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.

4 files changed, 31 insertions, 26 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()