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

From: Jens Axboe
Date: Thu May 28 2020 - 13:06:34 EST

On 5/28/20 11:02 AM, Sedat Dilek wrote:
> On Tue, May 26, 2020 at 10:59 PM Jens Axboe <axboe@xxxxxxxxx> 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:
>> or pull from:
>> git:// 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