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Additional threads has no effect on T-Spice performance
T-Spice shows no difference in performance with threads=4 and threads=16
There are two things in T-Spice that are multi-threaded; model evaluation at each step (i.e. creating of the matrix to solve) and the Matrix Solution functions used to solve the matrix (get the result for all voltages and currents at each step). Due to overhead with multi-threading, there is a limit to performance improvements by adding threads. Typically, after threads=3 or threads=4, there is little improvement in performance. In T-Spice, multi-threading affects only one sweep point/Monte Carlo at time and that is why performance rolls off at 4 threads.