Converting form .m to .mex is no longer efficient?
in [Matlab]
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From: James Tursa on 29 Jun 2010 18:54 "Max " <gm.pantagruel(a)gmail.com> wrote in message <i0ds1o$7p5$1(a)fred.mathworks.com>... > > I can't run the requested test since the earliest version I have is R2006b and mcc -x isn't >supported. However, I don't see anything in your constructor that would lead me to >believe that a compiled version would run any faster. All you are doing is creating a >structure and then tagging that structure with a class name ... neither of these activities >is going to run any faster in a mex routine. And the m-file version runs so fast (0.000074 >sec) that any tic-toc timing is pointless. > > > > James Tursa > > Thank you James, you made me look into it again and realize that I forget to mention one important thing. I was getting 0.001505 sec just because I had 'clear all' statement before 'tic', so the actual code was: > > clear all; > a=rand(1,10); > tic; > b = myclass(a); > toc; > > And if you add 'clear all' you'll probably also have something around 0.001 sec. Very strange behavior. Even though it is still a complete mystery to me I discovered that by changing from 'clear all' to 'clear variables' I could get timings similar to yours ( around 0.00007 sec). > > And now everything makes more sense. I started from the simple test: > > tic;my_empty_class(a);toc; > tic;some_mex_class(a);toc; > > and was puzzled after getting: > > 0.001505 sec - my 'empty' class > 0.000122 sec - external mex class from 3rd party commercial library > > The fact that my empty class was more that 10 times slower made me think that by converting to mex I could gain performance improvement. But now I believe that mex files treat 'clear all' statement somewhat differently and that is what makes such a big difference in timings between plain .m file and its .mex version in my case. One important difference between calling m-files and calling mex-files is the following: - For m-files, MATLAB creates a temporary shared data copy of the argument(s) and passes the addresses of these to the m-function. So there is some overhead in creating the shared data copies. - For mex files, MATLAB passes the addresses of the original argument(s) to the mex-function. There is *no* extra overhead in creating shared data copies involved. I suspect that it is simply a slight timing difference between creating & clearing these temporary shared data copies that is showing up in your timing tests. e.g., for these two tests: clear all tic b = somefunction(some arguments); toc vs this b = something; tic b = somefunction(some arguments); toc The timings will be different. That is because the 2nd example has to clear the pre-existing b before it can assign it a new value. That extra overhead will show up in the tic-toc timing. Since the actual time associated with this overhead should always be quite small, I probably wouldn't worry about it in the grand scheme of things. Other things going on inside your class constructors & member functions will likely swamp that tiny amount as far as timing goes. I would advise spending your time optimizing other parts of your code. James Tursa |