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<2014-05-14> by Lorenzo

Experiments in pickling

pickle is a standard library module to serialize and deserialize Python objects. Being written in pure Python, it's fairly slow, so the standard library provides a pure-C implementation, called cPickle, with the limitation that it cannot be subclassed.

What's interesting about cPickle are two little known settings that can speed up serialization quite substantially.

  1. cPickle.HIGHEST_PROTOCOL basically dumps a Python object using a binary protocol, rather than the default ASCII-based, more portable, protocol 0. If portability or backward compatibility are not an issue for you, you should use it: it's documented and probably here to stay.
  2. Pickler's undocumented fast flag. It turns out that cPickle implementation has a "fast mode" that is enabled by setting this fast flag to True. It's undocumented so probably subject to change, but it makes dumping objects way faster.

Looking at the implementation of cPickle, the comments in the code have something to say about "fast mode":

The fast mode disable the usage of memo, therefore speeding the pickling process by not generating superfluous PUT opcodes. It should not be used if with self-referential objects.

memo is basically a cache, within the pickler, that remembers what objects have already been processed, used mainly to avoid infinite loops when dumping self-referential data structures. But if you are dumping data structures that do not reference themselves, you can spare some time disabling this caching.

To test these settings, I compared "vanilla" cPickle.dumps, with "highest\protocol" and "fast mode", for 3 different objects: a small, a medium and a large list of dictionaries. The code is available in gist:

Clone it and run it:

$> git clone https://gist.github.com/1bec1b70ef9c8e254b57.git pickle_experiments
$> cd pickle_experiments
$> sh run.sh

On my machine, I get these results:

10 loops, best of 3: 7.39 usec per loop
10 loops, best of 3: 2.5 usec per loop
10 loops, best of 3: 4.1 usec per loop

10 loops, best of 3: 705 usec per loop
10 loops, best of 3: 206 usec per loop
10 loops, best of 3: 111 usec per loop

3 loops, best of 3: 1.34 sec per loop
10 loops, best of 3: 823 msec per loop
10 loops, best of 3: 135 msec per loop

Note: I've reduced the number of iterations to 3 for "vanilla LARGE" because it was taking too long…

"high\protocol" is between 2 and 3 times faster than "vanilla" for all objects, and "fast mode" is 6 times faster than "high\protocol" and 10 times faster than "vanilla" for large objects!