And this one is v3, obviously, not v2... On 5/23/20 12:57 PM, Jens Axboe wrote: > 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. If you want to test yourself, you can just use buffered=1 > with ioengine=io_uring with fio. No application changes are needed to > use the more optimized buffered async read. > > preadv for comparison: > real 1m13.821s > user 0m0.558s > sys 0m11.125s > CPU ~13% > > Mainline: > 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. > > Been beating on this and it's solid for me, and I'm now pretty happy > with how it all turned out. Not aware of any missing bits/pieces or > code cleanups that need doing. > > Series can also be found here: > > https://git.kernel.dk/cgit/linux-block/log/?h=async-buffered.3 > > or pull from: > > git://git.kernel.dk/linux-block async-buffered.3 > > fs/block_dev.c | 2 +- > fs/btrfs/file.c | 2 +- > fs/ext4/file.c | 2 +- > fs/io_uring.c | 99 ++++++++++++++++++++++++++++++++++ > fs/xfs/xfs_file.c | 2 +- > include/linux/blk_types.h | 3 +- > include/linux/fs.h | 5 ++ > include/linux/pagemap.h | 64 ++++++++++++++++++++++ > mm/filemap.c | 111 ++++++++++++++++++++++++-------------- > 9 files changed, 245 insertions(+), 45 deletions(-) > > Changes since v2: > - Get rid of unnecessary wait_page_async struct, just use wait_page_async > - Add another prep handler, adding wake_page_match() > - Use wake_page_match() in both callers > Changes since v1: > - Fix an issue with inline page locking > - Fix a potential race with __wait_on_page_locked_async() > - Fix a hang related to not setting page_match, thus missing a wakeup > -- Jens Axboe
