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The number of CPU Threads defines the number of parallel used CPU cores. The threads are used to build up the FE-matrix and the defined value is important for CPU as well as GPU computing. It is recommended to leave at least 1 or 2 threads unused for the operating system.
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You can choose between auto, where the program decides how many threads are best suited for your system, and manual, where you can decide by yourself how many threads should be used. 2-8 threads have proven to reach good performance values (depending on your hardware setup and the optimization optimisation model).