Does multithreading always yield better performance than single threading?

Question

I know the answer is No, here is an example Why single thread is faster than multithreading in Java? .

So when processing a task in a thread is triv

Answer 1

As already mentionened in a comment by @Jim Mischel, you can use

Amdahl's law

to calculate this. Amdahl's law states that the speedup gained from adding processors to solve a task is

where

N is the number of processors, and

P is the fraction of the code that can be executed in parallel (0 .. 1)

Now if T is the time it takes to execute the task on a single processor, and O is the total 'overhead' time (create and set up a second thread, communication, ...), a single thread is faster if

T < T/S(2) + O

or, after reordering, if

O/T > P/2

When the ratio Overhead / Execution Time is greater than P/2, a single thread is faster.