Re: [PATCHSET v5 0/12] Add support for async buffered reads

Sedat Dilek
2020-05-28T17:12:35Z

On Thu, May 28, 2020 at 7:06 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> 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.
>

Thanks for answering my questions.

How can I "watch io_uring instances"?

FIO?
Debian has fio version 3.19-2 in its apt repositories.
Version OK?

- Sedat -

> > 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
>