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I am using a Windows machine with 8 cores. No other process than Mathematica 8 is using CPU time right now - still the MathKernel.exe CPU usage does not go beyond 13%. My current computation takes forever and I feel that might be the reason. How can I adjust the CPU usage of Mathematica to use, say, up to 90% of CPU?

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It sounds like you're using 1 processor. Try things with Parallelize to see if you can use all cores. –  b.gatessucks Apr 22 '13 at 10:27
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13% means that it uses the 100% of one of your 8 cores (or one of the thread in case of hyperthreding). Some core functions of Mathematica might use more than one core but if you want actively to use more cores you might need to enable this functionality and use commands that can parallelize. Look at the manual for parallelize for more. There is a tab at Mathematica's options panel dedicated to parallelization look there too. –  Spawn1701D Apr 22 '13 at 10:28
    
Might look up functions like ParallelTable or ParallelDo. –  Sjoerd C. de Vries Apr 22 '13 at 15:30
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Although the previous commentators are undoubtedly correct, you also need to know that efficient parallelization is not simply a matter of adjusting some option setting, and the fact that Mathematica is attacking a problem serially is a good indication that parallelization may be a highly non-trivial undertaking in this case. In general, you might have to use different algorithms, explicitly decompose the problem into multiple concurrent tasks, or even rewrite your code completely according to an entirely different approach. But without further details it's difficult to make recommendations. –  Oleksandr R. Apr 22 '13 at 15:31
    
The problem at hand was a really big expression involving trigonometric functions which had to be FullSimplified. Probably, in such a case one has to look for workarounds... –  Kagaratsch Apr 22 '13 at 21:23

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