A problem with threads is that they are a finite resource expensive in terms of CPU capabilities and system overhead. In the background, a lot of work is involved in creating, scheduling and destroying a thread. Although modern CPUs are able to run large numbers of threads, the actual number of threads that can be run in parallel at any one time is limited by the number of CPU cores (though newer CPUs have 8 cores, most Android devices contain CPUs with 4 cores). When more threads are required than there CPU cores, the system has to perform thread scheduling to decide how execution of these threads is to be shared between the available cores.
To avoid these overheads, instead of starting a new thread for each coroutine and then destroying it when the coroutine exits, Kotlin maintains a pool of active threads and manages how coroutines are assigned to those threads. When an active coroutine is suspended it is saved by the Kotlin runtime and another coroutine...
