Really need help to speed up piece of code
in [Matlab]
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From: David Romero-Antequera on 5 Aug 2010 10:48 "Steven_Lord" <slord(a)mathworks.com> wrote in message <i3eghr$2l2$1(a)fred.mathworks.com>... > > > "Jan Simon" <matlab.THIS_YEAR(a)nMINUSsimon.de> wrote in message > news:i3clg8$d8r$1(a)fred.mathworks.com... > > Dear David, > > > >> Most of the time is spent in computing the fft2 and ifft2. So I guess > >> there is not much that I can do here :( > > > > Why not? Create a copy of IFFT2 and look, what could be omitted: > > - error(nargchk(1, 4, nargin, 'struct')) - remove it! > > - Usage of the powerful but slow VARARGIN? Replace it by the hard coded > > inputs. > > - "[m_in, n_in] = size(f)" is faster than calling SIZE twice > > - numel(varargin) - not needed, you know the number of inputs already > > - Omit the check for 'nonsymmetric' > > - No need to cast the inputs to DOUBLE > > - Use the debugger to check, if the (m_out~=m_in) part is used at all > > - Finally the acutual calculation is "ifft(ifft(...". > > If you can reduce the number of lines to 6, inline them to your function, > > because calling a funktion has a certain overhead ever. > > > > Do the same for FFT2, which is unfortunately very lean already. > > > > Do not hesitate to dig in Matlab's toolbox functions. You can accelerate > > even trivial functions like NOW, FILEPARTS, BLANKS and IND2RGB. > > But if you choose to do this, make sure you make a backup copy of the > unmodified function and/or copy the function to a file with a different name > and change the new file. While ripping out error checking code and code to > handle the most general cases can improve performance, those sections of > code weren't in there simply to take up space in the file -- the function > will no longer have a "safety net" to catch you with a (hopefully > informative) error message if you make a mistake. > > -- > Steve Lord > slord(a)mathworks.com > comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ > To contact Technical Support use the Contact Us link on > http://www.mathworks.com Yeah, I realized this. As this is a very special case for this routine only, I've created a new "local" function in the same file as the main function. Thanks for the advice anyways.
From: David Romero-Antequera on 5 Aug 2010 10:51
Walter Roberson <roberson(a)hushmail.com> wrote in message <i3cmcr$r42$1(a)canopus.cc.umanitoba.ca>... > David Romero-Antequera wrote: > > Is there any starting point to read about the accuracy? > > http://docs.sun.com/source/806-3568/ncg_goldberg.html > > http://docs.sun.com/source/806-3568/ncg_math.html > > > For example, how do I know how accurate is the > > exp function in MATLAB? > > I am finding mixed information on that. One recent IEEE paper about arbitrary > precision indicates that IEEE 754 does not mandate any precision for the > transcendential functions; the other places I look indicate that IEEE 754 > requires the calculation to be correct down to the last bit. > > A paper with several different implementations of exp() can be found at > http://www.convict.lu/Jeunes/ultimate_stuff/exp_ln_2.htm I have to made some more testing, specially to see if I am not compromising the accuracy of the whole computation. However, up to now, the implementation of the modified exponential have saved me TWO THIRDS of the time!!! WOW! I mean, I know that this is probably best case scenario, but... WOW!!!! |