simulator: Optimize bottlenecks in resource layer

This pull request addresses several bottlenecks that were present in the 
`opendc-simulator-resources` layer and  `TimerScheduler`.
These changes result into a 4x performance improvement for the energy experiments 
we are currently doing.

* The use of `StateFlow` has been removed where possible. Profiling shows that 
emitting changes to `StateFlow` becomes a bottleneck in a single-thread context.
* `SimSpeedConsumerAdapter` is an alternative for obtaining the changes in 
speed of a resource.

 **Breaking API Changes**
* `SimResourceSource` does not expose `speed` as `StateFlow` anymore. To monitor speed changes, use `SimSpeedConsumerAdapter`.
* Power draw in `SimBareMetalMachine` is not exposed as `StateFlow` anymore.

16 files changed, 117 insertions, 84 deletions

diff --git a/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/SimAbstractMachine.kt b/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/SimAbstractMachine.kt
index 2127b066..1f26c9c9 100644
--- a/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/SimAbstractMachine.kt
+++ b/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/SimAbstractMachine.kt
@@ -25,14 +25,13 @@ package org.opendc.simulator.compute
 import kotlinx.coroutines.*
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
-import kotlinx.coroutines.flow.launchIn
-import kotlinx.coroutines.flow.onEach
 import org.opendc.simulator.compute.model.MemoryUnit
 import org.opendc.simulator.compute.model.ProcessingUnit
 import org.opendc.simulator.compute.workload.SimWorkload
 import org.opendc.simulator.resources.SimResourceProvider
 import org.opendc.simulator.resources.SimResourceSource
 import org.opendc.simulator.resources.consume
+import org.opendc.simulator.resources.consumer.SimSpeedConsumerAdapter
 import java.time.Clock
 import kotlin.coroutines.CoroutineContext
 
@@ -47,9 +46,9 @@ public abstract class SimAbstractMachine(private val clock: Clock) : SimMachine
     /**
      * The speed of the CPU cores.
      */
-    public val speed: List<Double>
+    public val speed: DoubleArray
         get() = _speed
-    private var _speed = mutableListOf<Double>()
+    private var _speed = doubleArrayOf()
 
     /**
      * A flag to indicate that the machine is terminated.
@@ -94,29 +93,32 @@ public abstract class SimAbstractMachine(private val clock: Clock) : SimMachine
         val ctx = Context(resources, meta)
         val totalCapacity = model.cpus.sumByDouble { it.frequency }
 
-        _speed = MutableList(model.cpus.size) { 0.0 }
+        _speed = DoubleArray(model.cpus.size) { 0.0 }
+        var totalSpeed = 0.0
 
         workload.onStart(ctx)
 
         for ((cpu, source) in resources) {
             val consumer = workload.getConsumer(ctx, cpu)
-            val job = source.speed
-                .onEach {
-                    _speed[cpu.id] = it
-                    _usage.value = _speed.sum() / totalCapacity
-                }
-                .launchIn(this)
-
-            launch {
-                try {
-                    source.consume(consumer)
-                } finally {
-                    job.cancel()
-                }
+            val adapter = SimSpeedConsumerAdapter(consumer) { newSpeed ->
+                val oldSpeed = _speed[cpu.id]
+                _speed[cpu.id] = newSpeed
+                totalSpeed = totalSpeed - oldSpeed + newSpeed
+
+                updateUsage(totalSpeed / totalCapacity)
             }
+
+            launch { source.consume(adapter) }
         }
     }
 
+    /**
+     * This method is invoked when the usage of the machine is updated.
+     */
+    protected open fun updateUsage(usage: Double) {
+        _usage.value = usage
+    }
+
     override fun close() {
         if (!isTerminated) {
             isTerminated = true
diff --git a/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/SimBareMetalMachine.kt b/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/SimBareMetalMachine.kt
index 51b807d2..d5577279 100644
--- a/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/SimBareMetalMachine.kt
+++ b/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/SimBareMetalMachine.kt
@@ -84,12 +84,15 @@ public class SimBareMetalMachine(
     /**
      * The power draw of the machine.
      */
-    public val powerDraw: StateFlow<Double> = usage
-        .map {
-            this.scalingGovernors.forEach { it.onLimit() }
-            this.scalingDriver.computePower()
-        }
-        .stateIn(scope, SharingStarted.Eagerly, 0.0)
+    public var powerDraw: Double = 0.0
+        private set
+
+    override fun updateUsage(usage: Double) {
+        super.updateUsage(usage)
+
+        scalingGovernors.forEach { it.onLimit() }
+        powerDraw = scalingDriver.computePower()
+    }
 
     override fun close() {
         super.close()
diff --git a/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/cpufreq/DemandScalingGovernor.kt b/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/cpufreq/DemandScalingGovernor.kt
index b4bbf9fb..4d62c383 100644
--- a/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/cpufreq/DemandScalingGovernor.kt
+++ b/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/cpufreq/DemandScalingGovernor.kt
@@ -28,7 +28,7 @@ package org.opendc.simulator.compute.cpufreq
 public class DemandScalingGovernor : ScalingGovernor {
     override fun createLogic(ctx: ScalingContext): ScalingGovernor.Logic = object : ScalingGovernor.Logic {
         override fun onLimit() {
-            ctx.setTarget(ctx.resource.speed.value)
+            ctx.setTarget(ctx.resource.speed)
         }
 
         override fun toString(): String = "DemandScalingGovernor.Logic"
diff --git a/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/cpufreq/PStateScalingDriver.kt b/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/cpufreq/PStateScalingDriver.kt
index d109e4d8..1c82253c 100644
--- a/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/cpufreq/PStateScalingDriver.kt
+++ b/simulator/opendc-simulator/opendc-simulator-compute/src/main/kotlin/org/opendc/simulator/compute/cpufreq/PStateScalingDriver.kt
@@ -59,7 +59,7 @@ public class PStateScalingDriver(states: Map<Double, PowerModel>) : ScalingDrive
 
             for (ctx in contexts) {
                 targetFreq = max(ctx.target, targetFreq)
-                totalSpeed += ctx.resource.speed.value
+                totalSpeed += ctx.resource.speed
             }
 
             val maxFreq = states.lastKey()
diff --git a/simulator/opendc-simulator/opendc-simulator-compute/src/test/kotlin/org/opendc/simulator/compute/cpufreq/DemandScalingGovernorTest.kt b/simulator/opendc-simulator/opendc-simulator-compute/src/test/kotlin/org/opendc/simulator/compute/cpufreq/DemandScalingGovernorTest.kt
index 19c06126..c02b6285 100644
--- a/simulator/opendc-simulator/opendc-simulator-compute/src/test/kotlin/org/opendc/simulator/compute/cpufreq/DemandScalingGovernorTest.kt
+++ b/simulator/opendc-simulator/opendc-simulator-compute/src/test/kotlin/org/opendc/simulator/compute/cpufreq/DemandScalingGovernorTest.kt
@@ -35,7 +35,7 @@ internal class DemandScalingGovernorTest {
     fun testSetDemandLimit() {
         val ctx = mockk<ScalingContext>(relaxUnitFun = true)
 
-        every { ctx.resource.speed.value } returns 2100.0
+        every { ctx.resource.speed } returns 2100.0
 
         val logic = DemandScalingGovernor().createLogic(ctx)
 
diff --git a/simulator/opendc-simulator/opendc-simulator-compute/src/test/kotlin/org/opendc/simulator/compute/cpufreq/PStateScalingDriverTest.kt b/simulator/opendc-simulator/opendc-simulator-compute/src/test/kotlin/org/opendc/simulator/compute/cpufreq/PStateScalingDriverTest.kt
index 5c30bc1f..c6f233a6 100644
--- a/simulator/opendc-simulator/opendc-simulator-compute/src/test/kotlin/org/opendc/simulator/compute/cpufreq/PStateScalingDriverTest.kt
+++ b/simulator/opendc-simulator/opendc-simulator-compute/src/test/kotlin/org/opendc/simulator/compute/cpufreq/PStateScalingDriverTest.kt
@@ -59,7 +59,7 @@ internal class PStateScalingDriverTest {
         val resource = mockk<SimResourceSource>()
 
         every { cpu.frequency } returns 4100.0
-        every { resource.speed.value } returns 1200.0
+        every { resource.speed } returns 1200.0
 
         val driver = PStateScalingDriver(
             sortedMapOf(
@@ -84,7 +84,7 @@ internal class PStateScalingDriverTest {
         val resource = mockk<SimResourceSource>()
 
         every { cpu.frequency } returns 4100.0
-        every { resource.speed.value } returns 1200.0
+        every { resource.speed } returns 1200.0
 
         val driver = PStateScalingDriver(
             sortedMapOf(
@@ -125,11 +125,11 @@ internal class PStateScalingDriverTest {
 
         val scalingContext = logic.createContext(cpu, resource)
 
-        every { resource.speed.value } returns 1400.0
+        every { resource.speed } returns 1400.0
         scalingContext.setTarget(1400.0)
         assertEquals(150.0, logic.computePower())
 
-        every { resource.speed.value } returns 1400.0
+        every { resource.speed } returns 1400.0
         scalingContext.setTarget(4000.0)
         assertEquals(235.0, logic.computePower())
     }
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimAbstractResourceAggregator.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimAbstractResourceAggregator.kt
index e5991264..c7fa6a17 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimAbstractResourceAggregator.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimAbstractResourceAggregator.kt
@@ -102,7 +102,18 @@ public abstract class SimAbstractResourceAggregator(private val clock: Clock) :
 
     private val context = object : SimAbstractResourceContext(inputContexts.sumByDouble { it.capacity }, clock, _output) {
         override val remainingWork: Double
-            get() = inputContexts.sumByDouble { it.remainingWork }
+            get() {
+                val now = clock.millis()
+
+                return if (_remainingWorkFlush < now) {
+                    _remainingWorkFlush = now
+                    _inputContexts.sumByDouble { it.remainingWork }.also { _remainingWork = it }
+                } else {
+                    _remainingWork
+                }
+            }
+        private var _remainingWork: Double = 0.0
+        private var _remainingWorkFlush: Long = Long.MIN_VALUE
 
         override fun interrupt() {
             super.interrupt()
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimAbstractResourceContext.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimAbstractResourceContext.kt
index 5c5ee038..05ed0714 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimAbstractResourceContext.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimAbstractResourceContext.kt
@@ -272,6 +272,7 @@ public abstract class SimAbstractResourceContext(
                 require(deadline >= now) { "Deadline already passed" }
 
                 speed = 0.0
+                consumer.onConfirm(this, 0.0)
 
                 onIdle(deadline)
             }
@@ -283,6 +284,7 @@ public abstract class SimAbstractResourceContext(
                 require(deadline >= now) { "Deadline already passed" }
 
                 speed = min(capacity, limit)
+                consumer.onConfirm(this, speed)
 
                 onConsume(work, limit, deadline)
             }
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceConsumer.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceConsumer.kt
index 672a3e9d..38672b13 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceConsumer.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceConsumer.kt
@@ -23,7 +23,7 @@
 package org.opendc.simulator.resources
 
 /**
- * A [SimResourceConsumer] characterizes how a [SimResource] is consumed.
+ * A [SimResourceConsumer] characterizes how a resource is consumed.
  *
  * Implementors of this interface should be considered stateful and must be assumed not to be re-usable (concurrently)
  * for multiple resource providers, unless explicitly said otherwise.
@@ -46,6 +46,14 @@ public interface SimResourceConsumer {
     public fun onNext(ctx: SimResourceContext): SimResourceCommand
 
     /**
+     * This method is invoked when the resource provider confirms that the consumer is running at the given speed.
+     *
+     * @param ctx The execution context in which the consumer runs.
+     * @param speed The speed at which the consumer runs.
+     */
+    public fun onConfirm(ctx: SimResourceContext, speed: Double) {}
+
+    /**
      * This is method is invoked when the capacity of the resource changes.
      *
      * After being informed of such an event, the consumer might decide to adjust its consumption by interrupting the
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceDistributorMaxMin.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceDistributorMaxMin.kt
index 9df333e3..dfdd2c2e 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceDistributorMaxMin.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceDistributorMaxMin.kt
@@ -106,7 +106,7 @@ public class SimResourceDistributorMaxMin(
                 val output = iterator.next()
 
                 // Remove the output from the outputs to prevent ConcurrentModificationException when removing it
-                // during the call tou output.close()
+                // during the call to output.close()
                 iterator.remove()
 
                 output.close()
@@ -251,8 +251,8 @@ public class SimResourceDistributorMaxMin(
             totalAllocatedWork - totalRemainingWork,
             totalOvercommittedWork.toLong(),
             totalInterferedWork.toLong(),
-            totalRequestedSpeed,
             totalAllocatedSpeed,
+            totalRequestedSpeed
         )
 
         totalInterferedWork = 0.0
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceSource.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceSource.kt
index 9b10edaf..025b0406 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceSource.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceSource.kt
@@ -22,8 +22,6 @@
 
 package org.opendc.simulator.resources
 
-import kotlinx.coroutines.flow.MutableStateFlow
-import kotlinx.coroutines.flow.StateFlow
 import org.opendc.utils.TimerScheduler
 import java.time.Clock
 import kotlin.math.ceil
@@ -42,11 +40,10 @@ public class SimResourceSource(
     private val scheduler: TimerScheduler<Any>
 ) : SimResourceProvider {
     /**
-     * The resource processing speed over time.
+     * The current processing speed of the resource.
      */
-    public val speed: StateFlow<Double>
-        get() = _speed
-    private val _speed = MutableStateFlow(0.0)
+    public val speed: Double
+        get() = ctx?.speed ?: 0.0
 
     /**
      * The capacity of the resource.
@@ -101,8 +98,6 @@ public class SimResourceSource(
      */
     private inner class Context(consumer: SimResourceConsumer) : SimAbstractResourceContext(capacity, clock, consumer) {
         override fun onIdle(deadline: Long) {
-            _speed.value = speed
-
             // Do not resume if deadline is "infinite"
             if (deadline != Long.MAX_VALUE) {
                 scheduler.startSingleTimerTo(this, deadline) { flush() }
@@ -110,15 +105,12 @@ public class SimResourceSource(
         }
 
         override fun onConsume(work: Double, limit: Double, deadline: Long) {
-            _speed.value = speed
-
             val until = min(deadline, clock.millis() + getDuration(work, speed))
 
             scheduler.startSingleTimerTo(this, until, ::flush)
         }
 
         override fun onFinish(cause: Throwable?) {
-            _speed.value = speed
             scheduler.cancel(this)
             cancel()
 
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceTransformer.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceTransformer.kt
index 73f18c7c..de455021 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceTransformer.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/SimResourceTransformer.kt
@@ -124,6 +124,10 @@ public class SimResourceTransformer(
         }
     }
 
+    override fun onConfirm(ctx: SimResourceContext, speed: Double) {
+        delegate?.onConfirm(ctx, speed)
+    }
+
     override fun onCapacityChanged(ctx: SimResourceContext, isThrottled: Boolean) {
         delegate?.onCapacityChanged(ctx, isThrottled)
     }
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/consumer/SimSpeedConsumerAdapter.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/consumer/SimSpeedConsumerAdapter.kt
index fd4a9ed5..114c7312 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/consumer/SimSpeedConsumerAdapter.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/main/kotlin/org/opendc/simulator/resources/consumer/SimSpeedConsumerAdapter.kt
@@ -22,8 +22,6 @@
 
 package org.opendc.simulator.resources.consumer
 
-import kotlinx.coroutines.flow.MutableStateFlow
-import kotlinx.coroutines.flow.StateFlow
 import org.opendc.simulator.resources.SimResourceCommand
 import org.opendc.simulator.resources.SimResourceConsumer
 import org.opendc.simulator.resources.SimResourceContext
@@ -32,37 +30,52 @@ import kotlin.math.min
 /**
  * Helper class to expose an observable [speed] field describing the speed of the consumer.
  */
-public class SimSpeedConsumerAdapter(private val delegate: SimResourceConsumer) : SimResourceConsumer by delegate {
+public class SimSpeedConsumerAdapter(
+    private val delegate: SimResourceConsumer,
+    private val callback: (Double) -> Unit = {}
+) : SimResourceConsumer by delegate {
     /**
-     * The resource processing speed over time.
+     * The resource processing speed at this instant.
      */
-    public val speed: StateFlow<Double>
-        get() = _speed
-    private val _speed = MutableStateFlow(0.0)
+    public var speed: Double = 0.0
+        private set(value) {
+            if (field != value) {
+                callback(value)
+                field = value
+            }
+        }
+
+    init {
+        callback(0.0)
+    }
 
     override fun onNext(ctx: SimResourceContext): SimResourceCommand {
-        val command = delegate.onNext(ctx)
+        return delegate.onNext(ctx)
+    }
 
-        when (command) {
-            is SimResourceCommand.Idle -> _speed.value = 0.0
-            is SimResourceCommand.Consume -> _speed.value = min(ctx.capacity, command.limit)
-            is SimResourceCommand.Exit -> _speed.value = 0.0
-        }
+    override fun onConfirm(ctx: SimResourceContext, speed: Double) {
+        delegate.onConfirm(ctx, speed)
 
-        return command
+        this.speed = speed
     }
 
     override fun onCapacityChanged(ctx: SimResourceContext, isThrottled: Boolean) {
-        val oldSpeed = _speed.value
+        val oldSpeed = speed
 
         delegate.onCapacityChanged(ctx, isThrottled)
 
         // Check if the consumer interrupted the consumer and updated the resource consumption. If not, we might
         // need to update the current speed.
-        if (oldSpeed == _speed.value) {
-            _speed.value = min(ctx.capacity, _speed.value)
+        if (oldSpeed == speed) {
+            speed = min(ctx.capacity, speed)
         }
     }
 
+    override fun onFinish(ctx: SimResourceContext, cause: Throwable?) {
+        super.onFinish(ctx, cause)
+
+        speed = 0.0
+    }
+
     override fun toString(): String = "SimSpeedConsumerAdapter[delegate=$delegate]"
 }
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceAggregatorMaxMinTest.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceAggregatorMaxMinTest.kt
index de864c1c..bf8c6d1f 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceAggregatorMaxMinTest.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceAggregatorMaxMinTest.kt
@@ -26,12 +26,12 @@ import io.mockk.every
 import io.mockk.mockk
 import io.mockk.verify
 import kotlinx.coroutines.*
-import kotlinx.coroutines.flow.toList
 import kotlinx.coroutines.test.runBlockingTest
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertAll
 import org.junit.jupiter.api.assertThrows
+import org.opendc.simulator.resources.consumer.SimSpeedConsumerAdapter
 import org.opendc.simulator.resources.consumer.SimWorkConsumer
 import org.opendc.simulator.utils.DelayControllerClockAdapter
 import org.opendc.utils.TimerScheduler
@@ -47,15 +47,18 @@ internal class SimResourceAggregatorMaxMinTest {
         val scheduler = TimerScheduler<Any>(coroutineContext, clock)
 
         val aggregator = SimResourceAggregatorMaxMin(clock)
+        val forwarder = SimResourceForwarder()
         val sources = listOf(
-            SimResourceSource(1.0, clock, scheduler),
+            forwarder,
             SimResourceSource(1.0, clock, scheduler)
         )
         sources.forEach(aggregator::addInput)
 
         val consumer = SimWorkConsumer(1.0, 0.5)
         val usage = mutableListOf<Double>()
-        val job = launch { sources[0].speed.toList(usage) }
+        val source = SimResourceSource(1.0, clock, scheduler)
+        val adapter = SimSpeedConsumerAdapter(forwarder, usage::add)
+        source.startConsumer(adapter)
 
         try {
             aggregator.output.consume(consumer)
@@ -67,7 +70,6 @@ internal class SimResourceAggregatorMaxMinTest {
             )
         } finally {
             aggregator.output.close()
-            job.cancel()
         }
     }
 
@@ -85,18 +87,17 @@ internal class SimResourceAggregatorMaxMinTest {
 
         val consumer = SimWorkConsumer(2.0, 1.0)
         val usage = mutableListOf<Double>()
-        val job = launch { sources[0].speed.toList(usage) }
+        val adapter = SimSpeedConsumerAdapter(consumer, usage::add)
 
         try {
-            aggregator.output.consume(consumer)
+            aggregator.output.consume(adapter)
             yield()
             assertAll(
                 { assertEquals(1000, currentTime) },
-                { assertEquals(listOf(0.0, 1.0, 0.0), usage) }
+                { assertEquals(listOf(0.0, 2.0, 0.0), usage) }
             )
         } finally {
             aggregator.output.close()
-            job.cancel()
         }
     }
 
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceSourceTest.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceSourceTest.kt
index 58e19421..dbba6160 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceSourceTest.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceSourceTest.kt
@@ -27,10 +27,10 @@ import io.mockk.mockk
 import io.mockk.spyk
 import io.mockk.verify
 import kotlinx.coroutines.*
-import kotlinx.coroutines.flow.toList
 import kotlinx.coroutines.test.runBlockingTest
 import org.junit.jupiter.api.*
 import org.junit.jupiter.api.Assertions.assertEquals
+import org.opendc.simulator.resources.consumer.SimSpeedConsumerAdapter
 import org.opendc.simulator.resources.consumer.SimWorkConsumer
 import org.opendc.simulator.utils.DelayControllerClockAdapter
 import org.opendc.utils.TimerScheduler
@@ -54,11 +54,10 @@ class SimResourceSourceTest {
 
         try {
             val res = mutableListOf<Double>()
-            val job = launch { provider.speed.toList(res) }
+            val adapter = SimSpeedConsumerAdapter(consumer, res::add)
 
-            provider.consume(consumer)
+            provider.consume(adapter)
 
-            job.cancel()
             assertEquals(listOf(0.0, capacity, 0.0), res) { "Speed is reported correctly" }
         } finally {
             scheduler.close()
@@ -102,11 +101,10 @@ class SimResourceSourceTest {
 
         try {
             val res = mutableListOf<Double>()
-            val job = launch { provider.speed.toList(res) }
+            val adapter = SimSpeedConsumerAdapter(consumer, res::add)
 
-            provider.consume(consumer)
+            provider.consume(adapter)
 
-            job.cancel()
             assertEquals(listOf(0.0, capacity, 0.0), res) { "Speed is reported correctly" }
         } finally {
             scheduler.close()
diff --git a/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceSwitchExclusiveTest.kt b/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceSwitchExclusiveTest.kt
index edd60502..9a40edc4 100644
--- a/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceSwitchExclusiveTest.kt
+++ b/simulator/opendc-simulator/opendc-simulator-resources/src/test/kotlin/org/opendc/simulator/resources/SimResourceSwitchExclusiveTest.kt
@@ -25,14 +25,13 @@ package org.opendc.simulator.resources
 import io.mockk.every
 import io.mockk.mockk
 import kotlinx.coroutines.ExperimentalCoroutinesApi
-import kotlinx.coroutines.flow.toList
-import kotlinx.coroutines.launch
 import kotlinx.coroutines.test.runBlockingTest
 import kotlinx.coroutines.yield
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertAll
 import org.junit.jupiter.api.assertThrows
+import org.opendc.simulator.resources.consumer.SimSpeedConsumerAdapter
 import org.opendc.simulator.resources.consumer.SimTraceConsumer
 import org.opendc.simulator.utils.DelayControllerClockAdapter
 import org.opendc.utils.TimerScheduler
@@ -65,17 +64,17 @@ internal class SimResourceSwitchExclusiveTest {
 
         val switch = SimResourceSwitchExclusive()
         val source = SimResourceSource(3200.0, clock, scheduler)
-
-        switch.addInput(source)
+        val forwarder = SimResourceForwarder()
+        val adapter = SimSpeedConsumerAdapter(forwarder, speed::add)
+        source.startConsumer(adapter)
+        switch.addInput(forwarder)
 
         val provider = switch.addOutput(3200.0)
-        val job = launch { source.speed.toList(speed) }
 
         try {
             provider.consume(workload)
             yield()
         } finally {
-            job.cancel()
             provider.close()
         }