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Subject[PATCH v2 00/12] btrfs: add zstd compression level support
Hi everyone,

V2 had only a handful of changes outside of minor feedback.
- use functions over macros
- BTRFS_NR_WORKSPACE_MANAGERS is added instead of overriding
0011 (new):
- address monotonic memory requirement for zstd workspaces
- increase reclaim timer to 307s from 67s
- move from keeping track of time in ns to jiffies
- remove timer in cleanup code
- use min_t() instead of if statements in .set_level()
- add header text to describe how workspaces are managed
- nofs_flag type -> unsigned long to unsigned int

From v1:
This is a respin of [1] which aims to add zstd compression level
support. V3 moves away from the using set_level() to resize workspaces
in favor of just allocating a workspace of the appropriate level and
using a timer to reclaim unused workspaces.

Zstd compression requires different amounts of memory for each level of
compression. The prior patches implemented indirection to allow for each
compression type to manage their workspaces independently. This patch
uses this indirection to implement compression level support for zstd.

As mentioned above, a requirement that differs zstd from zlib is that
higher levels of compression require more memory. To manage this, each
compression level has its own queue of workspaces. A global LRU is used
to help with reclaim. To guarantee forward progress, a max level
workspace is preallocated and hidden from the LRU.

When getting a workspace, it uses a bitmap to identify the levels that
are populated and scans up. If it finds a workspace that is greater than
it, it uses it, but does not update the last_used time and the
corresponding place in the LRU. This provides a mechanism to decrease
memory utilization as we only keep around workspaces that are sized
appropriately for the in use compression levels.

By knowing which compression levels have available workspaces, we can
recycle rather than always create new workspaces as well as take
advantage of the preallocated max level for forward progress. If we hit
memory pressure, we sleep on the max level workspace. We continue to
rescan in case we can use a smaller workspace, but eventually should be
able to obtain the max level workspace or allocate one again should
memory pressure subside. The memory requirement for decompression is the
same as level 1, and therefore can use any of available workspace.

The number of workspaces is bound by an upper limit of the workqueue's
limit which currently is 2 (percpu) limit). Second, a reclaim timer is
used to free inactive/improperly sized workspaces. The reclaim timer is
set to 67s to avoid colliding with transaction commit (every 30s) and
attempts to reclaim any unused workspace older than 45s.

Repeating the experiment from v2 [1], the Silesia corpus was copied to a
btrfs filesystem 10 times and then read back after dropping the caches.
The btrfs filesystem was on an SSD.

Level Ratio Compression (MB/s) Decompression (MB/s)
1 2.658 438.47 910.51
2 2.744 364.86 886.55
3 2.801 336.33 828.41
4 2.858 286.71 886.55
5 2.916 212.77 556.84
6 2.363 119.82 990.85
7 3.000 154.06 849.30
8 3.011 159.54 875.03
9 3.025 100.51 940.15
10 3.033 118.97 616.26
11 3.036 94.19 802.11
12 3.037 73.45 931.49
13 3.041 55.17 835.26
14 3.087 44.70 716.78
15 3.126 37.30 878.84


This patchset contains the following 12 patches:

0001 adds helper functions for type_level conversion. 0002 renames
workspaces_list to workspace_manager. 0003 moves back to managing the
heuristic workspaces as the index 0 compression level. 0004-0007 unify
operations with the workspace_manager with 0007 moving to compression
types owning their workspace_manager. 0008-0010 plumbs level throughout
the compression level getting interface and converts set_level() to be a
bounding function rather than setting level on a workspace. 0011
makes zstd compression level memory monotonic. 00012 adds zstd
compression level support.

This patchset is on top of kdave#master d73aba1115cf.

diffstats below:

Dennis Zhou (12):
btrfs: add helpers for compression type and level
btrfs: rename workspaces_list to workspace_manager
btrfs: manage heuristic workspace as index 0
btrfs: unify compression ops with workspace_manager
btrfs: add helper methods for workspace manager init and cleanup
btrfs: add compression interface in (get/put)_workspace()
btrfs: move to fn pointers for get/put workspaces
btrfs: plumb level through the compression interface
btrfs: change set_level() to bound the level passed in
btrfs: zstd use the passed through level instead of default
btrfs: make zstd memory requirements monotonic
btrfs: add zstd compression level support

fs/btrfs/compression.c | 249 +++++++++++++++------------------
fs/btrfs/compression.h | 53 ++++++-
fs/btrfs/lzo.c | 31 +++-
fs/btrfs/super.c | 10 +-
fs/btrfs/zlib.c | 45 ++++--
fs/btrfs/zstd.c | 311 +++++++++++++++++++++++++++++++++++++++--
6 files changed, 539 insertions(+), 160 deletions(-)


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