Hello, I have a question about `Dispatchers`. I cr...

# announcements

[Ka Wing Chin]

Hello, I have a question about Dispatchers. I created an endless loop to create threads and see how the program will behave.

class MainActivity : AppCompatActivity() {

    private val dispatcher: CoroutineContext = <http://Dispatchers.IO|Dispatchers.IO>

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val cores = Runtime.getRuntime().availableProcessors()
        println("Cores: $cores")
        println("===============================")

        endlessLoop()
    }

    fun endlessLoop() = runBlocking {
        withContext(dispatcher) {
            println(Thread.currentThread().name)
            while(true) {
                launch {
                    println(Thread.currentThread().name)
                    while(true) {
                        // endless loop
                    }
                }
            }
        }
    }
}

I got a phone with 4 cores and each core can handle 4 threads.
If I would run this Dispatchers.Default. Then output will be:

DefaultDispatcher-worker-1
DefaultDispatcher-worker-2
DefaultDispatcher-worker-3
DefaultDispatcher-worker-4

I get this, because all cores got occupied and working at 100% to get it done and no more tasks are allowed to be added.

However if I run it with <http://Dispatchers.IO|Dispatchers.IO> which has 64 threads in the pool, it will run all 64 threads.
I though it was 4 cores * 4 threads = 16 threads max. Can someone explain me why the program doesn't stop at DefaultDispatcher-worker-16?

[Adam Powell]

The io dispatcher is designed to run tasks that are going to call blocking platform APIs that will block a thread but not consume cpu resources since the kernel won't schedule the thread to run again until the io operation completes

[Adam Powell]

So it's ok to have many more of these operations pending than you have available cpu cores

[Adam Powell]

Things like reading a file or socket