why python is slower than java?

[Fábio Mendes]

Em Sáb, 2004-11-06 às 22:07 -0500, Roy Smith escreveu:
> Terry Hancock <hancock at anansispaceworks.com> wrote:
> > And I have certainly written some extremely poorly optimized
> > Python programs that positively *crawled*.
> 
> My guess is the poor performance had nothing to do with the language you 
> wrote it in, and everything to do with the algorithms you used.

Well, try to write a Linear Algebra algorithm in pure python... Then
you'll see that pyrex, scipy or the C API are your friends. Of course
algorithmis matters, but for some intensive CPU applications python can
be as slow as 1/100 C using the same algorith. This, of course, matters.
At least if your program is running this kind of operation most of the
time.

> Local optimizations rarely gets you more than a factor of 2 improvement.  
> Choice of language (at least within the same general catagory such as 
> comparing one native compiled language to another, or one virtual 
> machine language to another) probably has a somewhat broader range, but 
> still a factor of 10 would be quite surprising.

10x or 20x difference is likely to hurt you. If you have a 20x slower
computer were you be using the same apps as you use now? For scientific
simulations (which interest me most), it's the difference between get
your results after 1day calculation of one month...

> To get really bad performance, you need to pick the wrong algorithm.  A 
> project I worked on a while ago had a bit of quadratic behavior in it 
> when dealing with files in a directory.  Most of our customers dealt 
> with file sets in the 100's.  One customer had 50,000 files.  Going 
> from, say, 500 to 50,000 is a 100-fold increase in N, and gave a 
> 10,000-fold increase in execution time.

This is an extreme case, but is a typical behaviour of how a good
algorithm can modify the execution times of your program: scaling up
neatly. The language matters indeed, but python is well served in a
broad range of libs. For the most CPU intensive tasks there are usually
python bindings for C/C++/Fortran libraries, so it's usually possible to
execute python code at native speeds. This is why I use python in my
(simple) physics applications: scipy gives me a very good interface for
evaluating heavy numeric stuff. When I program in C++ I'm usually
tempted to write a lot of code from the scratch, for I'm lazy to search
for the write libs and learn how they work. I'm a physicist, not a real
programer, so this end up with C++ code slower than python's (which uses
scipy magic). 

The point I want to make here is that library programmers usually makes
much better (in the sense it's faster) code than an application
programmer. So, in any language, if you want to write a big project from
the scratch, you'll probably end up with a buggy, slow and cumbersome
library for the 'low level' stuff, and the high level interface will
never move on. If you pick up the existing libs and extend them to
tailor your needs, you have better chances of success. Python seems to
be better than java in this point (at least in the OSS arena), for there
is an enourmously bigger set of libraries you can start with, and is
easier to wrap C/C++/Fortran code to it. So the likely 10% or so runtime
penalty I get for running a C lib through python is more than paid in
application development time. To me, what matters is: can a __python
program__ run as fast as a __c program__? In lots of cases: yes, just
use the right lib. 

All this 'java interpreter is faster than python interpreter' nonsense
doesn't appeal to me. For intensive CPU operations, neither interpreters
are good, or even decent, so there is no subistitute for a good o' low
level implementation. Can java use those implementations as easyly as
python? --No-- Does java have a comparable set of low level (fast!) libs
wrapped to it? --Not as rich as python--. So forget the language
shootout contest! Those are not real world examples, they're complete
crap. Python has a richer set of faster libs than java. Use them,  so
when it matters, python usually can be faster than java. This is enough
for me for saying that python IS faster than java.

If you care much, a python program can be almost as fast as a C one,
just use a python wrapper to a C lib or use pyrex! This is not saying
that language doesn't matter. Not all so called scripting languages have
the same facilities as python. For instance perl is very optimized to
regex substitution, but lacks more broad biddings for, for example
numeric libraries. We're lucky to use python, we're lucky that it has so
many facilities. Other scripting languages have nice syntax, powerful
builtin objects and other gizmos that make them much more expressive and
productive than the low level counterparts. IMHO python is unique in the
fact that it also provides very nice set of wrapped low level libraries
to run runtime critical pieces of code, so in a lot of cases we can get
the best of both worlds. Java is too much a in between for me: execution
not so fast, but not so slow; development not so fast, but not so slow;
builtins not so expressive, but not so low level either; it is not that
good, neither that bad, etc... 

Cheers,
Fabio