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Subject[PATCHSET RFC 0/11] Add support for async buffered reads
We technically support this already through io_uring, but it's
implemented with a thread backend to support cases where we would
block. This isn't ideal.

After a few prep patches, the core of this patchset is adding support
for async callbacks on page unlock. With this primitive, we can simply
retry the IO operation. With io_uring, this works a lot like poll based
retry for files that support it. If a page is currently locked and
needed, -EIOCBQUEUED is returned with a callback armed. The callers
callback is responsible for restarting the operation.

With this callback primitive, we can add support for
generic_file_buffered_read(), which is what most file systems end up
using for buffered reads. XFS/ext4/btrfs/bdev is wired up, but probably
trivial to add more.

The file flags support for this by setting FMODE_BUF_RASYNC, similar
to what we do for FMODE_NOWAIT. Open to suggestions here if this is
the preferred method or not.

In terms of results, I wrote a small test app that randomly reads 4G
of data in 4K chunks from a file hosted by ext4. The app uses a queue
depth of 32.

preadv for comparison:
real 1m13.821s
user 0m0.558s
sys 0m11.125s
CPU ~13%

real 0m12.054s
user 0m0.111s
sys 0m5.659s
CPU ~32% + ~50% == ~82%

This patchset:
real 0m9.283s
user 0m0.147s
sys 0m4.619s
CPU ~52%

The CPU numbers are just a rough estimate. For the mainline io_uring
run, this includes the app itself and all the threads doing IO on its
behalf (32% for the app, ~1.6% per worker and 32 of them). Context
switch rate is much smaller with the patchset, since we only have the
one task performing IO.

The goal here is efficiency. Async thread offload adds latency, and
it also adds noticable overhead on items such as adding pages to the
page cache. By allowing proper async buffered read support, we don't
have X threads hammering on the same inode page cache, we have just
the single app actually doing IO.

Series can also be found here:

or pull from:

git:// async-buffered

fs/block_dev.c | 2 +-
fs/btrfs/file.c | 2 +-
fs/ext4/file.c | 2 +-
fs/io_uring.c | 101 ++++++++++++++++++++++++++++++++++++++
fs/xfs/xfs_file.c | 2 +-
include/linux/blk_types.h | 3 +-
include/linux/fs.h | 5 ++
include/linux/pagemap.h | 40 +++++++++++++++
mm/filemap.c | 71 +++++++++++++++++++++------
9 files changed, 207 insertions(+), 21 deletions(-)

Jens Axboe

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