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Subject: Re: [PATCHSET v2 0/12] Add support for async buffered reads
From:   Jens Axboe <axboe@kernel.dk>
To:     io-uring@vger.kernel.org
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20200523185755.8494-1-axboe@kernel.dk>
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Date:   Sat, 23 May 2020 13:20:42 -0600
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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