why python is slower than java?
ianb at colorstudy.com
Fri Nov 5 18:05:07 CET 2004
Maurice LING wrote:
> This may be a dumb thing to ask, but besides the penalty for dynamic
> typing, is there any other real reasons that Python is slower than Java?
Python is generally late-bound, which means much more happens at runtime
compared to Java. For instance, consider accessing an attribute,
"self.x". In Java you know what the class is shaped like, exactly.
Usually this turns into some bytecode like "access the variable of self,
at offset 3". I don't know any of the details, but that's the general gist.
Similarly, there's many other optimizations where you can know at
compile time exactly what functions are called, how variables are
stored, etc. Using that information, you can do optimizations like
inline code. In the same places in Python, you are typically doing one
(or several!) hashtable lookups based on the variable or attribute name.
But even in Java there are limits. Because of interfaces and subclasses
you can know the shape of self/this, but you can't know the exact shape
of the objects around you (C++ is more aggressive in this respect, and
often can determine the exact shape; but in the process it's dangerous
to put together different pieces of compiled code when they don't know
about each other, which is why binary interfaces there are fragile).
Anyway, Java does a lot of the same stuff as Python when the exact type
is determined at runtime (which is frequently). This is where JIT comes
in, doing the optimizations at runtime; it is still limited, as it
cannot guarantee the type of the objects in the system, but must check
them each time before using the optimized path. And, actually, Python
can do the same sort of things with psyco. It's still harder in Python,
and the end result not as effective, but it's one among many tools.
If Java wasn't doing any optimizations, I don't think it would be
significantly faster than Python.
Also note that Python assimilates external (C, Fortan, etc) libraries
much better than Java seems to. In an entire system, Python can easily
be faster because Java includes many slow libraries (slow compared to
equivalent libraries available in Python). E.g., Swing is much slower
Anyway, that's my take. I'm no authority, as I've never seriously used
Java, and haven't done any tests, nor spent anytime looking at the
Ian Bicking / ianb at colorstudy.com / http://blog.ianbicking.org
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