[Psyco-devel] RE: [Numpy-discussion] Interesting Psyco/Numeric/Numarray comparison

Tim Hochberg tim.hochberg at ieee.org
Wed Feb 5 08:54:05 CST 2003


Perry Greenfield wrote:

>The "psymeric" results are indeed interesting. However, I'd like to
>make some remarks about numarray benchmarks. At this stage, most of
>the focus has been on large, contiguous array performance (and as
>can be seen that is where numarray does best). There are a number
>of other improvements that can and will be made to numarray performance
>so some of the other benchmarks are bound to improve (how much is
>uncertain). For example, the current behavior with strided arrays
>results in looping over subblocks of the array, and that looping is
>done on relatively small blocks in Python. We haven't done any tuning
>yet to see what the optimum size of block should be (it may be machine
>dependent as well), and it is likely that the loop will eventually be
>moved into C. Small array performance should improve quite a bit, we
>are looking into how to do that now and should have a better idea
>soon of whether we can beat Numeric's performance or not.
>  
>
I fully expect numarray to beat Numeric for large arrays eventually just 
based on the fact the psymeric tends to be slightly faster Numeric now 
for many cases. However, for small arrays it seems that you're likely to 
be fighting the function call overhead of Python unless you go 
completely, or nearly completely, to C. But that would be a shame as it 
would make modifying/extending numarray that much harder.

>But "psymeric" approach raises an obvious question (implied I guess, but
>not explicitly stated). With Psyco, is there a need for Numeric or
>numarray at all? I haven't thought this through in great detail, but at
>least one issue seems tough to address in this approach, and that is
>handling numeric types not supported by Python (e.g., Int8, Int16 UInt16,
>Float32, etc.).  Are you offering the possiblity of the "pysmeric"
>approach as being the right way to go,
>  
>
I think there are too many open questions at this point to be a serious 
contender. It's interesting enough and the payoff would be big enough 
that I think it's worth throwing out some of the questions and see if 
anything interesting pops out.

> and if so, how would you handle
>this issue?
>  
>

The types issue may not be a problem. Python's array.array supports a 
full set of types 
(http://www.python.org/doc/current/lib/module-array.html). However, 
psyco does not currently support fast operations on types 'f', 'I' and 
'L'. I don't know if this is a technical problem, or something that's 
likely to be resolved in time. The 'f' (Float32) case is critical, the 
others less so.

Armin, if you're reading this perhaps you'd like to comment?

>On the other hand, there are lots of algorithms that cannot be handled
>well with array manipulations.
>
This is where the Psyco approach would shine. One occasionally runs into 
cases where some part of the computation just cannot be done naturaly 
with array operations. A common case is the equivalent of this bit of C 
code: "A[i] = (C[i]<TOL) ? B[i]+5 : C[i]-5". This can be done using 
take, but it requires a bunch of extra memory (3 arrays worth) and 
calculations. In principle at least this could be done using psymeric in 
a more natural way without the extra memory and calculations.

> It would seem that psyco would be a natural
>alternative in such cases (as long as one is content to use Float64 or
>Int32), but it isn't obivious that these require arrays as anything but
>data structures (e.g. places to obtain and store scalars).
>  
>
That's not been my experience. When I've run into awkward cases like 
this it's been in situations where nearly all of my computations could 
be vectorized.

Anyway, here are what I see as the issues with this type of approach:

* Types: I believe that this should not be a problem

* Interfacing with C/Fortran:  This seems necessary for any Numeric 
wannabe. It seems that it must be possible, but it may require a bit of 
C-code, so it may not be possible to get completely away from C.

* Speed: It's not clear to me at this point whether psymeric would get 
any faster than it currently is. It's pretty fast now, but the factor of 
two difference between it and numarray for contiguous arrays (a common 
case) is nothing to sneeze at.

Cross-platform: This is the reall killer. Psyco only runs on x86 
machines. I don't know if or when that's likely to change. Not being 
cross platform seems nix this from being a serious contender as a 
Numeric replacement for the time being.


-tim









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