[PATCH v2 0/5] btrfs: improve stalls under sudden writeback

[Boris Burkov <boris@bur.io>]

If you have a system with very large memory (TiBs) and a normal
percentage based dirty_ratio/dirty_background_ratio like the defaults of
20%/10%, then we can theoretically rack up 100s of GiB of dirty pages
before doing any writeback. This is further exacerbated if we also see a
sudden drop in the free memory due to a large allocation. If we
(relatively likely for a large ram system) also have a large disk, we are
unlikely to do trigger much preemptive metadata reclaim either.

Once we do start doing writeback with such a large supply, the results
are somewhat ugly. The delalloc work generates a huge amount of delayed
refs without proper reservations which sends the metadata space system
into a tailspin trying to run yet more delalloc to free space.
Ultimately, the system stalls waiting for huge amounts of ordered
extents and delayed refs blocking all users in start_transaction() on
tickets in reserve_space().

This patch series aims to address these issues in a relatively targeted
way by improving our reservations for delalloc delayed refs and by doing
some very basic smoothing of the work in flush_space(). Further work
could be done to improve flush_space() heuristics and latency but this
is already a big help on my observed workloads.

I was able to reproduce stalls on a more "modest" system with 264GiB of
ram by using a somewhat silly 80% dirty_ratio.

I was unfortunately unable to reproduce any stalls on a yet smaller
system with only 32GiB of ram.

The first 3 patches do the delayed_ref rsv accounting on btrfs_inode,
mirroring inode->block_rsv.
The 4th patch is a cleanup to the types counting max extents
The 5th patch reduces the size of the unit of work in shrink_delalloc()
to further reduce stalls.
---
Changelog:
v2:
- patch 1 no longer embeds a new block_rsv on btrfs_inode for the
  delayed reservation. Instead it does the reservation on
  inode->block_rsv and migrates it to trans->delayed_rsv at the moment
  of truth.

Boris Burkov (5):
  btrfs: reserve space for delayed_refs in delalloc
  btrfs: account for csum delayed_refs in delalloc
  btrfs: account for compression in delalloc extent reservation
  btrfs: make inode->outstanding_extents a u64
  btrfs: cap shrink_delalloc iterations to 128M

 fs/btrfs/btrfs_inode.h       | 17 +++++--
 fs/btrfs/delalloc-space.c    | 78 +++++++++++++++++++++++-------
 fs/btrfs/delalloc-space.h    |  3 ++
 fs/btrfs/fs.h                | 13 -----
 fs/btrfs/inode.c             | 93 ++++++++++++++++++++++++++++--------
 fs/btrfs/ordered-data.c      |  4 +-
 fs/btrfs/space-info.c        | 31 ++++++++----
 fs/btrfs/tests/inode-tests.c | 18 +++----
 fs/btrfs/transaction.c       | 36 ++++++--------
 include/trace/events/btrfs.h |  8 ++--
 10 files changed, 201 insertions(+), 100 deletions(-)

-- 
2.53.0