It is very common in Astrophysics to work with sky pixels. The sky is tassellated in patches with specific properties and a sky map is then a collection of intensity values for each pixel. The most common pixelization used in Cosmology is HEALPix.
Measurements from telescopes are then represented as an array of pixels that encode the pointing of the instrument at each timestamp and the measurement output.
import pandas as pd import numba import numpy as np
For simplicity let’s assume we have a sky with 50K pixels:
NPIX = 50000
And we have 50 million measurement from our instrument:
NTIME = int(50 * 1e6)
The pointing of our instrument is an array of pixels, random in our sample case:
pixels = np.random.randint(0, NPIX-1, NTIME)
Our data are also random:
timeline = np.random.randn(NTIME)
Create a map of the sky with pandas
One of the most common operations is to sum all of our measurements in a sky map, so the value of each pixel in our sky map will be the sum of each individual measurement.
The easiest way is to use the
groupby operation in
timeline_pandas = pd.Series(timeline, index=pixels) timeline_pandas.head() 46889 0.407097 3638 1.300001 6345 0.174931 15742 -0.255958 34308 1.147338 dtype: float64 %time m = timeline_pandas.groupby(level=0).sum() CPU times: user 4.09 s, sys: 471 ms, total: 4.56 s Wall time: 4.55 s
Create a map of the sky with numba
We would like to improve the performance of this operation using
numba, which allows to produce automatically C-speed compiled code from pure python functions.
First we need to develop a pure python version of the code, test it, and then have
numba optimize it:
def groupby_python(index, value, output): for i in range(index.shape): output[index[i]] += value[i] m_python = np.zeros_like(m) %time groupby_python(pixels, timeline, m_python) CPU times: user 37.5 s, sys: 0 ns, total: 37.5 s Wall time: 37.6 s np.testing.assert_allclose(m_python, m)
Pure Python is slower than the
pandas version implemented in
Optimize the function with numba.jit
numba.jit gets an input function and creates an compiled version with does not depend on slow Python calls, this is enforced by
numba would throw an error if it would not be possible to run in
groupby_numba = numba.jit(groupby_python, nopython=True) m_numba = np.zeros_like(m) %time groupby_numba(pixels, timeline, m_numba) CPU times: user 274 ms, sys: 5 ms, total: 279 ms Wall time: 278 ms np.testing.assert_allclose(m_numba, m)
Performance improvement is about 100x compared to Python and 20x compared to Pandas, pretty good!
Use numba.jit as a decorator
The exact same result is obtained if we use
numba.jit as a decorator:
@numba.jit(nopython=True) def groupby_numba(index, value, output): for i in range(index.shape): output[index[i]] += value[i]