[Python-Dev] this is what happens if you freeze all the modules required for startup

[Brett Cannon]

It was realized during PyCon that since we are freezing importlib we could
now consider freezing all the modules to cut out having to stat or read
them from disk. So for day 1 of the sprints I I decided to hack up a
proof-of-concept to see what kind of performance gain it would get.

Freezing everything except encodings.__init__, os, and _sysconfigdata, it
speeds up startup by 11% compared to default. Compared to 2.7 it shaves 14%
from the slowdown (27% slower vs. 41% slower). The full results are at the
end of the email.

Now the question is whether the maintenance cost of having to rebuild
Python for a select number of stdlib modules is enough to warrant putting
in the effort to make this work. My guess is the best approach would be
adding a Lib/_frozen directory where any modules that we treat like this
would be kept to act as a reminder that you need to rebuild for them (I
would probably move importlib/_boostrap.py as well to make this consistent).

Thoughts?


--------------------------------------

default vs the freezing:

### normal_startup ###

Min: 0.524812 -> 0.473339: 1.11x faster

Avg: 0.534403 -> 0.481245: 1.11x faster

Significant (t=61.80)

Stddev: 0.00466 -> 0.00391: 1.1909x smaller


### startup_nosite ###

Min: 0.307359 -> 0.291939: 1.05x faster

Avg: 0.317667 -> 0.300156: 1.06x faster

Significant (t=26.29)

Stddev: 0.00543 -> 0.00385: 1.4099x smaller

---------

2.7 vs the freezing:

### normal_startup ###

Min: 0.367571 -> 0.465264: 1.27x slower

Avg: 0.374404 -> 0.476662: 1.27x slower

Significant (t=-90.26)

Stddev: 0.00313 -> 0.00738: 2.3603x larger


### startup_nosite ###

Min: 0.164510 -> 0.290544: 1.77x slower

Avg: 0.169833 -> 0.301109: 1.77x slower

Significant (t=-286.30)

Stddev: 0.00211 -> 0.00407: 1.9310x larger

---------

As a baseline, 2.7 vs default:

### normal_startup ###

Min: 0.368916 -> 0.521758: 1.41x slower

Avg: 0.376784 -> 0.531883: 1.41x slower

Significant (t=-172.82)

Stddev: 0.00423 -> 0.00474: 1.1207x larger


### startup_nosite ###

Min: 0.165156 -> 0.309090: 1.87x slower

Avg: 0.171516 -> 0.319004: 1.86x slower

Significant (t=-283.45)

Stddev: 0.00334 -> 0.00399: 1.1948x larger
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