Reducing the Python startup time

The python startup time always nagged me (17-30ms) and I just searched again for a way to reduce it, when I found this:

The Python-Launcher caches GTK imports and forks new processes to reduce the startup time of python GUI programs.

Python-launcher does not solve my problem directly, but it points into an interesting direction: If you create a small daemon which you can contact via the shell to fork a new instance, you might be able to get rid of your startup time.

To get an example of the possibilities, download the python-launcher and socat and do the following:

PYTHONPATH="../lib.linux-x86_64-2.7/" python python-launcher-daemon &
echo pass > 1
for i in {1..100}; do 
    echo 1 | socat STDIN UNIX-CONNECT:/tmp/python-launcher-daemon.socket & 

Todo: Adapt it to a given program and remove the GTK stuff. Note the & at the end: Closing the socket connection seems to be slow, so I just don’t wait for socat to finish. Breaks at somewhere over 200 simultaneous connections. Option: Use a datagram socket instead.

The essential trick is to just create a server which opens a socket. Then it reads all the data from the socket. Once it has the data, it forks like the following:

        pid = os.fork()
        if pid:

        signal.signal(signal.SIGPIPE, signal.SIG_DFL)
        signal.signal(signal.SIGCHLD, signal.SIG_DFL)

        glob = dict(__name__="__main__")
        print 'launching', program
        execfile(program, glob, glob)

        raise SystemExit

Running a program that way 100-times took just 0.23 seconds for me so the Python startup time of 17ms got reduced to 2.3ms.

You might have to switch from forking to just executing the code instead of forking if you want to be even faster and the code snippets are small. For example when running the same test without the fork and the signals, 100 executions of the same code took just 0.09s, cutting down the startup time to an impressing 0.9ms - with the cost of no longer running in parallel.

(That’s what I also do with emacsclient… My emacs takes ~30s to start (due to excessive use of additional libraries I added), but emacsclient -c shows up almost instantly.)

I tested the speed by just sending a file with the following snippet to the server:

import time
with open("2", "a") as f:
    f.write(str(time.time()) + "\n")

Note: If your script only needs the included python libraries (batteries) and no custom-installed libs, you can also reduce the startuptime by avoiding site initialization:

python -S [script]

Without -S python -c '' takes 0.018s for me. With -S I am down to

time python -S -c '' → 0.004s. 

Note that you might miss some installed packages that way. This is slower than the daemon method by up to factor 4 (4ms instead of 0.9), but still faster than the default way. Note that cold disk buffers can make the difference much bigger on the first run which is not relevant in this case but very much relevant in general for the impression of startup speed.

PS: I attached the python-launcher 0.1.0 in case its website goes down. License: GPL and MIT; included. This message was originally written at stackoverflow.

python-launcher-0.1.0.tar.gz11.11 KB
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