[Python-ideas] Length hinting and preallocation for container types

[Antoine Pitrou]

Le Wed, 06 Mar 2013 11:23:26 +0900,
"Stephen J. Turnbull" <stephen at xemacs.org>
a écrit :
> 
> Not at all coincidentally.  As a compiler writer (which he refers to
> in the slides several times) he is offended by poor performance, as
> measured in CPU/invocation.  Good for him!  I'm glad he's working on
> PyPy!  But when compiler writers start talking language design for
> performance, we inevitably end up with C<0.5 wink/>.

I think that's a strong argument indeed.

>  > > One of his assertions is about memory (re)allocation. C is faster
>  > > in some cases because C code usually does fewer allocations and
>  > > reallocations. Python has no API to easily reallocate a list with
>  > > 100 items.
> 
> And it shouldn't.<wink/>
> 
> But why do you think allocation is slow in the general case?  Sure, it
> involves system calls which indeed do slow things down.

It depends what one calls a system call. A memory allocation shouldn't
always incur a call to the kernel. Library calls can be quite fast.

Moreover, in the CPython case, there's also a custom allocator which
handles all allocation requests smaller than 512 bytes.
(I'm sure PyPy has their own allocator too)

Regards

Antoine.