kotlin - Why Dispatchers.Default runs faster than Dispatchers.IO in coroutines for simple iteration when launched 100 coroutines

I was trying to understand if I use Dispatchers.IO instead of Dispatchers.Default using this code

runBlocking {
            val list = mutableListOf<Job>()
            val time1 = System.currentTimeMillis()
            for (i in 1..100){
                val job = lifecycleScope.launch(Dispatchers.IO) {
                    val c = "c$i"
                    Log.i("CoroutineRunnerLog", "Launched a coroutine $c on Thread : ${Thread.currentThread().name}")
                    for (j in 1..10){
                        Log.d("CoroutineRunnerLog", "Coroutine $c, j = $j, Thread : ${Thread.currentThread().name}")
                    }
                }
                list.add(job)
            }
            list.joinAll()
            val time2 = System.currentTimeMillis()
            Log.e("CoroutineRunnerLog", "difference is = ${time2 - time1}")
        }

Since Dispatchers.IO can expand upto 64 threads,and Dispatchers.Default has max threads equal to CPU cores(8 in my case)

Expected behaviour should be Dispatcher.IO should have better performance(I am not sure here)

I just want to understand why Dispatchers.Default is running faster than Dispatchers.IO despite of having lesser number of threads in its thread pool.

Output :
Dispatchers.IO = 200 - 300 ms
Dispatchers.Default = 80 - 100 ms

I was trying to understand if I use Dispatchers.IO instead of Dispatchers.Default using this code

runBlocking {
            val list = mutableListOf<Job>()
            val time1 = System.currentTimeMillis()
            for (i in 1..100){
                val job = lifecycleScope.launch(Dispatchers.IO) {
                    val c = "c$i"
                    Log.i("CoroutineRunnerLog", "Launched a coroutine $c on Thread : ${Thread.currentThread().name}")
                    for (j in 1..10){
                        Log.d("CoroutineRunnerLog", "Coroutine $c, j = $j, Thread : ${Thread.currentThread().name}")
                    }
                }
                list.add(job)
            }
            list.joinAll()
            val time2 = System.currentTimeMillis()
            Log.e("CoroutineRunnerLog", "difference is = ${time2 - time1}")
        }

Since Dispatchers.IO can expand upto 64 threads,and Dispatchers.Default has max threads equal to CPU cores(8 in my case)

Expected behaviour should be Dispatcher.IO should have better performance(I am not sure here)

I just want to understand why Dispatchers.Default is running faster than Dispatchers.IO despite of having lesser number of threads in its thread pool.

Output :
Dispatchers.IO = 200 - 300 ms
Dispatchers.Default = 80 - 100 ms

• kotlin
• kotlin-coroutines

Share Follow asked Jan 30 at 11:38 Roop KishoreRoop Kishore 34844 silver badges1414 bronze badges 1

• Each job is not really doing much. I expect more time is taken up creating threads than running them. Also, please read stackoverflow.com/questions/504103/… – k314159 Commented Jan 30 at 12:55

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3 Answers 3

Sorted by: Reset to default 3

Running CPU-bound jobs on more threads than the number of CPU cores results in excessive context switching and thread management overhead, increasing the total execution time.

Since your jobs only compute without waiting, using Dispatchers.Default reduces the total execution time.

However, if you introduce waiting (e.g., Thread.sleep(500)) in your jobs, Dispatchers.IO will complete the total work faster by allowing other tasks to utilize the freed-up threads while waiting.

Expected behaviour should be Dispatcher.IO should have better performance

This is fundamentally incorrect. If you have 8 cores running CPU bound tasks you should expect worse performance when using more than 8 threads simultaneously. Every extra thread entails context switching that would not occur if there were 8 threads or less running on the CPU.

Dispatchers.IO can expand upto 64 threads

Dispatchers.IO has a lot of threads because these threads are designed to do blocking calls (e.g network, reading from the disk)

Using a lot of threads only has an advantage if those threads are actually running on the CPU a short time each.

To sum up, A CPU running on 100% has no benefit from more threads than cores, quite literally the opposite.

On a final note, I'd like to stress that it is imperative that you don't block threads on the Dispatchers.Default dispatcher. Since there are a limit amount of threads on that dispatcher, any blocked thread will very much hurt the performance of your code.

If you plan to call a blocking operation, use withContext as such:

withContext(Dispatchers.IO){
    // blocking code goes here
}

I would say it because the Context Switching Overhead in Dispatchers.IO

• Dispatchers.IO can create more threads than CPU cores, meaning some threads will be suspended and resumed frequently.
• Thread context switching is expensive: If a thread is paused and resumed frequently, it adds overhead.
• Dispatchers.Default, on the other hand, sticks to the number of CPU cores and avoids unnecessary thread swapping.

In your case:

• Default has fewer threads = less context switching = faster execution.
• IO has more threads = more context switching = slower execution.

What Should You Use?

• Since your workload is CPU-bound (loops, logging, minor operations), Dispatchers.Default is the better choice.
• If you were making network requests, database queries, or file I/O, then Dispatchers.IO would be better.

If your workload doesn't heavily rely on I/O blocking, Dispatchers.Default will usually be faster due to fewer threads and less context switching overhead.