C++ performance got worse going from Ubuntu 20 to Ubuntu 21

[Peter Taraba]

This is not a bug, more of a degradation of performance report.

Going from Ubuntu 20 to Ubuntu 21 (and same on Ubuntu 22 pre-release), I noticed that C++ performance got worse, while it stayed the same for pypy:

Ubuntu 21:
pypy 1.419s 1.387s 8.527% 2.469%
c++ 1.843s 1.719s 8.935% 2.465%

Ubuntu 20:
PyPy 1.410s 1.389s 8.463% 2.290%
C++ 1.588s 1.507s 8.781% 2.284%

details:
https://www.frisky.world/2022/03/what-happened-to-c-performance-going.html

software I use:
https://github.com/pepe78/python-performance

[Chris Guiver (guiverc)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. Unfortunately, we cannot work on this bug because your description didn't include enough information. You may find it helpful to read "How to report bugs effectively" http://www.chiark.greenend.org.uk/~sgtatham/bugs.html. We'd be grateful if you would then provide a more complete description of the problem.

We have instructions on debugging some types of problems at http://wiki.ubuntu.com/DebuggingProcedures.

At a minimum, we need:

1. The specific steps or actions you took that caused you to encounter the problem.
2. The behavior you expected.
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Please also ensure that you include the release and flavour of Ubuntu that you are using.

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Thank you!

[Peter Taraba (pt-pepe78)]

That's a bummer! I think it's not too challenging to download Ubuntu 20 and Ubuntu 21 and test it out. :) (I really don't care that much that C++ gets slow... while it's a bummer, it's not a show stopper).

[Peter Taraba (pt-pepe78)]

And if you really need repro steps, which I think are quite obvious

A.1. Install Ubuntu 20 (https://ubuntu.com/download/desktop/thank-you?version=20.04.4&architecture=amd64)
A.2. git clone https://github.com/pepe78/python-performance
A.3. cd python-performance
A.4. ./run.sh

B.1. Install Ubuntu 21 (https://ubuntu.com/download/desktop/thank-you/?version=21.10&architecture=amd64)
B.2. git clone https://github.com/pepe78/python-performance
B.3. cd python-performance
B.4. ./run.sh

compare results for C++ and PyPy. PyPy is around same, C++ got worse.

Have a nice day!

[Matthias Klose (doko)]

would you be able to track that down to some specific gcc-11 package version?

[Peter Taraba (pt-pepe78)]

Hi Matthias,

it seems like the "bug" (performance slow down) is not dependent on gcc version (it happens with both g++9 and g++11):
https://www.frisky.world/2022/04/testing-ubuntu-2204-lts-jammy-jellyfish.html

but rather dependent on kernel version (5.13 linux kernel was faster than 5.15 linux kernel):
https://www.frisky.world/2022/04/its-probably-in-kernel.html

not really sure how many code commits there were between 5.13 and 5.15 linux kernel - that might be quite hard to find which commit caused this... if it was necessary or if it is possible to improve for performance.

Cheers

[Peter Taraba (pt-pepe78)]

It also looks like it already happened on 5.14, so I would be looking at commits between 5.13 and 5.14:

Linux-5.13.0-40-generic-x86_64-with-glibc2.29

c++ g++ (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0 1.524 1.507 8.431 2.278

Linux-5.14.10-300.fc35.x86_64-x86_64-with-glibc2.34

c++ g++ (GCC) 11.3.1 20220421 (Red Hat 11.3.1-2) 1.800 1.707 8.785 2.633

Linux-5.15.0-27-generic-x86_64-with-glibc2.35

c++ g++ (Ubuntu 11.2.0-19ubuntu1) 11.2.0 1.728 1.713 8.406 2.266

more results:
https://github.com/pepe78/python-performance

[Launchpad Janitor (janitor)]

[Expired for gcc-11 (Ubuntu) because there has been no activity for 60 days.]