[Numpy-discussion] Interesting timing results

Sasha ndarray at mac.com
Thu Jan 19 12:57:12 CST 2006


On 1/17/06, Travis Oliphant <oliphant.travis at ieee.org> wrote:
> ...                        Currently all array scalar math goes through the
> entire ufunc machinery.  This is clearly sub-optimal.   It is the reason
> for the scalarmath module that I've started in the src directory.
> Eventually, we should be able to have scalar math as fast as Python
> scalars.

I have implemented "nonzero", &, | and ^ for scalar bools. (See
http://projects.scipy.org/scipy/numpy/changeset/1946). Here are the
timings:

Before:
> python -m timeit -s "from numpy import bool_; x = bool_(0)" "x & x"
100000 loops, best of 3: 3.85 usec per loop

Now:
> python -m timeit -s "from numpy import bool_; x = bool_(0)" "x & x"
10000000 loops, best of 3: 0.174 usec per loop

This is close to python bool:
> python -m timeit -s "x = bool(0)" "x & x"
10000000 loops, best of 3: 0.142 usec per loop

and faster than python int:
> python -m timeit -s "from numpy import bool_; x = 0" "x & x"
10000000 loops, best of 3: 0.199 usec per loop

But it is in fact all within the timing error now.

I did not put it in the scalarmath module for two reasons: first,
scalarmath is not hooked to numpy yet and second because C-API does
not provide access to scalar bools yet.  I have posted a proposal for
C-API changes and did not hear any opposition (well, no support
either), so I will implement that soon.

There are a few issues with the new APIs that I proposed.  First is a
simple one: I proposed to expose boolean scalars as named constants to
Python, the question is
how to call them.

1. True_ and False_
2. true and false

The second issue is whether to add new numbers to _ARRAY_API or add a
separate _ARRAY_SCALAR_API .

I've only optimized scalar-scalar case in binary ops and fall back to
old for everything else.  I would like to optimize scalar-array and
array-scalar cases as well, but I wonder if it is apropriate to make
"(bool_(0) | x) is x" true when x is an array. Alternatively 
(bool_(0) | x)  can become a new array that shares data with x.

-- sasha




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