[Python-Dev] [ python-Patches-876206 ] scary frame speed hacks

[Raymond Hettinger]

> > The drawback is that recursive functions would be slower now.
> 
> How come?

With the current freelist approach, there can be a large pool of
pre-made frames available for each level of recursion.  On the
plus side, this means fewer calls to malloc().  On the minus side,
the pooled frames are generic to any code block and take a long
time to initialize.

The patch eliminates the freelist in favor of keeping a single
pre-made frame for each code block.  In addition to saving 
a call to malloc(), the advantage is that the pre-made frame
is custom to the code block and only half of fields need to
be updated each time the code block is called.  

This is a nice net win for normal code blocks.  However, recursive
functions use up their one pre-made frame on the first level of
recursion.  On subsequent calls, they have to call malloc() resulting
in a net slowdown.

As is, the patch is slim and elegant.  However, it could built
out to have both a code block specific pre-made frame and a freelist.
The patch would then be much larger and somewhat ugly, but it would
avoid the speed loss for recursive functions.

Armin's quick and dirty timings for the current patch show a 10%
speedup for non-recursive functions and a 20% slowdown of 
recursive functions.

The question is whether to clutter the patch in order to save the
20% on recursive functions.


Raymond Hettinger