[PATCH 00/13] Parallelizing filesystem writeback

Kundan Kumar
2025-05-29T11:39:49Z

Currently, pagecache writeback is performed by a single thread. Inodes
are added to a dirty list, and delayed writeback is triggered. The single
writeback thread then iterates through the dirty inode list, and executes
the writeback.

This series parallelizes the writeback by allowing multiple writeback
contexts per backing device (bdi). These writebacks contexts are executed
as separate, independent threads, improving overall parallelism.

Would love to hear feedback in-order to move this effort forward.

Design Overview
================
Following Jan Kara's suggestion [1], we have introduced a new bdi
writeback context within the backing_dev_info structure. Specifically,
we have created a new structure, bdi_writeback_context, which contains
its own set of members for each writeback context.

struct bdi_writeback_ctx {
struct bdi_writeback wb;
struct list_head wb_list; /* list of all wbs */
struct radix_tree_root cgwb_tree;
struct rw_semaphore wb_switch_rwsem;
wait_queue_head_t wb_waitq;
};

There can be multiple writeback contexts in a bdi, which helps in
achieving writeback parallelism.

struct backing_dev_info {
...
int nr_wb_ctx;
struct bdi_writeback_ctx **wb_ctx_arr;
...
};

FS geometry and filesystem fragmentation
========================================
The community was concerned that parallelizing writeback would impact
delayed allocation and increase filesystem fragmentation.
Our analysis of XFS delayed allocation behavior showed that merging of
extents occurs within a specific inode. Earlier experiments with multiple
writeback contexts [2] resulted in increased fragmentation due to the
same inode being processed by different threads.

To address this, we now affine an inode to a specific writeback context
ensuring that delayed allocation works effectively.

Number of writeback contexts
===========================
The plan is to keep the nr_wb_ctx as 1, ensuring default single threaded
behavior. However, we set the number of writeback contexts equal to
number of CPUs in the current version. Later we will make it configurable
using a mount option, allowing filesystems to choose the optimal number
of writeback contexts.

IOPS and throughput
===================
We see significant improvement in IOPS across several filesystem on both
PMEM and NVMe devices.

Performance gains:
- On PMEM:
Base XFS : 544 MiB/s
Parallel Writeback XFS : 1015 MiB/s (+86%)
Base EXT4 : 536 MiB/s
Parallel Writeback EXT4 : 1047 MiB/s (+95%)

- On NVMe:
Base XFS : 651 MiB/s
Parallel Writeback XFS : 808 MiB/s (+24%)
Base EXT4 : 494 MiB/s
Parallel Writeback EXT4 : 797 MiB/s (+61%)

We also see that there is no increase in filesystem fragmentation
# of extents:
- On XFS (on PMEM):
Base XFS : 1964
Parallel Writeback XFS : 1384

- On EXT4 (on PMEM):
Base EXT4 : 21
Parallel Writeback EXT4 : 11

[1] Jan Kara suggestion :
https://lore.kernel.org/all/gamxtewl5yzg4xwu7lpp7obhp44xh344swvvf7tmbiknvbd3ww@jowphz4h4zmb/
[2] Writeback using unaffined N (# of CPUs) threads :
https://lore.kernel.org/all/20250414102824.9901-1-kundan.kumar@samsung.com/

Kundan Kumar (13):
writeback: add infra for parallel writeback
writeback: add support to initialize and free multiple writeback ctxs
writeback: link bdi_writeback to its corresponding bdi_writeback_ctx
writeback: affine inode to a writeback ctx within a bdi
writeback: modify bdi_writeback search logic to search across all wb
ctxs
writeback: invoke all writeback contexts for flusher and dirtytime
writeback
writeback: modify sync related functions to iterate over all writeback
contexts
writeback: add support to collect stats for all writeback ctxs
f2fs: add support in f2fs to handle multiple writeback contexts
fuse: add support for multiple writeback contexts in fuse
gfs2: add support in gfs2 to handle multiple writeback contexts
nfs: add support in nfs to handle multiple writeback contexts
writeback: set the num of writeback contexts to number of online cpus

fs/f2fs/node.c | 11 +-
fs/f2fs/segment.h | 7 +-
fs/fs-writeback.c | 146 +++++++++++++-------
fs/fuse/file.c | 9 +-
fs/gfs2/super.c | 11 +-
fs/nfs/internal.h | 4 +-
fs/nfs/write.c | 5 +-
include/linux/backing-dev-defs.h | 32 +++--
include/linux/backing-dev.h | 45 +++++--
include/linux/fs.h | 1 -
mm/backing-dev.c | 225 ++++++++++++++++++++-----------
mm/page-writeback.c | 5 +-
12 files changed, 333 insertions(+), 168 deletions(-)

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2.25.1