeping for one jiffy in non-ssd mounts during log commit

Joining/starting a log transaction tracks if we ever had more than one task
concurrently logging by setting the flag BTRFS_ROOT_MULTI_LOG_TASKS in the
respective root. Once set, this flag remains for the rest of the lifetime
of the transaction, only cleared when we don't have a log root and need to
create a new one (transaction commits drop log roots).

During log commit, if we are not on a ssd mount (or use the -o nossd mount
option) and the BTRFS_ROOT_MULTI_LOG_TASKS flag is set, we sleep for one
jiffy with the excuse to allow future log writers to join and log inodes
and then commit a larger log transaction to reduce overall IO. However
this is extremely inefficient because:

1) If at some point we had multiple tasks logging concurrently but now
   we have only one task at a time, we force it to wait for 1 jiffy;

2) One jiffy can vary between 1ms to 10ms, depending on the kernel
   config option CONFIG_HZ, which by default has a value of 250HZ and
   that corresponds to 4ms - that is a lot.

This massively reduces the latency of fsyncs for non-ssd mounts, even
on consumer grade spinning disks.

Remove this mechanism to track if we have (or ever had) multiple tasks
logging and wait for 1 jiffy.

The following fio test was used to benchmark:

   $ cat fio-buffered-fsync.sh
   DEV=/dev/sdj
   MNT=/mnt/sdj
   MOUNT_OPTIONS=""
   MKFS_OPTIONS=""

   if [ $# -ne 6 ]; then
      echo "Use $0 NUM_JOBS FILE_SIZE IO_SIZE FSYNC_FREQ BLOCK_SIZE [write|randwrite]"
      exit 1
   fi

   NUM_JOBS=$1
   FILE_SIZE=$2
   IO_SIZE=$3
   FSYNC_FREQ=$4
   BLOCK_SIZE=$5
   WRITE_MODE=$6

   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=$BLOCK_SIZE
   ioengine=psync
   filesize=$FILE_SIZE
   io_size=$IO_SIZE
   directory=$MNT
   numjobs=$NUM_JOBS
   EOF

   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

Running the script as:  ./fio-buffered-fsync.sh 8 64M 64M 1 4K randwrite

Before patch:

  WRITE: bw=2647KiB/s (2711kB/s), 2647KiB/s-2647KiB/s (2711kB/s-2711kB/s), io=512MiB (537MB), run=198055-198055msec

After patch:

  WRITE: bw=14.9MiB/s (15.6MB/s), 14.9MiB/s-14.9MiB/s (15.6MB/s-15.6MB/s), io=512MiB (537MB), run=34471-34471msec

That's about 5.7 times faster.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

0 files changed, 0 insertions, 0 deletions