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emmanuelle
64 lines
1.9 KiB
ReStructuredText
64 lines
1.9 KiB
ReStructuredText
========================
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How to parallelize loops
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========================
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In image processing, we frequently apply the same algorithm
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on a large batch of images. In this paragraph, we propose to
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use `joblib <https://pythonhosted.org/joblib/>`_ to parallelize
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loops. Here is an example of such repetitive tasks:
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.. code-block:: python
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from skimage import data, color, util
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from skimage.restoration import denoise_tv_chambolle
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from skimage.feature import hog
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def task(image):
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"""
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Apply some functions and return an image.
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"""
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image = denoise_tv_chambolle(image[0][0], weight=0.1, multichannel=True)
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fd, hog_image = hog(color.rgb2gray(image), orientations=8,
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pixels_per_cell=(16, 16), cells_per_block=(1, 1),
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visualise=True)
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return hog_image
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# Prepare images
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hubble = data.hubble_deep_field()
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width = 10
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pics = util.view_as_windows(hubble, (width, hubble.shape[1], hubble.shape[2]), step=width)
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To call the function ``task`` on each element of the list ``pics``, it is
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usual to write a for loop. To measure the execution time of this loop, you can
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use ipython and measure the execution time with ``%timeit``.
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.. code-block:: python
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def classic_loop():
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for image in pics:
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task(image)
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%timeit classic_loop()
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Another equivalent way to code this loop is to use a comprehension list which has the same efficiency.
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.. code-block:: python
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def comprehension_loop():
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[task(image) for image in pics]
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%timeit comprehension_loop()
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``joblib`` is a library providing an easy way to parallelize for loops once we have a comprehension list.
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The number of jobs can be specified.
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.. code-block:: python
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from joblib import Parallel, delayed
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def joblib_loop():
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Parallel(n_jobs=4)(delayed(task)(i) for i in pics)
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%timeit joblib_loop()
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