[pypy-dev] Confusion with meaning of "Stackless"

Armin Rigo arigo at tunes.org
Thu Dec 14 07:48:58 CET 2006

Hi Lenard,

On Wed, Dec 13, 2006 at 03:50:21PM -0800, Lenard Lindstrom wrote:
> Tasklets are nice, but I just wish to update my understanding of 
> "Stackless". Does the old motto "A Python Implementation That Does 
> Not Use The C Stack" [1] still apply? Is it relevant to PyPy?

Yes and no.  A bit confusingly, the usage of the term has evolved, along
with the Stackless Python project itself.  Now, "a Stackless feature"
means a feature that cannot be implemented without some form of explicit
stack control, either by being very careful about the C code (e.g.
avoiding recursive calls) or by saving and restoring pieces of the C
stack by a brute force 'memcpy' approach.  In PyPy, we use a variant of
the former approach: a "stackless build" of pypy-c is a compiled version
of some C code that was generated with careful systematic tweaks.

The motto of a Stackless PyPy is probably: "A Python implementation that
uses the C Stack as a cache".  Indeed, the C code supports saving its
own stack of frames away into the heap, and restoring it from there,
frame by frame.  So the C stack is really just a cache for the heap.  If
the cache is full (stack overflow), we save some more frames into the
heap to free some cache space.  If a context switch occurs (coroutine
switch) the cache is invalidated (entierely flushed to the heap) and the
new context's frames are copied from the heap to the cache as needed
(the function's frames are resumed one by one).

A bientot,


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