Terrible FPU performance

[David Cournapeau]

On Wed, Apr 27, 2011 at 4:14 AM, Dan Goodman <dg.gmane at thesamovar.net> wrote:
> Hi,
>
> On 26/04/2011 15:40, Mihai Badoiu wrote:
>> I have terrible performance for multiplication when one number gets very
>> close to zero.  I'm using cython by writing the following code:
>
> This might be an issue with denormal numbers:
>
> http://en.wikipedia.org/wiki/Denormal_number
>
> I don't know much about them though, so I can't advise any further than
> that...

This indeed sounds like it. Mihai, which CPU are you using ? Pentium4
are especially known to have terrible (read order of magnitude slower)
performance with denormal numbers.

There is unfortunately no simple way to know whether a float is
denormal or not in python, but since you are using cython, if you are
under posix you should be able to use fpclassify to check this,

>From there, if you see a difference between cython/python and C, it
will be easier to debug.

cheers,

David