[PATCH v4 0/2] btrfs: go extent-by-extent submission for buffered reads and writes

[Qu Wenruo <wqu@suse.com>]

[CHANGELOG]
v4:
- Make the writeback_bio_size limit check more robust
  Only clamp @cur_len and do round_up() when the writeback_bio_size is
  larger than the current bio size.

  This will handle unaligned writeback_bio_size more robustly.

v3:
- Enhance the writeback_bio_size limit check
  Now we limit the writeback size early.

  This will follow the writeback_bio_size better.

- Do a better microbenchmark for the writeback patch
  It turns out that writeback throttle is making submit_bio() sleep,
  masking the improvement in extent_writepage_io().

  With wbt_late_nsec set to 0, now the improvement is way more obvious.
  Now it's over 90% reduce in average runtime, other than no improvement
  in the average runtime.

v2:
- Fix the length of advancement when no OE is found
  We should still retry the next block, as there may be only a block of
  gap.

  Exposed by Sashiko on the 2nd patch.

  Although it also exposed a false alert on the
  truncate_ordered_extents_beyond_eof().
  Where all the blocks in the range should have an OE, or we're having
  a bigger problem.

Although we have large data folio support for a while, the buffered
reads and writes are still iterating a large folio block-by-block.

For a large buffered IO, the large folio has a very high chance to
contain only one single extent.

In that case, although doing block-by-block checks is good for
readability, it's not really performant.

The series changes the behavior to do extent-by-extent iteration
instead, this can bring a very slight performance improve.

For best case scenario, the runtime to submit a folio read can be
reduced from 32us to 2.5us, and a much better distribution.
The runtime to submit a folio write can be reduced from 177us to 9us.

Qu Wenruo (2):
  btrfs: read a folio extent-by-extent instead of block-by-block
  btrfs: write back a folio extent-by-extent instead of block-by-block

 fs/btrfs/extent_io.c | 323 ++++++++++++++++++++++++++-----------------
 fs/btrfs/subpage.c   |  49 +++++++
 fs/btrfs/subpage.h   |   4 +
 3 files changed, 247 insertions(+), 129 deletions(-)

-- 
2.55.0