Millis and os

Hi everyone,I am constructing a system which has ui and some data transfer etc..I am using arduino uno and with millis library ,it has some delays so ui has delays. I have readed about pre-emptive and co-operative OS .is The arduino`s millis is pre-emptive or co-operative. is there any other tecnique to reduce this delays. thank you

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I know how to use millis actually my question is, if there is a solution for reducing the delay of tasks in main loop.because when transferring data, displaying function freezes until transfer ends.

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Because there is no operating system in Arduino, dividing processor time "fairly" between different tasks is a common problem. Task division must be explicit in the code in Arduino rather than inherent in the OS. This thread gives a detailed example of how one might partition subtasks and make the pieces run at particular intervals: https://forum.arduino.cc/index.php?topic=223286.0