linux-kernel - Re: [PATCHSET v2 0/12] Add support for async buffered reads

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Date:   Sun, 24 May 2020 10:46:10 +0100
From:   Chris Panayis <chris@...ency.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCHSET v2 0/12] Add support for async buffered reads

Yes! Jens & Team! Yes!

My code has never looked so beautiful, been so efficient and run so well 
since switching to io_uring/async awesome-ness.. Really, really is a 
game-changer in terms of software design, control, performance, 
expressiveness... so many levels. Really, really great work! Thank you!

Chris


On 23/05/2020 20:20, Jens Axboe wrote:
> 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
>>
>