[Python-Dev] Startup time

[Guido van Rossum]

While Python's general speed has gone up, its startup speed has slowed
down!

I timed this two different ways.  The first way is to run

  python<version> -c "import time; print time.clock()"

On Unix, this prints the CPU time used since the process was created.
The second way is to run

  time python<version> -c pass

which shows CPU and real time to complete running the process.  I did
this on a 633 MHz PC running Red Hat Linux 7.3.  The Python builds
were standard non-debug builds.  I tried with and without the -S
option, which is supposed to suppress loading of site.py and hence
most startup overhead; it didn't exist in Python 1.3 and 1.4.

Results for the first way are pretty inaccurate because it's such a
small number and is only measured in 1/100 of a second, yet revealing.
Some times are printed as two values; I didn't do enough runs to
compute a careful average, so I'm just showing the range:

  Version	CPU Time	CPU Time with -S
  1.3		0.00		N/A
  1.4		0.00		N/A
  1.5.2		0.01		0.00
  2.0		0.01-0.02	0.00
  2.1		0.01-0.02	0.00
  2.2		0.02		0.00
  2.3		0.04		0.03-0.04

Now using time:

  Version	CPU Time	CPU Time with -S
  1.3		0.004		N/A
  1.4		0.004		N/A
  1.5		0.018		0.006
  2.0		0.021		0.006
  2.1		0.018		0.004
  2.2		0.025		0.004
  2.3		0.045		0.045

Note two things: (a) the start time goes up over time, and (b) for
Python 2.3, -S doesn't make any difference.

Given that we often run very short Python programs, and e.g. Python's
popularity for CGI scripts, I find this increase in startup time very
worrysome, and worthy of our attention (more than gaining nanoseconds
on dict operations or even socket I/O speed).

My goal: I'd like Python 2.3(final) to start up at least as fast as
Python 2.2, and I'd like the much faster startup time back with -S.

I have no time to investigate the cause right now, although I have a
suspicion that the problem might be in loading too much of the
encoding framework at start time (I recall Marc-Andre and Martin
debating this earlier).

--Guido van Rossum (home page: http://www.python.org/~guido/)