[Numpy-discussion] lazy evaluation

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Wed Jun 6 05:22:28 EDT 2012

On 06/06/2012 12:06 AM, mark florisson wrote:
> On 5 June 2012 22:36, Dag Sverre Seljebotn<d.s.seljebotn at astro.uio.no>  wrote:
>> On 06/05/2012 10:47 PM, mark florisson wrote:
>>> On 5 June 2012 20:17, Nathaniel Smith<njs at pobox.com>    wrote:
>>>> On Tue, Jun 5, 2012 at 7:08 PM, mark florisson
>>>> <markflorisson88 at gmail.com>    wrote:
>>>>> On 5 June 2012 17:38, Nathaniel Smith<njs at pobox.com>    wrote:
>>>>>> On Tue, Jun 5, 2012 at 4:12 PM, mark florisson
>>>>>> <markflorisson88 at gmail.com>    wrote:
>>>>>>> On 5 June 2012 14:58, Nathaniel Smith<njs at pobox.com>    wrote:
>>>>>>>> On Tue, Jun 5, 2012 at 12:55 PM, mark florisson
>>>>>>>> <markflorisson88 at gmail.com>    wrote:
>>>>>>>>> It would be great if we implement the NEP listed above, but with a few
>>>>>>>>> extensions. I think Numpy should handle the lazy evaluation part, and
>>>>>>>>> determine when expressions should be evaluated, etc. However, for each
>>>>>>>>> user operation, Numpy will call back a user-installed hook
>>>>>>>>> implementing some interface, to allow various packages to provide
>>>>>>>>> their own hooks to evaluate vector operations however they want. This
>>>>>>>>> will include packages such as Theano, which could run things on the
>>>>>>>>> GPU, Numexpr, and in the future
>>>>>>>>> https://github.com/markflorisson88/minivect (which will likely have an
>>>>>>>>> LLVM backend in the future, and possibly integrated with Numba to
>>>>>>>>> allow inlining of numba ufuncs). The project above tries to bring
>>>>>>>>> together all the different array expression compilers together in a
>>>>>>>>> single framework, to provide efficient array expressions specialized
>>>>>>>>> for any data layout (nditer on steroids if you will, with SIMD,
>>>>>>>>> threaded and inlining capabilities).
>>>>>>>> A global hook sounds ugly and hard to control -- it's hard to tell
>>>>>>>> which operations should be deferred and which should be forced, etc.
>>>>>>> Yes, but for the user the difference should not be visible (unless
>>>>>>> operations can raise exceptions, in which case you choose the safe
>>>>>>> path, or let the user configure what to do).
>>>>>>>> While it would be less magical, I think a more explicit API would in
>>>>>>>> the end be easier to use... something like
>>>>>>>>    a, b, c, d = deferred([a, b, c, d])
>>>>>>>>    e = a + b * c  # 'e' is a deferred object too
>>>>>>>>    f = np.dot(e, d)  # so is 'f'
>>>>>>>>    g = force(f)  # 'g' is an ndarray
>>>>>>>>    # or
>>>>>>>>    force(f, out=g)
>>>>>>>> But at that point, this could easily be an external library, right?
>>>>>>>> All we'd need from numpy would be some way for external types to
>>>>>>>> override the evaluation of ufuncs, np.dot, etc.? We've recently seen
>>>>>>>> several reasons to want that functionality, and it seems like
>>>>>>>> developing these "improved numexpr" ideas would be much easier if they
>>>>>>>> didn't require doing deep surgery to numpy itself...
>>>>>>> Definitely, but besides monkey-patch-chaining I think some
>>>>>>> modifications would be required, but they would be reasonably simple.
>>>>>>> Most of the functionality would be handled in one function, which most
>>>>>>> ufuncs (the ones you care about, as well as ufunc (methods) like add)
>>>>>>> call. E.g. if ((result = NPy_LazyEval("add", op1, op2)) return result;
>>>>>>> , which is inserted after argument unpacking and sanity checking. You
>>>>>>> could also do a per-module hook, and have the function look at
>>>>>>> sys._getframe(1).f_globals, but that is fragile and won't work from C
>>>>>>> or Cython code.
>>>>>>> How did you have overrides in mind?
>>>>>> My vague idea is that core numpy operations are about as fundamental
>>>>>> for scientific users as the Python builtin operations are, so they
>>>>>> should probably be overrideable in a similar way. So we'd teach numpy
>>>>>> functions to check for methods named like "__numpy_ufunc__" or
>>>>>> "__numpy_dot__" and let themselves be overridden if found. Like how
>>>>>> __gt__ and __add__ and stuff work. Or something along those lines.
>>>>>>> I also found this thread:
>>>>>>> http://mail.scipy.org/pipermail/numpy-discussion/2011-June/056945.html
>>>>>>> , but I think you want more than just to override ufuncs, you want
>>>>>>> numpy to govern when stuff is allowed to be lazy and when stuff should
>>>>>>> be evaluated (e.g. when it is indexed, slice assigned (although that
>>>>>>> itself may also be lazy), etc). You don't want some funny object back
>>>>>>> that doesn't work with things which are not overridden in numpy.
>>>>>> My point is that probably numpy should *not* govern the decision about
>>>>>> what stuff should be lazy and what should be evaluated; that should be
>>>>>> governed by some combination of the user and
>>>>>> Numba/Theano/minivect/whatever. The toy API I sketched out would make
>>>>>> those decisions obvious and explicit. (And if the funny objects had an
>>>>>> __array_interface__ attribute that automatically forced evaluation
>>>>>> when accessed, then they'd work fine with code that was expecting an
>>>>>> array, or if they were assigned to a "real" ndarray, etc.)
>>>>> That's disappointing though, since the performance drawbacks can
>>>>> severely limit the usefulness for people with big data sets. Ideally,
>>>>> you would take your intuitive numpy code, and make it go fast, without
>>>>> jumping through hoops. Numpypy has lazy evaluation,  I don't know how
>>>>> good a job it does, but it does mean you can finally get fast numpy
>>>>> code in an intuitive way (and even run it on a GPU if that is possible
>>>>> and beneficial).
>>>> All of these proposals require the user to jump through hoops -- the
>>>> deferred-ufunc NEP has the extra 'with deferredstate' thing, and more
>>>> importantly, a set of rules that people have to learn and keep in mind
>>>> for which numpy operations are affected, which ones aren't, which
>>>> operations can't be performed while deferredstate is True, etc. So
>>>> this has two problems: (1) these rules are opaque, (2) it's far from
>>>> clear what the rules should be.
>>> Right, I guess I should have commented on that. I don't think the
>>> deferredstate stuff is needed at all, execution can always be deferred
>>> as long as it does not affect semantics. So if something is marked
>>> readonly because it is used in an expression and then written to, you
>>> evaluate the expression and then perform the write. The only way to
>>> break stuff, I think, would be to use pointers through the buffer
>>> interface or PyArray_DATA and not respect the sudden readonly
>>> property. A deferred expression is only evaluated once in any valid
>>> GIL-holding context (so it shouldn't break threads either).
>> I think Nathaniel's point is that the point where you get a 10-second
>> pause to wait for computation is part of the semantics of current NumPy:
>> print 'Starting computation'
>> z = (x + y).sum()
>> print 'Computation done'
>> print 'Result was', z
>> I think that if this wasn't the case, newbies would be be tripped up a
>> lot and things would feel a lot less intuitive. Certainly when working
>> from the IPython command line.
>> Also, to remain sane in IPython (or when using a debugger, etc.), I'd want
>> "print z"
>> to print something like "unevaluated array", not to trigger a
>> computation. Same with str(z) and so on.
> I guess you could detect that at runtime, or just make it
> configurable. As for triggering computation somewhere else, I guess I
> find it preferable to horrible performance :)

My problem might be that I don't use NumPy wherever I need performance 
(except as a glorified double*, i.e. I don't use it for computation). 
NumPy is for interactive "play with a reduced dataset" work.

>> I don't think a context manager modifying thread-local global state like
>> with np.lazy:
>>      ...
>> would be horribly intrusive.
>> But I also think it'd be good to start with being very explicit (x =
>> np.lazy_multiply(a, b); compute(x)) -- such an API should be available
>> anyway -- and then have the discussion once that works.
> Maybe that's the best way forward. I guess I'd prefer an import
> numpy.lazy_numpy as numpy in that case. I don't really like the with
> statement here, since ideally you'd just experiment with swapping in
> another module and see if your code still runs fine.

Or just "import lazyarray as np". As I said, I think it's important to 
refactor NumPy so that things can happen outside of the NumPy project.

NumPy needs to be very conservative. You've seen the recent NA semantics 
debate. If NumPy was to decide on *the* final blessed semantics for lazy 
evaluation, even as an experimental sub-module, you'd never see the end 
of it.

One part of this is a polymorphic C API targeted for lazy evaluation and 
get current NumPy to support that. Another part is, as Nathaniel has 
commented, making things like "np.dot" have some kind of polymorphic 
dispatch-on-the-objects behaviour.

(I'd like something based on multiple dispatch rather than just calling 
something on the left operand. Then use that multiple dispatch for 
implementing +, - and so on as well when you want anything to interact 
with NumPy arrays.)


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