PicoNumpy

An experiment about Numpy and pyhandle/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 promising 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

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"

Few results

As of today (5 March 2020), HPy is not yet ready!

At work (meige8pcpa79, Intel(R) Xeon(R) CPU E5-1603 v3 @ 2.80GHz)

• With CPython

Julia                      :     1 * norm = 0.00534 s
Transonic-Pythran          : 0.564 * norm
Numpy                      :  18.2 * norm
PicoNumpy (purepy)         :  23.7 * norm
PicoNumpy (purepy_array)   :  21.7 * norm
PicoNumpy (Cython)         :  15.6 * norm
PicoNumpy (CPython C-API)  :   4.7 * norm

• With PyPy3

Julia                      :     1 * norm = 0.00534 s
Transonic-Pythran          : 0.603 * norm
Numpy                      :   183 * norm
PicoNumpy (purepy)         :   2.4 * norm
PicoNumpy (purepy_array)   :  3.73 * norm
PicoNumpy (Cython)         :   135 * norm
PicoNumpy (CPython C-API)  :  17.8 * norm

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

• With CPython

Julia                      :     1 * norm = 0.00176 s
Transonic-Pythran          :  0.73 * norm
Numpy                      :  37.6 * norm
PicoNumpy (purepy)         :  46.4 * norm
PicoNumpy (purepy_array)   :  41.9 * norm
PicoNumpy (Cython)         :  31.4 * norm
PicoNumpy (CPython C-API)  :  9.22 * norm

• With PyPy3

Julia                      :     1 * norm = 0.00176 s
Transonic-Pythran          : 0.797 * norm
Numpy                      :   326 * norm
PicoNumpy (purepy)         :  4.71 * norm
PicoNumpy (purepy_array)   :  7.48 * norm
PicoNumpy (Cython)         :   243 * norm
PicoNumpy (CPython C-API)  :  36.6 * norm

CPython C-API usage in PicoNumpy

It is the first time that I wrote a Python extension by hand (without Cython or Pythran) and moreover, there are nearly no checks and I didn't pay attention to reference counting!

List obtained from grep -rwoh 'Py[_A-Z].[a-zA-Z_]*' | sort | uniq:

PyArg_ParseTupleAndKeywords
PyCFunction
Py_DECREF
PyErr_NoMemory
PyErr_SetString
PyExc_TypeError
PyFloat_AsDouble
PyFloat_FromDouble
PyFloatObject
Py_INCREF
PyList_Check
PyList_GET_ITEM
PyList_New
PyList_SetItem
PyList_Size
PyLong_AsLong
PyMemberDef
PyMethodDef
PyMODINIT_FUNC
PyModule_AddObject
PyModule_Create
PyModuleDef
PyModuleDef_HEAD_INIT
PyNumber_Check
PyNumberMethods
PyObject
PyObject_HEAD
PyObject_New
PySequenceMethods
Py_ssize_t
Py_TPFLAGS_DEFAULT
Py_TYPE
PyType_GenericNew
PyTypeObject
PyType_Ready
Py_UNUSED
PyVarObject_HEAD_INIT