Dispatcher
A Dispatcher determines which thread or thread pool executes coroutines, controlling the execution context for concurrent operations.
Built-in Dispatchers
From Dispatchers.Default
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Dispatchers.Default
- CPU-intensive work (calculations, data processing)
- Thread pool size equals CPU cores
Questions that come up
Should we limit max concurrency by one less thread for heavy workloads to leave some for UI?
From Q: Should you limit the concurrency of Dispatchers.Default? (A: No, for Linux multi core CPUs)
Go to text →
Should we limit the concurrency of Dispatchers.Default by creating a limitedParallelism dispatcher?
// Like this:
val maxConcurrency = (Runtime.getRuntime().availableProcessors() - 1).coerceAtLeast(1)
val limitedDispatcher = dispatcher.limitedParallelism(maxConcurrency)
Why would we want to limitedParallelism? Is due to the following question: if you use up all threads will operating system (OS) create cooperative environment for other processes that are running?.
Experiment aimed to roughly answer this
Experiment: load up CPU and try to use other apps in the meantime.
When I tried this on 16-core/32-thread AMD Ryzen 9 7945HX on Linux it seems at least Linux OS was creating a cooperative environment between the threads to not show perceivable slow down in other processes while the CPU was shown to be running at 100% per htop. Hence, as of now I don't think it's worth artificially lowering the parallelism and rather let OS take care of scheduling threads.
From code-example loading up CPU for 30 seconds
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GT-Sandbox-Snapshot: example loading up all the CPU cores for 30 seconds doing computation in a loop
Code
package com.glassthought.sandbox
import kotlinx.coroutines.*
import kotlin.system.measureTimeMillis
fun main(): Unit = runBlocking(CoroutineName("main-runBlocking")) {
val availableProcessors = Runtime.getRuntime().availableProcessors()
println("Starting CPU stress test on $availableProcessors cores...")
println("This will run for 30 seconds. System may become less responsive.")
val duration = 30_000L // 30 seconds in milliseconds
val elapsedTime = measureTimeMillis {
// Create a list of jobs, one for each processor
val jobs = List(availableProcessors) { coreIndex ->
// Use Dispatchers.Default which has a thread pool sized to CPU cores
// Each coroutine will pin to a thread and consume CPU
launch(Dispatchers.Default + CoroutineName("cpu-loader-$coreIndex")) {
println("Core $coreIndex: Starting intensive computation...")
val startTime = System.currentTimeMillis()
var counter = 0L
var sum = 0.0
// Tight loop that runs for 30 seconds
while (System.currentTimeMillis() - startTime < duration) {
// Perform some CPU-intensive operations
counter++
sum += Math.sqrt(counter.toDouble())
// Add some more operations to ensure CPU usage
if (counter % 1000000 == 0L) {
// Occasionally perform more complex calculations
for (i in 1..100) {
sum += Math.sin(sum) * Math.cos(counter.toDouble())
}
}
// Check for cancellation periodically (every million iterations)
if (counter % 10000000 == 0L) {
yield() // Allow coroutine cancellation if needed
}
}
println("Core $coreIndex: Completed. Counter=$counter, Sum=$sum")
}
}
// Wait for all jobs to complete
jobs.joinAll()
}
println("\nCPU stress test completed in ${elapsedTime}ms")
println("All $availableProcessors cores were loaded for approximately 30 seconds")
}
Command to reproduce:
gt.sandbox.checkout.commit 552c189a3d508ae510c9 \
&& cd "${GT_SANDBOX_REPO}" \
&& cmd.run.announce "./gradlew run --quiet"
Recorded output of command:
Picked up JAVA_TOOL_OPTIONS: -Dkotlinx.coroutines.debug
Picked up JAVA_TOOL_OPTIONS: -Dkotlinx.coroutines.debug
Starting CPU stress test on 32 cores...
This will run for 30 seconds. System may become less responsive.
Core 0: Starting intensive computation...
Core 1: Starting intensive computation...
Core 2: Starting intensive computation...
Core 3: Starting intensive computation...
Core 4: Starting intensive computation...
Core 5: Starting intensive computation...
Core 6: Starting intensive computation...
Core 7: Starting intensive computation...
Core 8: Starting intensive computation...
Core 9: Starting intensive computation...
Core 10: Starting intensive computation...
Core 11: Starting intensive computation...
Core 12: Starting intensive computation...
Core 13: Starting intensive computation...
Core 14: Starting intensive computation...
Core 15: Starting intensive computation...
Core 16: Starting intensive computation...
Core 17: Starting intensive computation...
Core 18: Starting intensive computation...
Core 19: Starting intensive computation...
Core 20: Starting intensive computation...
Core 21: Starting intensive computation...
Core 22: Starting intensive computation...
Core 23: Starting intensive computation...
Core 24: Starting intensive computation...
Core 25: Starting intensive computation...
Core 26: Starting intensive computation...
Core 27: Starting intensive computation...
Core 29: Starting intensive computation...
Core 28: Starting intensive computation...
Core 30: Starting intensive computation...
Core 31: Starting intensive computation...
Core 3: Completed. Counter=1209229201, Sum=2.8033135773977207E13
Core 11: Completed. Counter=1211769364, Sum=2.8121513705789645E13
Core 8: Completed. Counter=1199590272, Sum=2.7698620755711055E13
Core 2: Completed. Counter=1203021176, Sum=2.7817535389470645E13
Core 13: Completed. Counter=1190825648, Sum=2.7395612274237375E13
Core 31: Completed. Counter=1218795961, Sum=2.83664671475123E13
Core 21: Completed. Counter=1212393233, Sum=2.814323367875016E13
Core 19: Completed. Counter=1204196135, Sum=2.785829831628206E13
Core 12: Completed. Counter=1223621706, Sum=2.8535106632545105E13
Core 20: Completed. Counter=1223951172, Sum=2.854663222930776E13
Core 26: Completed. Counter=1209303550, Sum=2.803572122383962E13
Core 29: Completed. Counter=1225677066, Sum=2.8607033934887254E13
Core 0: Completed. Counter=1222882977, Sum=2.8509269567738004E13
Core 23: Completed. Counter=1209603905, Sum=2.8046166730119613E13
Core 24: Completed. Counter=1189998737, Sum=2.7367081901189492E13
Core 9: Completed. Counter=1215672477, Sum=2.8257492284361145E13
Core 25: Completed. Counter=996223257, Sum=2.0962532819551504E13
Core 30: Completed. Counter=1199697911, Sum=2.770234892705607E13
Core 5: Completed. Counter=1214359549, Sum=2.821172744882745E13
Core 28: Completed. Counter=1206475954, Sum=2.7937448955144168E13
Core 18: Completed. Counter=1213988582, Sum=2.8198801103505355E13
Core 4: Completed. Counter=1208161491, Sum=2.7996015452241723E13
Core 7: Completed. Counter=1173755242, Sum=2.6808657016430223E13
Core 22: Completed. Counter=1193089284, Sum=2.7473763708917047E13
Core 16: Completed. Counter=1216669588, Sum=2.8292265177977305E13
Core 6: Completed. Counter=1214497392, Sum=2.8216531091415805E13
Core 14: Completed. Counter=1202774004, Sum=2.7808962768959176E13
Core 27: Completed. Counter=1213167406, Sum=2.817019424997135E13
Core 17: Completed. Counter=1204046012, Sum=2.785308898096554E13
Core 1: Completed. Counter=1208010559, Sum=2.7990769428434086E13
Core 15: Completed. Counter=1215303167, Sum=2.8244616717065184E13
Core 10: Completed. Counter=1215761769, Sum=2.8260605640605434E13
CPU stress test completed in 30043ms
All 32 cores were loaded for approximately 30 seconds
Note: Would be interesting to try this out on Mac & Windows later.
Dispatchers.IO
- Optimized for I/O operations (file, network, database)
- Backed by shared thread pool that grows as needed
Dispatchers.Main
- UI thread dispatcher (Android main thread, JavaFX Application Thread)
- Use for UI updates and lightweight operations
Dispatchers.Unconfined
Dispatchers.Unconfined
- Starts in caller thread, resumes in any thread
- In general should NOT be used.
Usage
// Switch context
withContext(Dispatchers.IO) {
// Network call or file operation
}
// Launch with specific dispatcher
launch(Dispatchers.Default) {
// CPU-intensive work
}
Key Points
- Dispatchers are inherited by child coroutines
- Use
withContext()to switch dispatchers temporarily - Choose dispatcher based on operation type, not arbitrary preference
- Main dispatcher requires platform-specific dependency
Relationships
- Uses: Thread pools for execution
- Enables: Context switching with
withContext() - Inherited by: Child coroutines
Examples
From example
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With Dispatchers.IO
From With Dispatchers.IO
Go to text →
Simple dispatcher IO
From Simple usage of CoRoutines with IO Dispatcher
Go to text →
About
Here we show example of simple usage of dispatcher io.
We can see that different OS threads are being used.
Code
package gt.sandbox
import gt.sandbox.internal.output.Out
import kotlinx.coroutines.*
val out = Out.standard()
suspend fun fetchData(s: String) {
out.println("Fetching data... $s")
delay(1000) // This suspends the coroutine for 1 second without blocking the thread
out.println("Data fetched $s")
}
fun main(): Unit = runBlocking {
launch(Dispatchers.IO) {
fetchData("a-1")
fetchData("a-2")
}
launch(Dispatchers.IO) {
fetchData("b")
}
launch(Dispatchers.IO) {
fetchData("c")
}
}
Command to reproduce:
gt.sandbox.checkout.commit 1fc85e0 \
&& cd "${GT_SANDBOX_REPO}" \
&& cmd.run.announce "./gradlew run"
Recorded output of command:
> Task :app:checkKotlinGradlePluginConfigurationErrors SKIPPED
> Task :app:processResources NO-SOURCE
> Task :app:compileKotlin
> Task :app:compileJava NO-SOURCE
> Task :app:classes UP-TO-DATE
> Task :app:run
[2024-06-09T04:18:21.291646Z][ms-elapsed-since-start: 50][tname:DefaultDispatcher-worker-2/tid:22] Fetching data... c
[2024-06-09T04:18:21.291456Z][ms-elapsed-since-start: 50][tname:DefaultDispatcher-worker-1/tid:21] Fetching data... a-1
[2024-06-09T04:18:21.291454Z][ms-elapsed-since-start: 50][tname:DefaultDispatcher-worker-3/tid:23] Fetching data... b
[2024-06-09T04:18:22.323347Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-2/tid:22] Data fetched c
[2024-06-09T04:18:22.323736Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-3/tid:23] Data fetched a-1
[2024-06-09T04:18:22.323347Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-1/tid:21] Data fetched b
[2024-06-09T04:18:22.324494Z][ms-elapsed-since-start: 1072][tname:DefaultDispatcher-worker-3/tid:23] Fetching data... a-2
[2024-06-09T04:18:23.325568Z][ms-elapsed-since-start: 2073][tname:DefaultDispatcher-worker-3/tid:23] Data fetched a-2
BUILD SUCCESSFUL in 2s
2 actionable tasks: 2 executed
Increasing beyond double number of cores
From beyond-number-of-cores
Go to text →
About
In this exame we illustrate the Dispatchers.IO re-using the same OS thread for 2 different executions.
This is illustrated in these lines of output:
[2024-06-09T04:23:29.653692Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-21/tid:41] Fetching data... task 19
[2024-06-09T04:23:29.653867Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-21/tid:41] Fetching data... task 22
Code
package gt.sandbox
import gt.sandbox.internal.output.Out
import kotlinx.coroutines.*
val out = Out.standard()
suspend fun fetchData(s: String) {
out.println("Fetching data... $s")
delay(1000) // This suspends the coroutine for 1 second without blocking the thread
out.println("Data fetched $s")
}
fun main(): Unit = runBlocking {
val availableCores = Runtime.getRuntime().availableProcessors()
out.println("Number of available cores: $availableCores")
for (i in 1..(availableCores* 2) + 2) {
launch(Dispatchers.IO) {
fetchData("task $i")
}
}
}
Command to reproduce:
gt.sandbox.checkout.commit 95c0ae2 \
&& cd "${GT_SANDBOX_REPO}" \
&& cmd.run.announce "./gradlew run"
Recorded output of command:
> Task :app:checkKotlinGradlePluginConfigurationErrors SKIPPED
> Task :app:processResources NO-SOURCE
> Task :app:compileKotlin
> Task :app:compileJava NO-SOURCE
> Task :app:classes UP-TO-DATE
> Task :app:run
[2024-06-09T04:23:29.630413Z][ms-elapsed-since-start: 47][tname:main/tid:1] Number of available cores: 10
[2024-06-09T04:23:29.651359Z][ms-elapsed-since-start: 59][tname:DefaultDispatcher-worker-1/tid:21] Fetching data... task 1
[2024-06-09T04:23:29.651570Z][ms-elapsed-since-start: 59][tname:DefaultDispatcher-worker-3/tid:23] Fetching data... task 2
[2024-06-09T04:23:29.651777Z][ms-elapsed-since-start: 59][tname:DefaultDispatcher-worker-5/tid:25] Fetching data... task 3
[2024-06-09T04:23:29.651815Z][ms-elapsed-since-start: 59][tname:DefaultDispatcher-worker-2/tid:22] Fetching data... task 4
[2024-06-09T04:23:29.651846Z][ms-elapsed-since-start: 59][tname:DefaultDispatcher-worker-4/tid:24] Fetching data... task 5
[2024-06-09T04:23:29.652016Z][ms-elapsed-since-start: 60][tname:DefaultDispatcher-worker-8/tid:28] Fetching data... task 6
[2024-06-09T04:23:29.652160Z][ms-elapsed-since-start: 60][tname:DefaultDispatcher-worker-7/tid:27] Fetching data... task 7
[2024-06-09T04:23:29.652243Z][ms-elapsed-since-start: 60][tname:DefaultDispatcher-worker-6/tid:26] Fetching data... task 8
[2024-06-09T04:23:29.652418Z][ms-elapsed-since-start: 60][tname:DefaultDispatcher-worker-11/tid:31] Fetching data... task 9
[2024-06-09T04:23:29.652762Z][ms-elapsed-since-start: 60][tname:DefaultDispatcher-worker-10/tid:30] Fetching data... task 10
[2024-06-09T04:23:29.652811Z][ms-elapsed-since-start: 60][tname:DefaultDispatcher-worker-14/tid:34] Fetching data... task 11
[2024-06-09T04:23:29.652945Z][ms-elapsed-since-start: 60][tname:DefaultDispatcher-worker-13/tid:33] Fetching data... task 12
[2024-06-09T04:23:29.653149Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-18/tid:38] Fetching data... task 13
[2024-06-09T04:23:29.653555Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-9/tid:29] Fetching data... task 15
[2024-06-09T04:23:29.653556Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-21/tid:41] Fetching data... task 14
[2024-06-09T04:23:29.653623Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-7/tid:27] Fetching data... task 16
[2024-06-09T04:23:29.653651Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-9/tid:29] Fetching data... task 17
[2024-06-09T04:23:29.653676Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-13/tid:33] Fetching data... task 18
[2024-06-09T04:23:29.653717Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-7/tid:27] Fetching data... task 20
[2024-06-09T04:23:29.653692Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-21/tid:41] Fetching data... task 19
[2024-06-09T04:23:29.653867Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-21/tid:41] Fetching data... task 22
[2024-06-09T04:23:29.653732Z][ms-elapsed-since-start: 61][tname:DefaultDispatcher-worker-9/tid:29] Fetching data... task 21
[2024-06-09T04:23:30.661370Z][ms-elapsed-since-start: 1069][tname:DefaultDispatcher-worker-17/tid:37] Data fetched task 7
[2024-06-09T04:23:30.661588Z][ms-elapsed-since-start: 1069][tname:DefaultDispatcher-worker-15/tid:35] Data fetched task 4
[2024-06-09T04:23:30.662263Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-15/tid:35] Data fetched task 8
[2024-06-09T04:23:30.661870Z][ms-elapsed-since-start: 1069][tname:DefaultDispatcher-worker-19/tid:39] Data fetched task 2
[2024-06-09T04:23:30.662439Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-15/tid:35] Data fetched task 3
[2024-06-09T04:23:30.662540Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-19/tid:39] Data fetched task 6
[2024-06-09T04:23:30.662606Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-15/tid:35] Data fetched task 1
[2024-06-09T04:23:30.662685Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-19/tid:39] Data fetched task 10
[2024-06-09T04:23:30.662824Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-19/tid:39] Data fetched task 11
[2024-06-09T04:23:30.662870Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-24/tid:45] Data fetched task 13
[2024-06-09T04:23:30.662972Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-19/tid:39] Data fetched task 15
[2024-06-09T04:23:30.663027Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-24/tid:45] Data fetched task 14
[2024-06-09T04:23:30.661376Z][ms-elapsed-since-start: 1069][tname:DefaultDispatcher-worker-18/tid:38] Data fetched task 9
[2024-06-09T04:23:30.663206Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-24/tid:45] Data fetched task 17
[2024-06-09T04:23:30.663273Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-18/tid:38] Data fetched task 18
[2024-06-09T04:23:30.663533Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-6/tid:26] Data fetched task 19
[2024-06-09T04:23:30.663124Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-19/tid:39] Data fetched task 16
[2024-06-09T04:23:30.662265Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-17/tid:37] Data fetched task 5
[2024-06-09T04:23:30.662858Z][ms-elapsed-since-start: 1070][tname:DefaultDispatcher-worker-15/tid:35] Data fetched task 12
[2024-06-09T04:23:30.663355Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-24/tid:45] Data fetched task 20
[2024-06-09T04:23:30.663540Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-18/tid:38] Data fetched task 22
[2024-06-09T04:23:30.663689Z][ms-elapsed-since-start: 1071][tname:DefaultDispatcher-worker-6/tid:26] Data fetched task 21
BUILD SUCCESSFUL in 1s
2 actionable tasks: 2 executed
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