Support custom start-up and clean-up time for VMs (#112)

This pull request implements customizable startup and clean-up time for virtual machine. 
We achieve this by implementing `SimWorkload` chaining and modelling start-up and 
clean-up time using `SimWorkload` as well.

Implements #33

## Implementation Notes :hammer_and_pick:

* Store method parameters in class files
* Add completion parameter to startWorkload
* Provide workload constructors in SimWorkloads 
* Add support for resetting machine context 
* Add support for chaining workloads 
* Use workload chaining for boot delay 
* Model host boot time 
* Do not suspend on guest start 
* Use static logger field 

## Breaking API Changes :warning:

* `SimMachine#startWorkload` now has a third parameter `completion` which is invoked when
   the workload finishes executing (either due to failure or success).
* `SimFlopsWorkload` and `SimRuntimeWorkload` can be instantiated via `SimWorkloads`.

10 files changed, 341 insertions, 100 deletions

diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimAbstractMachine.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimAbstractMachine.java
index cf5aed03..d968d884 100644
--- a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimAbstractMachine.java
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimAbstractMachine.java
@@ -25,6 +25,7 @@ package org.opendc.simulator.compute;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Consumer;
 import org.opendc.simulator.compute.device.SimNetworkAdapter;
 import org.opendc.simulator.compute.model.MachineModel;
 import org.opendc.simulator.compute.model.MemoryUnit;
@@ -62,12 +63,13 @@ public abstract class SimAbstractMachine implements SimMachine {
     }
 
     @Override
-    public final SimMachineContext startWorkload(SimWorkload workload, Map<String, Object> meta) {
+    public final SimMachineContext startWorkload(
+            SimWorkload workload, Map<String, Object> meta, Consumer<Exception> completion) {
         if (activeContext != null) {
             throw new IllegalStateException("A machine cannot run multiple workloads concurrently");
         }
 
-        final Context ctx = createContext(workload, new HashMap<>(meta));
+        final Context ctx = createContext(workload, new HashMap<>(meta), completion);
         ctx.start();
         return ctx;
     }
@@ -83,10 +85,12 @@ public abstract class SimAbstractMachine implements SimMachine {
     /**
      * Construct a new {@link Context} instance representing the active execution.
      *
-     * @param workload The workload to start on the machine.
-     * @param meta The metadata to pass to the workload.
+     * @param workload   The workload to start on the machine.
+     * @param meta       The metadata to pass to the workload.
+     * @param completion A block that is invoked when the workload completes carrying an exception if thrown by the workload.
      */
-    protected abstract Context createContext(SimWorkload workload, Map<String, Object> meta);
+    protected abstract Context createContext(
+            SimWorkload workload, Map<String, Object> meta, Consumer<Exception> completion);
 
     /**
      * Return the active {@link Context} instance (if any).
@@ -102,7 +106,9 @@ public abstract class SimAbstractMachine implements SimMachine {
         private final SimAbstractMachine machine;
         private final SimWorkload workload;
         private final Map<String, Object> meta;
+        private final Consumer<Exception> completion;
         private boolean isClosed;
+        private Exception cause;
 
         /**
          * Construct a new {@link Context} instance.
@@ -110,11 +116,17 @@ public abstract class SimAbstractMachine implements SimMachine {
          * @param machine The {@link SimAbstractMachine} to which the context belongs.
          * @param workload The {@link SimWorkload} to which the context belongs.
          * @param meta The metadata passed to the context.
+         * @param completion A block that is invoked when the workload completes carrying an exception if thrown by the workload.
          */
-        public Context(SimAbstractMachine machine, SimWorkload workload, Map<String, Object> meta) {
+        public Context(
+                SimAbstractMachine machine,
+                SimWorkload workload,
+                Map<String, Object> meta,
+                Consumer<Exception> completion) {
             this.machine = machine;
             this.workload = workload;
             this.meta = meta;
+            this.completion = completion;
         }
 
         @Override
@@ -123,6 +135,28 @@ public abstract class SimAbstractMachine implements SimMachine {
         }
 
         @Override
+        public void reset() {
+            final FlowGraph graph = getMemory().getInput().getGraph();
+
+            for (SimProcessingUnit cpu : getCpus()) {
+                final Inlet inlet = cpu.getInput();
+                graph.disconnect(inlet);
+            }
+
+            graph.disconnect(getMemory().getInput());
+
+            for (SimNetworkInterface ifx : getNetworkInterfaces()) {
+                ((NetworkAdapter) ifx).disconnect();
+            }
+
+            for (SimStorageInterface storage : getStorageInterfaces()) {
+                StorageDevice impl = (StorageDevice) storage;
+                graph.disconnect(impl.getRead());
+                graph.disconnect(impl.getWrite());
+            }
+        }
+
+        @Override
         public final void shutdown() {
             if (isClosed) {
                 return;
@@ -136,11 +170,19 @@ public abstract class SimAbstractMachine implements SimMachine {
             // Cancel all the resources associated with the machine
             doCancel();
 
+            Exception e = this.cause;
+
             try {
                 workload.onStop(this);
             } catch (Exception cause) {
-                LOGGER.warn("Workload failed during onStop callback", cause);
+                if (e != null) {
+                    e.addSuppressed(cause);
+                } else {
+                    e = cause;
+                }
             }
+
+            completion.accept(e);
         }
 
         /**
@@ -151,7 +193,7 @@ public abstract class SimAbstractMachine implements SimMachine {
                 machine.activeContext = this;
                 workload.onStart(this);
             } catch (Exception cause) {
-                LOGGER.warn("Workload failed during onStart callback", cause);
+                this.cause = cause;
                 shutdown();
             }
         }
@@ -160,24 +202,7 @@ public abstract class SimAbstractMachine implements SimMachine {
          * Run the stop procedures for the resources associated with the machine.
          */
         protected void doCancel() {
-            final FlowGraph graph = getMemory().getInput().getGraph();
-
-            for (SimProcessingUnit cpu : getCpus()) {
-                final Inlet inlet = cpu.getInput();
-                graph.disconnect(inlet);
-            }
-
-            graph.disconnect(getMemory().getInput());
-
-            for (SimNetworkInterface ifx : getNetworkInterfaces()) {
-                ((NetworkAdapter) ifx).disconnect();
-            }
-
-            for (SimStorageInterface storage : getStorageInterfaces()) {
-                StorageDevice impl = (StorageDevice) storage;
-                graph.disconnect(impl.getRead());
-                graph.disconnect(impl.getWrite());
-            }
+            reset();
         }
 
         @Override
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimBareMetalMachine.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimBareMetalMachine.java
index aa7502d6..11356eb2 100644
--- a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimBareMetalMachine.java
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimBareMetalMachine.java
@@ -26,6 +26,7 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Consumer;
 import org.opendc.simulator.compute.device.SimPeripheral;
 import org.opendc.simulator.compute.model.MachineModel;
 import org.opendc.simulator.compute.model.ProcessingUnit;
@@ -39,7 +40,7 @@ import org.opendc.simulator.flow2.Inlet;
  *
  * <p>
  * A {@link SimBareMetalMachine} is a stateful object, and you should be careful when operating this object concurrently. For
- * example, the class expects only a single concurrent call to {@link #startWorkload(SimWorkload, Map)}.
+ * example, the class expects only a single concurrent call to {@link #startWorkload(SimWorkload, Map, Consumer)} )}.
  */
 public final class SimBareMetalMachine extends SimAbstractMachine {
     /**
@@ -192,8 +193,9 @@ public final class SimBareMetalMachine extends SimAbstractMachine {
     }
 
     @Override
-    protected SimAbstractMachine.Context createContext(SimWorkload workload, Map<String, Object> meta) {
-        return new Context(this, workload, meta);
+    protected SimAbstractMachine.Context createContext(
+            SimWorkload workload, Map<String, Object> meta, Consumer<Exception> completion) {
+        return new Context(this, workload, meta, completion);
     }
 
     /**
@@ -206,8 +208,12 @@ public final class SimBareMetalMachine extends SimAbstractMachine {
         private final List<NetworkAdapter> net;
         private final List<StorageDevice> disk;
 
-        private Context(SimBareMetalMachine machine, SimWorkload workload, Map<String, Object> meta) {
-            super(machine, workload, meta);
+        private Context(
+                SimBareMetalMachine machine,
+                SimWorkload workload,
+                Map<String, Object> meta,
+                Consumer<Exception> completion) {
+            super(machine, workload, meta, completion);
 
             this.graph = machine.graph;
             this.cpus = machine.cpus;
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimMachine.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimMachine.java
index 59599875..1f86aa02 100644
--- a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimMachine.java
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimMachine.java
@@ -24,6 +24,7 @@ package org.opendc.simulator.compute;
 
 import java.util.List;
 import java.util.Map;
+import java.util.function.Consumer;
 import org.opendc.simulator.compute.device.SimPeripheral;
 import org.opendc.simulator.compute.model.MachineModel;
 import org.opendc.simulator.compute.workload.SimWorkload;
@@ -47,10 +48,11 @@ public interface SimMachine {
      *
      * @param workload The workload to start on the machine.
      * @param meta The metadata to pass to the workload.
+     * @param completion A block that is invoked when the workload completes carrying an exception if thrown by the workload.
      * @return A {@link SimMachineContext} that represents the execution context for the workload.
      * @throws IllegalStateException if a workload is already active on the machine or if the machine is closed.
      */
-    SimMachineContext startWorkload(SimWorkload workload, Map<String, Object> meta);
+    SimMachineContext startWorkload(SimWorkload workload, Map<String, Object> meta, Consumer<Exception> completion);
 
     /**
      * Cancel the active workload on this machine (if any).
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimMachineContext.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimMachineContext.java
index f6a3bd38..5d08e2b7 100644
--- a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimMachineContext.java
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/SimMachineContext.java
@@ -68,6 +68,11 @@ public interface SimMachineContext {
     List<? extends SimStorageInterface> getStorageInterfaces();
 
     /**
+     * Reset all resources of the machine.
+     */
+    void reset();
+
+    /**
      * Shutdown the workload.
      */
     void shutdown();
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/kernel/SimHypervisor.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/kernel/SimHypervisor.java
index 6e295837..f03a0c20 100644
--- a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/kernel/SimHypervisor.java
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/kernel/SimHypervisor.java
@@ -28,6 +28,7 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.SplittableRandom;
+import java.util.function.Consumer;
 import java.util.stream.Collectors;
 import org.opendc.simulator.compute.SimAbstractMachine;
 import org.opendc.simulator.compute.SimMachine;
@@ -471,7 +472,8 @@ public final class SimHypervisor implements SimWorkload {
         }
 
         @Override
-        protected Context createContext(SimWorkload workload, Map<String, Object> meta) {
+        protected Context createContext(
+                SimWorkload workload, Map<String, Object> meta, Consumer<Exception> completion) {
             if (isClosed) {
                 throw new IllegalStateException("Virtual machine does not exist anymore");
             }
@@ -482,7 +484,15 @@ public final class SimHypervisor implements SimWorkload {
             }
 
             return new VmContext(
-                    context, this, random, interferenceDomain, counters, SimHypervisor.this.counters, workload, meta);
+                    context,
+                    this,
+                    random,
+                    interferenceDomain,
+                    counters,
+                    SimHypervisor.this.counters,
+                    workload,
+                    meta,
+                    completion);
         }
 
         @Override
@@ -538,8 +548,9 @@ public final class SimHypervisor implements SimWorkload {
                 VmCounters vmCounters,
                 HvCounters hvCounters,
                 SimWorkload workload,
-                Map<String, Object> meta) {
-            super(machine, workload, meta);
+                Map<String, Object> meta,
+                Consumer<Exception> completion) {
+            super(machine, workload, meta, completion);
 
             this.context = context;
             this.random = random;
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimChainWorkload.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimChainWorkload.java
new file mode 100644
index 00000000..9304122a
--- /dev/null
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimChainWorkload.java
@@ -0,0 +1,182 @@
+/*
+ * 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.simulator.compute.workload;
+
+import java.util.List;
+import java.util.Map;
+import org.opendc.simulator.compute.SimMachineContext;
+import org.opendc.simulator.compute.SimMemory;
+import org.opendc.simulator.compute.SimNetworkInterface;
+import org.opendc.simulator.compute.SimProcessingUnit;
+import org.opendc.simulator.compute.SimStorageInterface;
+import org.opendc.simulator.flow2.FlowGraph;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * A {@link SimWorkload} that composes two {@link SimWorkload}s.
+ */
+final class SimChainWorkload implements SimWorkload {
+    private static final Logger LOGGER = LoggerFactory.getLogger(SimChainWorkload.class);
+
+    private final SimWorkload[] workloads;
+    private int activeWorkloadIndex;
+
+    private Context activeContext;
+
+    /**
+     * Construct a {@link SimChainWorkload} instance.
+     *
+     * @param workloads The workloads to chain.
+     */
+    SimChainWorkload(SimWorkload... workloads) {
+        this.workloads = workloads;
+    }
+
+    @Override
+    public void onStart(SimMachineContext ctx) {
+        final SimWorkload[] workloads = this.workloads;
+        final int activeWorkloadIndex = this.activeWorkloadIndex;
+
+        if (activeWorkloadIndex >= workloads.length) {
+            return;
+        }
+
+        final Context context = new Context(ctx);
+        activeContext = context;
+
+        if (!context.doStart(workloads[activeWorkloadIndex])) {
+            ctx.shutdown();
+        }
+    }
+
+    @Override
+    public void onStop(SimMachineContext ctx) {
+        final SimWorkload[] workloads = this.workloads;
+        final int activeWorkloadIndex = this.activeWorkloadIndex;
+
+        if (activeWorkloadIndex >= workloads.length) {
+            return;
+        }
+
+        final Context context = activeContext;
+        activeContext = null;
+
+        context.doStop(workloads[activeWorkloadIndex]);
+    }
+
+    /**
+     * A {@link SimMachineContext} that intercepts the shutdown calls.
+     */
+    private class Context implements SimMachineContext {
+        private final SimMachineContext ctx;
+
+        private Context(SimMachineContext ctx) {
+            this.ctx = ctx;
+        }
+
+        @Override
+        public FlowGraph getGraph() {
+            return ctx.getGraph();
+        }
+
+        @Override
+        public Map<String, Object> getMeta() {
+            return ctx.getMeta();
+        }
+
+        @Override
+        public List<? extends SimProcessingUnit> getCpus() {
+            return ctx.getCpus();
+        }
+
+        @Override
+        public SimMemory getMemory() {
+            return ctx.getMemory();
+        }
+
+        @Override
+        public List<? extends SimNetworkInterface> getNetworkInterfaces() {
+            return ctx.getNetworkInterfaces();
+        }
+
+        @Override
+        public List<? extends SimStorageInterface> getStorageInterfaces() {
+            return ctx.getStorageInterfaces();
+        }
+
+        @Override
+        public void reset() {
+            ctx.reset();
+        }
+
+        @Override
+        public void shutdown() {
+            final SimWorkload[] workloads = SimChainWorkload.this.workloads;
+            final int activeWorkloadIndex = ++SimChainWorkload.this.activeWorkloadIndex;
+
+            if (doStop(workloads[activeWorkloadIndex - 1]) && activeWorkloadIndex < workloads.length) {
+                ctx.reset();
+
+                if (doStart(workloads[activeWorkloadIndex])) {
+                    return;
+                }
+            }
+
+            ctx.shutdown();
+        }
+
+        /**
+         * Start the specified workload.
+         *
+         * @return <code>true</code> if the workload started successfully, <code>false</code> otherwise.
+         */
+        private boolean doStart(SimWorkload workload) {
+            try {
+                workload.onStart(this);
+            } catch (Exception cause) {
+                LOGGER.warn("Workload failed during onStart callback", cause);
+                doStop(workload);
+                return false;
+            }
+
+            return true;
+        }
+
+        /**
+         * Stop the specified workload.
+         *
+         * @return <code>true</code> if the workload stopped successfully, <code>false</code> otherwise.
+         */
+        private boolean doStop(SimWorkload workload) {
+            try {
+                workload.onStop(this);
+            } catch (Exception cause) {
+                LOGGER.warn("Workload failed during onStop callback", cause);
+                return false;
+            }
+
+            return true;
+        }
+    }
+}
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimFlopsWorkload.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimFlopsWorkload.java
index 255fd1b2..f3efbebb 100644
--- a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimFlopsWorkload.java
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimFlopsWorkload.java
@@ -49,9 +49,9 @@ public class SimFlopsWorkload implements SimWorkload, FlowStageLogic {
      * Construct a new {@link SimFlopsWorkload}.
      *
      * @param flops The number of floating point operations to perform for this task in MFLOPs.
-     * @param utilization A model of the CPU utilization of the application.
+     * @param utilization The CPU utilization of the workload.
      */
-    public SimFlopsWorkload(long flops, double utilization) {
+    SimFlopsWorkload(long flops, double utilization) {
         if (flops < 0) {
             throw new IllegalArgumentException("Number of FLOPs must be positive");
         } else if (utilization <= 0.0 || utilization > 1.0) {
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimRuntimeWorkload.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimRuntimeWorkload.java
index c3380b31..194efafd 100644
--- a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimRuntimeWorkload.java
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimRuntimeWorkload.java
@@ -48,9 +48,9 @@ public class SimRuntimeWorkload implements SimWorkload, FlowStageLogic {
      * Construct a new {@link SimRuntimeWorkload}.
      *
      * @param duration The duration of the workload in milliseconds.
-     * @param utilization A model of the CPU utilization of the application.
+     * @param utilization The CPU utilization of the workload.
      */
-    public SimRuntimeWorkload(long duration, double utilization) {
+    SimRuntimeWorkload(long duration, double utilization) {
         if (duration < 0) {
             throw new IllegalArgumentException("Duration must be positive");
         } else if (utilization <= 0.0 || utilization > 1.0) {
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimWorkloadLifecycle.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimWorkloadLifecycle.java
deleted file mode 100644
index f0e2561f..00000000
--- a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimWorkloadLifecycle.java
+++ /dev/null
@@ -1,60 +0,0 @@
-/*
- * 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.simulator.compute.workload;
-
-import java.util.HashSet;
-import org.opendc.simulator.compute.SimMachineContext;
-
-/**
- * A helper class to manage the lifecycle of a {@link SimWorkload}.
- */
-public final class SimWorkloadLifecycle {
-    private final SimMachineContext ctx;
-    private final HashSet<Runnable> waiting = new HashSet<>();
-
-    /**
-     * Construct a {@link SimWorkloadLifecycle} instance.
-     *
-     * @param ctx The {@link SimMachineContext} of the workload.
-     */
-    public SimWorkloadLifecycle(SimMachineContext ctx) {
-        this.ctx = ctx;
-    }
-
-    /**
-     * Register a "completer" callback that must be invoked before ending the lifecycle of the workload.
-     */
-    public Runnable newCompleter() {
-        Runnable completer = new Runnable() {
-            @Override
-            public void run() {
-                final HashSet<Runnable> waiting = SimWorkloadLifecycle.this.waiting;
-                if (waiting.remove(this) && waiting.isEmpty()) {
-                    ctx.shutdown();
-                }
-            }
-        };
-        waiting.add(completer);
-        return completer;
-    }
-}
diff --git a/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimWorkloads.java b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimWorkloads.java
new file mode 100644
index 00000000..82557d06
--- /dev/null
+++ b/opendc-simulator/opendc-simulator-compute/src/main/java/org/opendc/simulator/compute/workload/SimWorkloads.java
@@ -0,0 +1,70 @@
+/*
+ * 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.simulator.compute.workload;
+
+import java.time.Duration;
+
+/**
+ * Helper methods for constructing {@link SimWorkload}s.
+ */
+public class SimWorkloads {
+    private SimWorkloads() {}
+
+    /**
+     * Create a {@link SimWorkload} that executes a specified number of floating point operations (FLOPs) at the given
+     * utilization.
+     *
+     * @param flops The number of floating point operations to perform for this task in MFLOPs.
+     * @param utilization The CPU utilization of the workload.
+     */
+    public static SimWorkload flops(long flops, double utilization) {
+        return new SimFlopsWorkload(flops, utilization);
+    }
+
+    /**
+     * Create a {@link SimWorkload} that consumes the CPU resources for a specified duration at the given utilization.
+     *
+     * @param duration The duration of the workload in milliseconds.
+     * @param utilization The CPU utilization of the workload.
+     */
+    public static SimWorkload runtime(long duration, double utilization) {
+        return new SimRuntimeWorkload(duration, utilization);
+    }
+
+    /**
+     * Create a {@link SimWorkload} that consumes the CPU resources for a specified duration at the given utilization.
+     *
+     * @param duration The duration of the workload.
+     * @param utilization The CPU utilization of the workload.
+     */
+    public static SimWorkload runtime(Duration duration, double utilization) {
+        return runtime(duration.toMillis(), utilization);
+    }
+
+    /**
+     * Chain the specified <code>workloads</code> into a single {@link SimWorkload}.
+     */
+    public static SimWorkload chain(SimWorkload... workloads) {
+        return new SimChainWorkload(workloads);
+    }
+}