[pypy-dev] Question about extension support

Laura Creighton lac at openend.se
Wed Mar 26 23:29:33 CET 2014

Your C-extensions come all bundled up with a whole lot of gorp which
is designed to make them play nicely in a ref-counting environment.
Ref counting is a very slow way to do GC.  Sometimes -- really,
really, really hideously slow.  You are sometimes _way_
better off writing python code instead -- pypy with the jit turned off
outperforms CPython purely on the benefits of not doing ref-counting, and
pypy really needs the jit to  be fast.

There is a bit of conceptual confusion here -- on the one hand, because
C extensions often were written for reasons of performance when compared to
CPython, there is a tendency to believe that C-extensions are, pretty much
by definition fast.  And the other thing is a sort of reflexive belief that
'if it is in C (or C++) then it has to be fast'.  Both of these ideas are
wrong.  A whole lot of C extensions are actually really, really slow.  They 
are just faster than CPython -- or perhaps 'faster than CPython was when 
I wrote this thing' which isn't, after all, that hard a target to meet.

When PyPy finds a C extension which is working very hard to pretend it is
a set of Python objects that can be refcounted, it isn't brilliant enough
to be able to throw away all the ref-counting fakery, intuit what the code
here really is trying to do here, and just run that bit.  That's too hard.
Instead it decides to play along with the ref-counting faking.  So we are 
at 'watch the elephant tap-dance' time ... it doesn't have to do a very good
(read fast) job at this, it is amazing that it does it at all.


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