feat(utils): Add Pacer to pace scheduling cycles

This change adds a new Pacer class that can pace the incoming scheduling
requests into scheduling cycles by allowing the user to specify a
scheduling quantum.

6 files changed, 241 insertions, 64 deletions

diff --git a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ComputeServiceImpl.kt b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ComputeServiceImpl.kt
index 27a6ecae..6df3c4f9 100644
--- a/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ComputeServiceImpl.kt
+++ b/opendc-compute/opendc-compute-service/src/main/kotlin/org/opendc/compute/service/internal/ComputeServiceImpl.kt
@@ -35,7 +35,7 @@ import org.opendc.compute.service.driver.Host
 import org.opendc.compute.service.driver.HostListener
 import org.opendc.compute.service.driver.HostState
 import org.opendc.compute.service.scheduler.ComputeScheduler
-import org.opendc.utils.TimerScheduler
+import org.opendc.utils.Pacer
 import java.time.Clock
 import java.time.Duration
 import java.util.*
@@ -56,7 +56,7 @@ internal class ComputeServiceImpl(
     private val clock: Clock,
     meterProvider: MeterProvider,
     private val scheduler: ComputeScheduler,
-    private val schedulingQuantum: Duration
+    schedulingQuantum: Duration
 ) : ComputeService, HostListener {
     /**
      * The [CoroutineScope] of the service bounded by the lifecycle of the service.
@@ -147,9 +147,9 @@ internal class ComputeServiceImpl(
     private val _serversActiveAttr = Attributes.of(AttributeKey.stringKey("state"), "active")
 
     /**
-     * The [TimerScheduler] to use for scheduling the scheduler cycles.
+     * The [Pacer] to use for scheduling the scheduler cycles.
      */
-    private var timerScheduler: TimerScheduler<Unit> = TimerScheduler(scope.coroutineContext, clock)
+    private val pacer = Pacer(scope.coroutineContext, clock, schedulingQuantum.toMillis(), ::doSchedule)
 
     override val hosts: Set<Host>
         get() = hostToView.keys
@@ -354,28 +354,18 @@ internal class ComputeServiceImpl(
      * Indicate that a new scheduling cycle is needed due to a change to the service's state.
      */
     private fun requestSchedulingCycle() {
-        // Bail out in case we have already requested a new cycle or the queue is empty.
-        if (timerScheduler.isTimerActive(Unit) || queue.isEmpty()) {
+        // Bail out in case the queue is empty.
+        if (queue.isEmpty()) {
             return
         }
 
-        val quantum = schedulingQuantum.toMillis()
-
-        // We assume that the provisioner runs at a fixed slot every time quantum (e.g t=0, t=60, t=120).
-        // This is important because the slices of the VMs need to be aligned.
-        // We calculate here the delay until the next scheduling slot.
-        val delay = quantum - (clock.millis() % quantum)
-
-        timerScheduler.startSingleTimer(Unit, delay) {
-            doSchedule()
-        }
+        pacer.enqueue()
     }
 
     /**
      * Run a single scheduling iteration.
      */
-    private fun doSchedule() {
-        val now = clock.millis()
+    private fun doSchedule(now: Long) {
         while (queue.isNotEmpty()) {
             val request = queue.peek()
 
diff --git a/opendc-faas/opendc-faas-service/src/main/kotlin/org/opendc/faas/service/internal/FaaSServiceImpl.kt b/opendc-faas/opendc-faas-service/src/main/kotlin/org/opendc/faas/service/internal/FaaSServiceImpl.kt
index c285585a..82032718 100644
--- a/opendc-faas/opendc-faas-service/src/main/kotlin/org/opendc/faas/service/internal/FaaSServiceImpl.kt
+++ b/opendc-faas/opendc-faas-service/src/main/kotlin/org/opendc/faas/service/internal/FaaSServiceImpl.kt
@@ -37,7 +37,7 @@ import org.opendc.faas.service.deployer.FunctionInstance
 import org.opendc.faas.service.deployer.FunctionInstanceListener
 import org.opendc.faas.service.deployer.FunctionInstanceState
 import org.opendc.faas.service.router.RoutingPolicy
-import org.opendc.utils.TimerScheduler
+import org.opendc.utils.Pacer
 import java.lang.IllegalStateException
 import java.time.Clock
 import java.util.*
@@ -55,7 +55,7 @@ import kotlin.coroutines.resumeWithException
 internal class FaaSServiceImpl(
     context: CoroutineContext,
     private val clock: Clock,
-    private val meterProvider: MeterProvider,
+    meterProvider: MeterProvider,
     private val deployer: FunctionDeployer,
     private val routingPolicy: RoutingPolicy,
     private val terminationPolicy: FunctionTerminationPolicy
@@ -76,9 +76,9 @@ internal class FaaSServiceImpl(
     private val meter = meterProvider.get("org.opendc.faas.service")
 
     /**
-     * The [TimerScheduler] to use for scheduling the scheduler cycles.
+     * The [Pacer] to use for scheduling the scheduler cycles.
      */
-    private val scheduler: TimerScheduler<Unit> = TimerScheduler(scope.coroutineContext, clock)
+    private val pacer = Pacer(scope.coroutineContext, clock, quantum = 100) { doSchedule() }
 
     /**
      * The [Random] instance used to generate unique identifiers for the objects.
@@ -191,19 +191,12 @@ internal class FaaSServiceImpl(
      * Indicate that a new scheduling cycle is needed due to a change to the service's state.
      */
     private fun schedule() {
-        // Bail out in case we have already requested a new cycle or the queue is empty.
-        if (scheduler.isTimerActive(Unit) || queue.isEmpty()) {
+        // Bail out in case the queue is empty.
+        if (queue.isEmpty()) {
             return
         }
 
-        val quantum = 100
-
-        // We assume that the provisioner runs at a fixed slot every time quantum (e.g t=0, t=60, t=120).
-        // This is important because the slices of the VMs need to be aligned.
-        // We calculate here the delay until the next scheduling slot.
-        val delay = quantum - (clock.millis() % quantum)
-
-        scheduler.startSingleTimer(Unit, delay, ::doSchedule)
+        pacer.enqueue()
     }
 
     /**
diff --git a/opendc-utils/build.gradle.kts b/opendc-utils/build.gradle.kts
index 800b374d..858aa64c 100644
--- a/opendc-utils/build.gradle.kts
+++ b/opendc-utils/build.gradle.kts
@@ -26,6 +26,7 @@ description = "Utilities used across OpenDC modules"
 plugins {
     `kotlin-library-conventions`
     `testing-conventions`
+    `jacoco-conventions`
 }
 
 dependencies {
diff --git a/opendc-utils/src/main/kotlin/org/opendc/utils/Pacer.kt b/opendc-utils/src/main/kotlin/org/opendc/utils/Pacer.kt
new file mode 100644
index 00000000..07d3cb87
--- /dev/null
+++ b/opendc-utils/src/main/kotlin/org/opendc/utils/Pacer.kt
@@ -0,0 +1,92 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.utils
+
+import kotlinx.coroutines.*
+import java.lang.Runnable
+import java.time.Clock
+import kotlin.coroutines.ContinuationInterceptor
+import kotlin.coroutines.CoroutineContext
+
+/**
+ * Helper class to pace the incoming scheduling requests.
+ *
+ * @param context The [CoroutineContext] in which the pacer runs.
+ * @param clock The virtual simulation clock.
+ * @param quantum The scheduling quantum.
+ * @param process The process to invoke for the incoming requests.
+ */
+public class Pacer(
+    private val context: CoroutineContext,
+    private val clock: Clock,
+    private val quantum: Long,
+    private val process: (Long) -> Unit
+) {
+    /**
+     * The [Delay] instance that provides scheduled execution of [Runnable]s.
+     */
+    @OptIn(InternalCoroutinesApi::class)
+    private val delay =
+        requireNotNull(context[ContinuationInterceptor] as? Delay) { "Invalid CoroutineDispatcher: no delay implementation" }
+
+    /**
+     * The current [DisposableHandle] representing the pending scheduling cycle.
+     */
+    private var handle: DisposableHandle? = null
+
+    /**
+     * Determine whether a scheduling cycle is pending.
+     */
+    public val isPending: Boolean get() = handle != null
+
+    /**
+     * Enqueue a new scheduling cycle.
+     */
+    public fun enqueue() {
+        if (handle != null) {
+            return
+        }
+
+        val quantum = quantum
+        val now = clock.millis()
+
+        // We assume that the scheduler runs at a fixed slot every time quantum (e.g t=0, t=60, t=120).
+        // We calculate here the delay until the next scheduling slot.
+        val timeUntilNextSlot = quantum - (now % quantum)
+
+        @OptIn(InternalCoroutinesApi::class)
+        handle = delay.invokeOnTimeout(timeUntilNextSlot, {
+            process(now + timeUntilNextSlot)
+            handle = null
+        }, context)
+    }
+
+    /**
+     * Cancel the currently pending scheduling cycle.
+     */
+    public fun cancel() {
+        val handle = handle ?: return
+        this.handle = null
+        handle.dispose()
+    }
+}
diff --git a/opendc-utils/src/test/kotlin/org/opendc/utils/PacerTest.kt b/opendc-utils/src/test/kotlin/org/opendc/utils/PacerTest.kt
new file mode 100644
index 00000000..b8419e80
--- /dev/null
+++ b/opendc-utils/src/test/kotlin/org/opendc/utils/PacerTest.kt
@@ -0,0 +1,127 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.utils
+
+import kotlinx.coroutines.delay
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.assertThrows
+import org.opendc.simulator.core.runBlockingSimulation
+import java.time.Clock
+import kotlin.coroutines.EmptyCoroutineContext
+
+/**
+ * Test suite for the [Pacer] class.
+ */
+class PacerTest {
+    @Test
+    fun testEmptyContext() {
+        assertThrows<IllegalArgumentException> { Pacer(EmptyCoroutineContext, Clock.systemUTC(), 100) {} }
+    }
+
+    @Test
+    fun testSingleEnqueue() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            pacer.enqueue()
+        }
+
+        assertEquals(1, count) { "Process should execute once" }
+    }
+
+    @Test
+    fun testCascade() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            pacer.enqueue()
+            pacer.enqueue()
+
+            assertTrue(pacer.isPending)
+        }
+
+        assertEquals(1, count) { "Process should execute once" }
+    }
+
+    @Test
+    fun testCancel() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            pacer.enqueue()
+            pacer.cancel()
+
+            assertFalse(pacer.isPending)
+        }
+
+        assertEquals(0, count) { "Process should never execute " }
+    }
+
+    @Test
+    fun testCancelWithoutPending() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            assertFalse(pacer.isPending)
+            assertDoesNotThrow { pacer.cancel() }
+
+            pacer.enqueue()
+        }
+
+        assertEquals(1, count) { "Process should execute once" }
+    }
+
+    @Test
+    fun testSubsequent() {
+        var count = 0
+
+        runBlockingSimulation {
+            val pacer = Pacer(coroutineContext, clock, quantum = 100) {
+                count++
+            }
+
+            pacer.enqueue()
+            delay(100)
+            pacer.enqueue()
+        }
+
+        assertEquals(2, count) { "Process should execute twice" }
+    }
+}
diff --git a/opendc-workflow/opendc-workflow-service/src/main/kotlin/org/opendc/workflow/service/internal/WorkflowServiceImpl.kt b/opendc-workflow/opendc-workflow-service/src/main/kotlin/org/opendc/workflow/service/internal/WorkflowServiceImpl.kt
index 7b6d8651..cb4bfd45 100644
--- a/opendc-workflow/opendc-workflow-service/src/main/kotlin/org/opendc/workflow/service/internal/WorkflowServiceImpl.kt
+++ b/opendc-workflow/opendc-workflow-service/src/main/kotlin/org/opendc/workflow/service/internal/WorkflowServiceImpl.kt
@@ -26,7 +26,7 @@ import io.opentelemetry.api.metrics.Meter
 import io.opentelemetry.api.metrics.MeterProvider
 import kotlinx.coroutines.*
 import org.opendc.compute.api.*
-import org.opendc.utils.TimerScheduler
+import org.opendc.utils.Pacer
 import org.opendc.workflow.api.Job
 import org.opendc.workflow.api.WORKFLOW_TASK_CORES
 import org.opendc.workflow.service.*
@@ -187,9 +187,9 @@ public class WorkflowServiceImpl(
         .build()
 
     /**
-     * The [TimerScheduler] to use for scheduling the scheduler cycles.
+     * The [Pacer] to use for scheduling the scheduler cycles.
      */
-    private val timerScheduler: TimerScheduler<Unit> = TimerScheduler(scope.coroutineContext, clock)
+    private val pacer = Pacer(scope.coroutineContext, clock, schedulingQuantum.toMillis()) { doSchedule() }
 
     private val jobAdmissionPolicy: JobAdmissionPolicy.Logic
     private val taskEligibilityPolicy: TaskEligibilityPolicy.Logic
@@ -230,7 +230,7 @@ public class WorkflowServiceImpl(
         rootListener.jobSubmitted(jobInstance)
         submittedJobs.add(1)
 
-        requestSchedulingCycle()
+        pacer.enqueue()
     }
 
     override fun close() {
@@ -252,31 +252,6 @@ public class WorkflowServiceImpl(
     }
 
     /**
-     * Indicate that a new scheduling cycle is needed due to a change to the service's state.
-     */
-    private fun requestSchedulingCycle() {
-        // Bail out in case we have already requested a new cycle or the queue is empty.
-        if (timerScheduler.isTimerActive(Unit)) {
-            return
-        }
-
-        val quantum = schedulingQuantum.toMillis()
-        if (quantum == 0L) {
-            doSchedule()
-            return
-        }
-
-        // We assume that the provisioner runs at a fixed slot every time quantum (e.g t=0, t=60, t=120).
-        // This is important because the slices of the VMs need to be aligned.
-        // We calculate here the delay until the next scheduling slot.
-        val delay = quantum - (clock.millis() % quantum)
-
-        timerScheduler.startSingleTimer(Unit, delay) {
-            doSchedule()
-        }
-    }
-
-    /**
      * Perform a scheduling cycle immediately.
      */
     private fun doSchedule() {
@@ -409,10 +384,9 @@ public class WorkflowServiceImpl(
                     finishJob(job)
                 }
 
-                requestSchedulingCycle()
-            }
-            ServerState.DELETED -> {
+                pacer.enqueue()
             }
+            ServerState.DELETED -> {}
             else -> throw IllegalStateException()
         }
     }