Thursday, May 5, 2011

NumPy Follow up

Hi everyone. Since yesterday's blog post we got a ton of feedback, so we want to clarify a few things, as well as share some of the progress we've made, in only the 24 hours since the post.

Reusing the original NumPy

First, a lot of people have asked why we cannot just reuse the original NumPy through cpyext, our CPython C-API compatibility layer. We believe this is not the best approach, for a few reasons:

  1. cpyext is slow, and always will be slow. It has to emulate far too many details of the CPython object model that don't exist on PyPy (e.g., reference counting). Since people are using NumPy primarily for speed this would mean that even if we could have a working NumPy, no one would want to use it. Also, as soon as the execution crosses the cpyext boundary, it becomes invisible to the JIT, which means the JIT has to assume the worst and deoptimize stuff away.
  2. NumPy uses many obscure documented and undocumented details of the CPython C-API. Emulating these is often difficult or impossible (e.g. we can't fix accessing a struct field, as there's no function call for us to intercept).
  3. It's not much fun. Frankly, working on cpyext, debugging the crashes, and everything else that goes with it is not terribly fun, especially when you know that the end result will be slow. We've demonstrated we can build a much faster NumPy, in a way that's more fun, and given that the people working on this are volunteers, it's important to keep us motivated.

Finally, we are not proposing to rewrite the entirety of NumPy or, god forbid, BLAST, or any of the low level stuff that operates on C-level arrays, only the parts that interface with Python code directly.

C bindings vs. CPython C-API

There are two issues on C code, one has a very nice story, and the other not so much. First is the case of arbitrary C-code that isn't Python related, things like libsqlite, libbz2, or any random C shared library on your system. PyPy will quite happily call into these, and bindings can be developed either at the RPython level (using rffi) or in pure Python, using ctypes. Writing bindings with ctypes has the advantage that they can run on every alternative Python implementation, such as Jython and IronPython. Moreover, once we merge the jittypes2 branch ctypes calls will even be smoking fast.

On the other hand there is the CPython C-extension API. This is a very specific API which CPython exposes, and PyPy tries to emulate. It will never be fast, because there is far too much overhead in all the emulation that needs to be done.

One of the reasons people write C extensions is for speed. Often, with PyPy you can just forget about C, write everything in pure python and let the JIT to do its magic.

In case the PyPy JIT alone isn't fast enough, or you just want to use existing C code then it might make sense to split your C-extension into 2 parts, one which doesn't touch the CPython C-API and thus can be loaded with ctypes and called from PyPy, and another which does the interfacing with Python for CPython (where it will be faster).

There are also libraries written in C to interface with existing C codebases, but for whom performance is not the largest goal, for these the right solution is to try using CPyExt, and if it works that's great, but if it fails the solution will be to rewrite using ctypes, where it will work on all Python VMs, not just CPython.

And finally there are rare cases where rewriting in RPython makes more sense, NumPy is one of the few examples of these because we need to be able to give the JIT hints on how to appropriately vectorize all of the operations on an array. In general writing in RPython is not necessary for almost any libraries, NumPy is something of a special case because it is so ubiquitous that every ounce of speed is valuable, and makes the way people use it leads to code structure where the JIT benefits enormously from extra hints and the ability to manipulate memory directly, which is not possible from Python.

Progress

On a more positive note, after we published the last post, several new people came and contributed improvements to the numpy-exp branch. We would like to thank all of them:

  • nightless_night contributed: An implementation of __len__, fixed bounds checks on __getitem__ and __setitem__.
  • brentp contributed: Subtraction and division on NumPy arrays.
  • MostAwesomeDude contributed: Multiplication on NumPy arrays.
  • hodgestar contributed: Binary operations between floats and NumPy arrays.

Those last two were technically an outstanding branch we finally merged, but hopefully you get the picture. In addition there was some exciting work done by regular PyPy contributors. I hope it's clear that there's a place to jump in for people with any level of PyPy familiarity. If you're interested in contributing please stop by #pypy on irc.freenode.net, the pypy-dev mailing list, or send us pull requests on bitbucket.

Alex

19 comments:

Anonymous said...

How does this suggestion to use ctypes to interface with external C modules square with the python-dev antipathy towards doing that?

"Given the choice of using either ctypes or an external package, I prefer the external package." Martin v. Löwis

"If it means using ctypes to interface with system C libraries, I'm -10 on it :)" Antoine Pitrou

Alex said...

I don't know what to say for them, besides they apparently don't hate it so much as to remove it from the stdlib :)

Michael Foord said...

Isn't there another fairly major drawback to implementing in RPython - that you can only use it if it is compiled (translated) at the same time as pypy. So effectively pypy *has* to be distributed with all the RPython extensions you will ever use, or you have to retranslate *everything* whenever you add a new extension.

Developing cross-platform, cross-architecture, stuff with ctypes can also be a lot more painful than writing extensions using the Python C API (and having the compiler make some decisions at compile time rather than having to do it all at runtime).

Robert Kern said...

Most of python-dev's "antipathy" towards using ctypes is focused on using ctypes for stdlib modules, not on general principles. For security, stability, and portability reasons, many platforms need to disable ctypes when they build Python. Consequently, there is a policy that no stdlib module can use ctypes. They are not recommending against using ctypes in general.

Anonymous said...

One major worry is how well you will end up tracking NumPy development. Will you evenutally add an "RPython glue" subdir to NumPy's distribution?

cournape said...

thanks for the follow-up. I won't argue with points 1 and 3, but I think 2 can be reasonably addressed: I don't think the usage of internal details is pervasive in the code, and most of it is for historical reasons. We cannot remove them altogether from the numpy headers for backward compatibility reasons, but we can replace most of it inside numpy itself.

I am still a bit confused though: from your description, it seems that you intend to fork numpy to replace some pieces from C to RPython, but if I look at the numpy-ext branch, I see a rewrite of numpy in rpython. Maybe you are talking about another code ?

Anonymous said...

I think that the most important part of numpy is array operations (indexing, +-*/, broadcasting, etc). So it would be good enough to implement only array class in RPython and call to numpy using ctypes/cpyext for all other stuff. I've read somewhere about the plans to impose separation between numpy and scipy so numpy holds only implementation of fast arrays and scipy will hold all non-trivial operations on them. IMHO such separation will be ideal for pypy too.

Wladimir said...

Thanks for the clear explanation. I really wondered why it was so hard to re-use the existing numpy.

Antoine P. said...

Thanks Robert for clarifying our position :)

Another issue with ctypes is that it doesn't work on all systems.

Yet another issue with ctypes is that it is currently unmaintained (which won't help fixing portability issues :-)).

Anonymous said...

I am sory for the silly question, but how do I install this module in an existing pypy instalation ?

Thanks for the great job !

Anonymous said...

OK I see ...

hg clone https://bitbucket.org/pypy/pypy/src/numpy-exp .....

Anonymous said...

I like the idea of reimplementing part of Numpy in pypy to leverage the JIT in pypy. The existence of numexpr demonstrates the deficiency of Numpy as a Python library. A JIT is much more appropriate for what effectively should be a DSL.

But I would recommend something grander, perhaps for the longer term. I think if pypy could produce do for Python what McVM and McJIT propose to do for Matlab, it would be game-changing for Python and pypy. It would make pypy not only competitive with Matlab in ways that Numpy and Scipy are not yet and may never be, but also with F#. The rapid uptake of F# in financial industry in particular, despite the availability of Matlab, showcases the need for a fast prototyping language that does not rely on calling Fortran code for speed. I know I am looking for such language; Numpy and Python simply don't offer enough power and flexibility. I hope I can choose pypy.

Anonymous said...

Any idea about an eta on merging the jitypes2 branch (and/or a little more info on what it does to speed ctypes up so much)?

Antonio Cuni said...

@anonymous: the jitypes2 branch is mostly ready, but we are hunting two bugs and won't be merged until we fix them.

The speedup comes from the fact that ctypes call are seen by the JIT, and directly compiled into a call to the corresponding C function. Thus, most of the overhead of ctypes itself is optimized away.

CLM said...

I wonder if an RPython/cython backend might be possible. cython is already my favorite way to write CExtensions and it generates code for both python 2.x and 3.x. It would be great if it could be adapted for PyPy extensions.

Anonymous said...

Hi!

Thanks a lot for the previous post and the follow up! I really appreciate that you could find time to make a write up on the progress that you made so far on this extremely important feature.

This all sounds very cool, but also to me it seems that it's very important to work with NumPy / SciPy developers, so that the parts that have to be replaced would be isolated and maintained in parallel for RPython and C API, or rewritten in ctypes (not sure if this is even possible). This way this eternal catch-up trap that many seem to be afraid of will not happen.

Also, I wonder in how much money this would actually translate. Maybe Enthought could sponsor some development...

Regarding Cython... I also use it to write trivial extensions to implement computation kernels outside Python in C. It would be great if Cython were able to generate something that would work with PyPy as well...

Thanks!

Laura Creighton said...

CLM:We actually have a GSoC student proposal from Romain Guillebert to
investigate this idea.

Maciej Fijalkowski said...

@Anonymous the idea is that you should not use Cython at all and PyPy's JIT should handle the computational kernel just fine.

Anonymous said...

I don't know why do you decide to use ctypes - in numpy community it is considered as obsolete already for a long time (maybe several years), is not under active development, and now Cython is recommended by default tool for it:

http://docs.scipy.org/doc/numpy/user/misc.html?highlight=cython#interfacing-to-c

Also, I guess you could search for some volunteers to work on numpy-PYPY in numpy-user, scipy-user, scipy-dev mail lists.

I'm interested in operations like hstack, vstack, max, min, argmin, nanmax, nanargmin (along a given axis) etc - are they already available? Or when it will be done?