Re: [POC RFC 0/3] support graph like dependent sqes

[Christian Dietrich <stettberger@dokucode.de>]

Hi everyone!

We experimented with the BPF patchset provided by Pavel a few months
ago. And I had the exact same question: How can we compare the benefits
and drawbacks of a more flexible io_uring implementation? In that
specific use case, I wanted to show that a flexible SQE-dependency
generation with BPF could outperform user-space SQE scheduling. From my
experience with BPF, I learned that it is quite hard to beat
io_uring+userspace, if there is enough parallelism in your IO jobs.

For this purpose, I've built a benchmark generator that is able to
produce random dependency graphs of various shapes (isolated nodes,
binary tree, parallel-dependency chains, random DAC) and different
scheduling backends (usual system-call backend, plain io_uring,
BPF-enhanced io_uring) and different workloads.

At this point, I didn't have the time to polish the generator and
publish it, but I put the current state into this git:

https://collaborating.tuhh.de/e-exk4/projects/syscall-graph-generator

After running:

    ./generate.sh
    [sudo modprobe null_blk...]
    ./run.sh
    ./analyze.py

You get the following results (at least if you own my machine):

generator              iouring      syscall      iouring_norm
graph action size
chain read   128    938.563366  2019.199010   46.48%
flat  read   128    922.132673  2011.566337   45.84%
graph read   128   1129.017822  2021.905941   55.84%
rope  read   128   2051.763366  2014.563366  101.85%
tree  read   128   1049.427723  2015.254455   52.07%

For the userspace scheduler, I perform an offline analysis that finds
linear chains of operations that are not (anymore) dependent on other previous
unfinished results. These linear chains are then pushed into io_uring
with a SQE-link chain.

As I'm highly interested in this topic of pushing complex
IO-dependencies into the kernel space, I would be delighted to see how
your SQE-graph extension would compare against my rudimentary userspace
scheduler.

@Hao: Do you have a specific use case for your graph-like dependencies
      in mind? If you need assistance with the generator, please feel
      free to contact me.

chris
-- 
Prof. Dr.-Ing. Christian Dietrich
Operating System Group (E-EXK4)
Technische Universität Hamburg
Am Schwarzenberg-Campus 3 (E), 4.092
21073 Hamburg

eMail:  christian.dietrich@tuhh.de
Tel:    +49 40 42878 2188
WWW:    https://osg.tuhh.de/