Daniel Lemire's blog

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Ranged random-number generation is slow in Python…

A colleague has been running simulations using a library written in Python. She was having serious performance problems… Her application is parallelizable, but Python does not make parallelization easy. She could switch to another language, but that’s expensive.

Further investigations reveal that her simulation relies heavily on random-number generation. Every little step involves a random number. So how good is Python at generating random numbers?

Python has a nice framework to quickly benchmark functions: the timeit module.

How fast is the Python random-number generator?

$ python -m timeit -s 'import random' 'random.random()'
10000000 loops, best of 3: 0.0363 usec per loop

So over 100 CPU cycles to generate one random floating point numbers. However, timeit includes an overhead of about 30 cycles or so to every operation, related to the function-call overhead. It is not unreasonable.

What if you want to generate an integer in a range [0,1000]? It gets ugly.

$ python -m timeit -s 'import random' 'random.randint(0,1000)'
1000000 loops, best of 3: 0.847 usec per loop

Wow! We are now taking over 2000 CPU cycles per random integer. This can easily becoming a limiting factor when writing simulation code. I tried to read Python’s source code for random.randint, but I could not figure out quickly what it is doing.

If we accept a very small (negligible) bias, we can do it by multiplication instead…

$ python -m timeit -s 'import random' 'int(random.random() * 1001)'
1000000 loops, best of 3: 0.206 usec per loop

We are down to 400 CPU cycles per integer. It is still a lot… but it is four times faster to avoid Python’s default API (random.randint).

The nice thing with Python is that it is easy to write a C function and access it from Python. Of course, it comes with some significant overhead. I do not hope to use far fewer than 100 cycles per random value by calling a C function. However, the ranged random-number generators are expensive enough that a C function might help. So I took a simple function in C that generates a good-quality (unbiased) ranged random number and made it available to Python:

$python -m timeit -s 'import fastrand' 'fastrand.pcg32bounded(1001)'
10000000 loops, best of 3: 0.0693 usec per loop

That is about 10 times faster than Python’s native random.randint.

The lesson is that random.randint should probably not be used in performance-sensitive code.

My source code is available (Python and C).

Update: Marcel Ball reports in the comments that this performance problem does not affect PyPy, only the regular Python. David Andersen points out that using the numpy library via python -m timeit -s 'import numpy' 'numpy.random.randint(0, 1000)' is much faster though my own tests do not quite agree. Further reading: Daniel Lemire, Fast Random Integer Generation in an Interval, ACM Transactions on Modeling and Computer Simulation (to appear)

Credit: This blog post benefited from an exchange with Nathan Kurz.