mldivide take too much time

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yusuf albadry 2020-3-14

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回答： Precise Simulation 2020-5-5

Hi guys,

I am solving compressible NS using FEM , and I wrote my solver using MATLAB.

currently I have a problem with the running time ( 90x30 grid ). One time step takes about 22 seconds, 20 seconds of them are in the inverse line ! I dont know why it takes this much time !.

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John D'Errico 2020-3-14

Computers are not infinitely fast. People seem to think they are, but not so. Perhaps you watch too much TV, where the computers are always able to do amazing things in a fraction of a pico-second, when the user only had to click a couple of keys on the keyboard.

Since we cannot see into your computer, we cannot offer any useful advice. If you are unhappy with the speed of your code, then you could also get a faster computer, or change the overall algorithm (mldivide is, as has been said, already about as fast as it can be) or use less data (expect even worse results, since using less data is never a good idea.)

Changing the overall algorithm may mean to completely change what you are doing. That is, if mldivide is taking too much time for you, then restructure things so it needs to call mldivide less often.

Or, the answer may simply be to sit down with a good book and a cup of coffee, while the existing code finishes its work. What good book could I suggest, one that would be most appropriate here? Any good text on numerical methods in computing would be a good start, since it would help you to understand the algorithms involved.

Walter Roberson 2020-3-15

https://www.mathworks.com/help/matlab/ref/mldivide.html#bt4jslc-6

If your matrix is real-valued then if it is symmetric then it is also Hermitian -- equal to its complex transpose.

In such a case if the diagonal is all positive or all negative, then you might be able to use cholesky; if the diagonal does not meet that condition then LDL is used.

yusuf albadry 2020-3-15

thank you Walter, for helping, it decreased the time to 50% :)

Thanks for all of you

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回答（2 个）

Walter Roberson 2020-3-18

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To summarize: the most important issue was that the poster had a Ryzen, which is at a disadvantage when using the Intel high performance Math library MKL. Fortunately a user has posted information on how to speed it up; https://www.mathworks.com/matlabcentral/answers/396296-what-if-anything-can-be-done-to-optimize-performance-for-modern-amd-cpu-s#answer_401963 and when the poster did that, they doubled performance.

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Precise Simulation 2020-5-5

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You can also try the MUMPS linear solver which is also is up to 30-50% faster than mldivide (Umfpack) in serial mode (and also supports parallel), and is available as pre-compiled Matlab Mex files with the FEATool Multiphysics toolbox

https://github.com/precise-simulation/featool-multiphysics/tree/master/lib/mumps