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    SubjectRe: [PATCHSET v5 0/12] Add support for async buffered reads
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    On 5/28/20 11:02 AM, Sedat Dilek wrote:
    > On Tue, May 26, 2020 at 10:59 PM Jens Axboe <axboe@kernel.dk> 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.
    >>
    >> Also ran a simple fio based test case, varying the queue depth from 1
    >> to 16, doubling every time:
    >>
    >> [buf-test]
    >> filename=/data/file
    >> direct=0
    >> ioengine=io_uring
    >> norandommap
    >> rw=randread
    >> bs=4k
    >> iodepth=${QD}
    >> randseed=89
    >> runtime=10s
    >>
    >> QD/Test Patchset IOPS Mainline IOPS
    >> 1 9046 8294
    >> 2 19.8k 18.9k
    >> 4 39.2k 28.5k
    >> 8 64.4k 31.4k
    >> 16 65.7k 37.8k
    >>
    >> Outside of my usual environment, so this is just running on a virtualized
    >> NVMe device in qemu, using ext4 as the file system. NVMe isn't very
    >> efficient virtualized, so we run out of steam at ~65K which is why we
    >> flatline on the patched side (nvme_submit_cmd() eats ~75% of the test app
    >> CPU). Before that happens, it's a linear increase. Not shown is context
    >> switch rate, which is massively lower with the new code. The old thread
    >> offload adds a blocking thread per pending IO, so context rate quickly
    >> goes through the roof.
    >>
    >> 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.5
    >>
    >> or pull from:
    >>
    >> git://git.kernel.dk/linux-block async-buffered.5
    >>
    >
    > Hi Jens,
    >
    > I have pulled linux-block.git#async-buffered.5 on top of Linux v5.7-rc7.
    >
    > From first feelings:
    > The booting into the system (until sddm display-login-manager) took a
    > bit longer.
    > The same after login and booting into KDE/Plasma.

    There is no difference for "regular" use cases, only io_uring with
    buffered reads will behave differently. So I don't think you have longer
    boot times due to this.

    > I am building/linking with LLVM/Clang/LLD v10.0.1-rc1 on Debian/testing AMD64.
    >
    > Here I have an internal HDD (SATA) and my Debian-system is on an
    > external HDD connected via USB-3.0.
    > Primarily, I use Ext4-FS.
    >
    > As said above is the "emotional" side, but I need some technical instructions.
    >
    > How can I see Async Buffer Reads is active on a Ext4-FS-formatted partition?

    You can't see that. It'll always be available on ext4 with this series,
    and you can watch io_uring instances to see if anyone is using it.

    > Do I need a special boot-parameter (GRUB line)?
    >
    > Do I need to activate some cool variables via sysfs?
    >
    > Do I need to pass an option via fstab entry?

    No to all of these, you don't need anything to activate it. You need the
    program to use io_uring to do buffered reads.

    > Are any Async Buffer Reads related linux-kconfig options not set?
    > Which make sense?

    No kconfig options are needed.

    --
    Jens Axboe

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