First attempt to explain to Dethe. Comments? Improvements? I think that explaining clearly to edu-sig is very important, but I am far from the world's best explainer. I'd like a better job than what I did here, but am unsure what to change. I think it is too long, for one thing, but then maybe I am biased.
Laura -------- Happy New Year, Dethe! Thank you for your interest. You write: >PyPy uses a similar approach to Pyrex, called Psyco, which compiles >directly to 80x86 machine code (Pyrex compiles to cross-platform C >code). This allows PyPy to attempt to be faster than C-Python by >creating compiler optimizations. Not sure what the PyPy story is for >non-x86 platforms. There is also a project to recreate Python on top >of Smalltalk, by L. Peter Deutch, which he expects to be faster than >C-Python (and if anyone can do it, he could). > >Nice to see y'all again. Happy New Year (or Gnu Year?). > >--Dethe > I'd like to clarify a few misunderstandings I think you have. Psyco is not a technique, but rather a specialising compiler available as a Python extension module. It compiles directly to 386 machine code. PyPy, on the other hand, currently emits C code. Our previous version emitted Pyrex code. Some people in Korea are making a version that emits Lisp code. PyPy doesn't use Psyco, though many ideas are common to both. What's more there is nothing magic about machine code that makes it automatically fast -- an inefficiently-coded algorithm in assembler is still a turtle. The win in using PyPy is not about 'saving the time it takes to have a conversation with your C compiler', but instead about making such conversations more productive and useful. The more information you can tell your C compiler about your data, the better code it is prepared to generate. This is the tradeoff between flexibility and speed. When your Python system sees x = a + b it has no clue as to what types a and b are. They could be anything. This 'being ready to handle everything' has a large performance penalty. The runtime system has to do be prepared to do a _lot_, so it has to be fairly intelligent. All this intelligence is in the form of code instructions, and there are a lot of them that the runtime system has to execute, every time it wants to do anything at all. On the other hand, at code _reading_ time, the Python interpreter is purposefully stupid-but-straightforward. It doesn't have much to do, and so can be relatively quick in not doing it. A statically-typed compiled langauge works in precisely the other way. When the runtime system of a statically typed language sees x = a + b, it already knows all about x, a and b and their types. All the hard work was done in the compiling phase. There is very little left to worry about -- you might get an overflow exception or something -- but as an oversimplification, all the runtime system of a statically typed system has to know how to do is how to load and run. That's fast. So, one way you could speed up Python is to add type declarations, thus simplifying the life of the runtime system. This proposed solution more than a little drastic for those of us who like duck typing, signature based polymorphism, and the particular way coding in Python makes you think and feel. The PyPy alternative is to make the interpreter even smarter, and a whole lot better at remembering what it is doing. For instance, when it sees x = a + b instead of just adding this particular int to this particular int, it could generate some general purpose code for adding ints to ints. This code could be thus used for all ints that you wish to add this way. So while the first time is slow, the _next_ time will be a lot faster. And that is where the performance speedup happens -- in code where the same lines get run again, and again, and again. If all you have is a mainline where each line of code gets executed once, then we won't help you at all. Psyco is about as far as you can get with this approach and be left with a module you can import and use with regular python 2.2 or better. See: http://psyco.sourceforge.net/ PyPy is what you get when you pitch the old interpreter and write your own. See: http://codespeak.net/pypy/ And we should probably move further disussion to pypy-dev, here: http://codespeak.net/mailman/listinfo/pypy-dev Thanks for your interest, and thanks for writing, Laura Creighton (for PyPy) _______________________________________________ [email protected] http://codespeak.net/mailman/listinfo/pypy-dev
