Pex PEX perf regression

[jsirois]

Pex ships a PEX of itself for those who wish to download a Pex binary instead of pip installing it. Pants is an important user of the Pex PEX and f07ae47 (via git bisect) introduced a performance regression that induces ~300ms of startup overhead over a v2.1.16 baseline.

Tested with the following which emulates the simplest Pex PEX use case in Pants for interpreter selection for Pants tools:

$ python -mpex . -cpex --unzip -opex.pex
$ ./pex.pex --interpreter-constraint=">=3.6" -- -c 'import sys; print(sys.executable)'
$ multitime -n10 ./pex.pex --interpreter-constraint=">=3.6" -- -c 'import sys; print(sys.executable)'

v2.1.16

===> multitime results
1: ./pex.pex --interpreter-constraint=>=3.6 -- -c "import sys; print(sys.executable)"
            Mean        Std.Dev.    Min         Median      Max
real        0.861       0.007       0.851       0.859       0.874       
user        0.780       0.008       0.770       0.779       0.798       
sys         0.080       0.009       0.067       0.080       0.093 

f07ae47

===> multitime results
1: ./pex.pex --interpreter-constraint=>=3.6 -- -c "import sys; print(sys.executable)"
            Mean        Std.Dev.    Min         Median      Max
real        1.184       0.013       1.170       1.182       1.217       
user        1.076       0.012       1.056       1.080       1.091       
sys         0.101       0.012       0.080       0.099       0.123

[jsirois]

Noting that the specific mechanism at play here in the regression is --unzip. We ship the Pex PEX in --unzip mode since, without it:

$ python -mpex . -cpex -opex.pex
$ ./pex.pex --interpreter-constraint=">=3.6" -- -c 'import sys; print(sys.executable)'
$ multitime -n10 ./pex.pex --interpreter-constraint=">=3.6" -- -c 'import sys; print(sys.executable)'
===> multitime results
1: ./pex.pex --interpreter-constraint=>=3.6 -- -c "import sys; print(sys.executable)"
            Mean        Std.Dev.    Min         Median      Max
real        1.068       0.009       1.059       1.065       1.090       
user        0.982       0.011       0.971       0.976       1.006       
sys         0.082       0.010       0.057       0.085       0.094 

It makes sense that --unzip mode is sensitive to import scope since we always re-exec from the PEX file in that mode and the more imported code that is needed to re-exec the PEX, the slower it will be:
https://github.com/pantsbuild/pex/blob/cbb44f55f9e8f6293bd6bcd4f337e2aebdc1f1f5/pex/pex_builder.py#L32-L60