PicoNumpy

An experiment about Numpy and HPy

The C API of CPython is one of the cause of the success of Python in scientific computing. In particular, Numpy (and all the Python scientific stack) is built on top of this API. However, some characteristics of this API start to be an issue for the future of scientific Python (see 1, 2, HPy).

HPy is a very ambitious and promissing project to design a new and better C API for interacting with Python interpreters. It should allow people to write Python extensions efficient on different interpreters (CPython, PyPy, Jython, IronPython, GraalPython, RustPython, etc.).

PyPy would be especially useful for some scientific applications. For example for Integration and ODEs (scipy.integrate), for which there are a lot of callbacks of very small functions. This repository contains a tiny benchmark showing that as long as Numpy is not used, PyPy is very efficient for such task. Unfortunately, as soon as Numpy is used, PyPy becomes very slow!

With PicoNumpy, I'd like to study if HPy could help for codes using Numpy and callbacks of small Python functions.

We start by a simple but realistic benchmark (the slow loops only involve pure-Python and very simple Numpy). We then wrote a tiny ("pico") implementation of a Numpy like object (just sufficient to run the benchmark).

The next task is to reimplement PicoNumpy using HPy and to check if PyPy could efficiently accelerate our main benchmark.

PicoNumpy is really tiny. It just provides an array class (one-dimensional) supporting:

• Instantiation from a list of floats
• Elementwise multiplication and division by a float
• Elementwise addition (of 2 arrays)
• Indexing
• len

A good acceleration by PyPy of our example would be a great proof that the scientific Python community has to invest time and energy on HPy.

In the script bench_array1d.py, Transonic is used for the benchmark and comparison. With Transonic-Pythran, we typically get a 50 speedup compared to CPython (and ~400 versus PyPy, which is still very slow for such codes using Numpy).

Install and run the benchmarks

Warning: PicoNumpy now depends on HPy, which still has to be installed from the Git repository. For now, the installation is a bit more complex that what is described here (more about this here).

make should install the package in editable mode. cd bench; make should run the benchmarks. For the benchmarks, Julia is used for a good comparison point so the command julia has to be available.

For PyPy, the Makefiles are sensible to the environment variable PYTHON, so you could do:

export PYTHON=pypy3
make
cd bench
make

The benchmark code can be profiled for the different implementations with the commands (you need gprof2dot and graphviz):

cd bench
make profile METHOD="cpython-c-api"
make profile METHOD="purepy_array"
make profile METHOD="purepy"
make profile METHOD="cython"

More precise notes on how to install and run the benchmarks with PyPy

Download and extract a nightly PyPy build https://buildbot.pypy.org/nightly/. Add to the PATH environment variable the path of the directory containing the pypy executable (something like ~/opt/pypy-c-jit-101190-b661dc329618-linux64/bin). Then, you should be able to run:

pypy -m ensurepip
pypy -m pip install pip -U
pypy -m pip install numpy cython pytest transonic pythran

We need to install the correct version of HPy for the version of PyPy we are using:

pypy -c "import hpy.universal as u; print(u.get_version())"

gives ('0.0.2rc2.dev12+gc9660c2', 'c9660c2').

cd ~/Dev/hpy
# update to the correct commit
pypy setup.py develop

Now we can build-install PicoNumpy:

cd ~/Dev/piconumpy
pypy setup.py --hpy-abi=universal develop

And run the benchmarks with:

export PYTHON="pypy"
make clean
make bench_hpy
make

Few results

As of today (6 July 2021), HPy is not yet ready for high performance, but at least (with HPy 0.0.2) it runs !

At home (Intel(R) Core(TM) i5-8400 CPU @ 2.80GHz)

• With CPython

Julia                      :     1 * norm = 0.00196 s
PicoNumpy (CPython C-API)  :  9.42 * norm
PicoNumpy (HPy CPy ABI)    :  9.95 * norm
PicoNumpy (HPy Universal)  :  10.4 * norm
Transonic-Pythran          : 0.497 * norm
Numpy                      :  27.5 * norm
PicoNumpy (purepy)         :  37.3 * norm
PicoNumpy (purepy_array)   :  37.7 * norm
PicoNumpy (Cython)         :  28.9 * norm

• With PyPy3

Julia                      :     1 * norm = 0.00196 s
PicoNumpy (CPython C-API)  :  34.1 * norm
PicoNumpy (HPy Universal)  :  12.8 * norm
Transonic-Pythran          : 0.539 * norm
Numpy                      :   232 * norm
PicoNumpy (purepy)         :  4.39 * norm
PicoNumpy (purepy_array)   :  6.33 * norm
PicoNumpy (Cython)         :   274 * norm

Simpler benchmarks (bench/bench_cpy_vs_hpy.py)

• With CPython

CPython C-API:   1.92 seconds
HPy [Universal]: 2.08 seconds
HPy [CPy ABI]:   2.02 seconds

• With PyPy3

CPython C-API:   5.75 seconds
HPy [Universal]: 2.11 seconds