[Python-ideas] Why CPython is still behind in performance for some widely used patterns ?

Soni L. fakedme+py at gmail.com
Fri Jan 26 17:59:19 EST 2018

On 2018-01-26 08:18 PM, Chris Barker wrote:
> If there are robust and simple optimizations that can be added to 
> CPython, great, but:
>     This mail is the consequence of a true story, a story where CPython
>     got defeated by Javascript, Java, C# and Go.
> at least those last three are statically compiled languages -- they 
> are going to be faster than Python for this sort of thing -- 
> particularly for code written in a non-pythonic style...

Java and C#? Statically compiled? Haha.


Java has a bytecode. While yes, Java doesn't need to compile your code 
before running it, the compilation time in CPython is usually minimal, 
unless you're using eval. You can precompile your python into bytecode 
but it's usually not worth it. Java can also load bytecode at runtime 
and do bytecode manipulation stuff.

The only "real" benefit of Java is that object layout is pretty much 
static. (This can be simulated with __slots__ I think? idk.) See also, 
for example:

(The same goes for C#. Idk about Go.)

(Ofc, their JITs do also help. But even with the JIT off, it's still 
pretty good.)

>     def filter(rule, whatever):
>         if rule.x in whatever.x:
>             return True
>     rules = get_rules()
>     whatevers = get_whatevers()
>     for rule in rules:
>         for whatever in whatevers:
>             if filter(rule, whatever):
>                 cnt = cnt + 1
>     return cnt
>      It's true that they didn't optimize the code, but
>     they did not for any language having for all of them the same cost in
>     terms of iterations.
> sure, but I would argue that you do need to write code in a clean 
> style appropriate for the language at hand.
> For instance, the above creates a function that is a simple one-liner 
> -- there is no reason to do that, and the fact that function calls to 
> have significant overhead in Python is going to bite you.
>     for rule in rules:
>         x = rule.x
>         for whatever in whatevers:
>             if x in whatever.x:
>                 cnt += 1
>     The performance of the CPython boosted x3/x4 just doing these
>     "silly" things.
> "inlining" the filter call is making the code more pythonic and 
> readable -- a no brainer. I wouldn't call that a optimization.
> making rule.x local is an optimization -- that is, the only reason 
> you'd do it to to make the code go faster. how much difference did 
> that really make?
> I also don't know what type your "whatevers" are, but "x in something" 
> can be order (n) if they re sequences, and using a dict or set would 
> be a much better performance.
> and perhaps collections.Counter would help here, too.
> In short, it is a non-goal to get python to run as fast as static 
> langues for simple nested loop code like this :-)
>     The case of the rule cache IMHO is very striking, we have plenty
>     examples in many repositories where the caching of none local
>     variables is a widely used pattern, why hasn't been considered a way
>     to do it implicitly and by default?
> you can bet it's been considered -- the Python core devs are a pretty 
> smart bunch :-)
> The fundamental reason is that rule.x could change inside that loop -- 
> so you can't cache it unless you know for sure it won't. -- Again, 
> dynamic language.
>     The case of the slowness to call functions in CPython is quite
>     recurrent and looks like its an unsolved problem at all.
> dynamic language again ...
>      If the default code that you
>     can write in a language is by default slow and exists an alternative
>     to make it faster, this language is doing something wrong.
> yes, that's true -- but your example shouldn't be the default code you 
> write in Python.
>     BTW: pypy looks like is immunized [1]
>     [1]
>     https://gist.github.com/pfreixes/d60d00761093c3bdaf29da025a004582
>     <https://gist.github.com/pfreixes/d60d00761093c3bdaf29da025a004582>
> PyPy uses a JIT -- which is the way to make a dynamic language run 
> faster -- That's kind of why it exists....
> -CHB
> -- 
> Christopher Barker, Ph.D.
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