You pointed out how these methods effect the computation thread.
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I hope you are asking where to use the computation thread and where to use the IO thread. Computation thread is used to do any compute intensive work like loops etc. IO is used for web-service calls other IO tasks. The main difference between these 2 are that the Computation thread is backed by a bounded thread-pool with size equal to the number of available processors where the IO thread is backed by an unbounded thread-pool.