Terrible FPU performance

Alec Taylor alec.taylor6 at gmail.com
Wed Apr 27 07:11:40 CEST 2011


What's an FPU?

On Tue, Apr 26, 2011 at 11:40 PM, Mihai Badoiu <mbadoiu at gmail.com> wrote:
> Hi,
> I have terrible performance for multiplication when one number gets very
> close to zero.  I'm using cython by writing the following code:
>     cdef int i
>     cdef double x = 1.0
>     for 0 <= i < 10000000:
>         x *= 0.8
>         #x += 0.01
>     print x
> This code runs much much slower (20+ times slower) with the line x += 0.01
> uncommented.  I looked at the deassembled code and it looks correct.
>  Moreover, it's just a few lines and by writing a C code (without python on
> top), I get the same code, but it's much faster.  I've also tried using sse,
> but I get exactly the same behavior.  The best candidate that I see so far
> is that Python sets up the FPU in a different state than C.
> Any advice on how to solve this performance problem?
> thanks!
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>



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