Here is Jim's e-mail for more details:
It is best to create the wisdom ahead of time using the stand alone script
pyami/fft/fftwsetup.py. It will store the wisdom you create in your home
directory, but you can then copy it to any other home directory that wants
to use it, or copy it as root to a file called /etc/fftw/wisdom which any
user can access.
Using fftwsetup.py:
I just committed a fix to svn so it prints the usage message properly.
usage: ./fftwsetup.py N shape0 shape1
Make sure to use shape0 and shape1 as in numpy dimensions (row,col) not (X,Y)
N is the number of iterations. You only have to run 1 iteration to
generate wisdom. The only reason to run 2 or more is just to have it
verify that it actually runs faster after the first iteration, because only
the first iteration will have to generate the wisdom. Other iterations
will use that wisdom and be faster.
After you run fftwsetup.py, it will print out a timing report showing the
time it took to generate wisdom and run the fft for each iteration. It
will also save the new wisdom to your home directory as:
fftw3-wisdom-hostname
Every time you run fftwsetup.py, it will first read the home directory
wisdom if it exists, and also the /etc/fftw/wisdom if it exists. It will
combine those, plus the new wisdom it creates, and store it back to the
home directory wisdom. If you want /etc/fftw/wisdom to contain the new
wisdom, be sure to copy the new wisdom file from the home directory to
there (has to be called "wisdom" in /etc/fftw). Or if you are always sure
to run as root, then no need to copy to /etc/fftw and always just use the
home directory wisdom.
Power of two sized images will be faster to create wisdom, but odd sized
images will take longer. For example on defcon1 (4 core Xeon 2.5 GHz):
./fftwsetup.py 2 8192 8192
-> takes about 15 seconds to create wisdom the first time
-> if wisdom is already created, takes about 2 seconds to do fft
./fftwsetup.py 2 8191 8191
-> takes about 40 seconds to create wisdom the first time
-> if wisdom is already created, takes about 5 seconds to do fft