I'm using intel xeon harpertown (E5450) and Python 2.6.4. In the cython code, when I use fpclassify, in the slow case I get 3 (FP_SUBNORMAL) In the pure-C code, when I use fpclassify, in the case that's supposed to be slow but it's not, I get a 2 (FP_ZERO)
Somehow, the FPU's have different results for exactly the same asm code. --mihai On Tue, Apr 26, 2011 at 11:06 PM, David Cournapeau <courn...@gmail.com>wrote: > On Wed, Apr 27, 2011 at 4:14 AM, Dan Goodman <dg.gm...@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 > -- > http://mail.python.org/mailman/listinfo/python-list >
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