IO

val IO: CoroutineDispatcher

The CoroutineDispatcher that is designed for offloading blocking IO tasks to a shared pool of threads.

Additional threads in this pool are created and are shutdown on demand. The number of threads used by tasks in this dispatcher is limited by the value of "kotlinx.coroutines.io.parallelism" (IO_PARALLELISM_PROPERTY_NAME) system property. It defaults to the limit of 64 threads or the number of cores (whichever is larger).

Moreover, the maximum configurable number of threads is capped by the kotlinx.coroutines.scheduler.max.pool.size system property. If you need a higher number of parallel threads, you should use a custom dispatcher backed by your own thread pool.

Implementation note

This dispatcher shares threads with the Default dispatcher, so using withContext(Dispatchers.IO) { ... } when already running on the Default dispatcher does not lead to an actual switching to another thread — typically execution continues in the same thread. As a result of thread sharing, more than 64 (default parallelism) threads can be created (but not used) during operations over IO dispatcher.

Sources

jvm source
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