2021-04-20
PyPy starts fast enough for our Python 2 commands
Some day, Linux distributions like Ubuntu are not going to package Python 2 even in a limited version, the way they're doing now. One way for us to deal with this would be to migrate all of our remaining little Python 2 programs and scripts to Python 3. Another option is to run them under PyPy, which says that it will always support Python 2.7.
One of the potential issues with PyPy is that its JIT has a high warm-up cost, which means that small, short-running programs are going to be slower, perhaps significantly slower. Most of the Python 2 that we have left is in small administrative commands that are mostly run automatically, where on the one hand I would expect PyPy's overhead to be at its largest and on the other hand we probably don't really care about the overhead if it's not too big. So I decided to do some quick tests.
(I've been hit by the startup overhead of small programs in Python even without PyPy, but it was in an interactive situation.)
I did my tests on one of our Ubuntu 20.04 servers, which has PyPy version 7.3.1, and the results turned out to be more interesting than I expected. The artificial and irrelevant worst case was a Python 3 program that went from about 0.05 second to about 0.17 second (under pypy3) to actually do its work. Our typical small Python 2 commands seem to go from 0.01 or 0.02 second to about 0.07 second or so. The surprising best case was a central program used for managing our password file, where the runtime under PyPy actually dropped from around 0.40 second to 0.33 second. And a heavily multithreaded program that runs a lot of concurrent ssh commands had essentially the same runtime on a different 20.04 machine.
(In retrospect, the password file processing program does have to process several thousand lines of text, so perhaps I should not have been surprised that it's CPU-intensive enough for PyPy to speed it up. Somehow it's in my mind as a small, lightweight thing.)
All of this says that PyPy starts (and runs) our Python programs more than fast enough to serve us as an alternate implementation of Python 2 if we need to turn to it.