diff --git a/js/src/devtools/gc-ubench/harness.js b/js/src/devtools/gc-ubench/harness.js
index 7d7a64dc92f5..c522ddce1e4e 100644
--- a/js/src/devtools/gc-ubench/harness.js
+++ b/js/src/devtools/gc-ubench/harness.js
@@ -5,6 +5,7 @@
 // Global defaults
 var gDefaultGarbagePiles = "8M";
 var gDefaultGarbagePerFrame = "8K";
+var gDefaultTestDuration = 8.0;
 
 function parse_units(v) {
   if (!v.length) {
@@ -81,6 +82,47 @@ class AllocationLoad {
   }
 }
 
+class LoadCycle {
+  constructor(tests_to_run, duration) {
+    this.queue = [...tests_to_run];
+    this.duration = duration;
+    this.idx = -1;
+  }
+
+  get current() {
+    return this.queue[this.idx];
+  }
+
+  start(now = performance.now()) {
+    this.idx = 0;
+    this.cycleStart = this.started = now;
+  }
+
+  tick(now = performance.now()) {
+    if (this.currentLoadElapsed(now) < this.duration) {
+      return;
+    }
+
+    this.idx++;
+    this.started = now;
+    if (this.idx >= this.queue.length) {
+      this.idx = -1;
+    }
+  }
+
+  done() {
+    return this.idx == -1;
+  }
+
+  cycleElapsed(now = performance.now()) {
+    return now - this.cycleStart;
+  }
+
+  currentLoadElapsed(now = performance.now()) {
+    return now - this.started;
+  }
+}
+
 class AllocationLoadManager {
   constructor(tests) {
     this._loads = new Map();
@@ -89,6 +131,17 @@ class AllocationLoadManager {
     }
     this._active = undefined;
     this._paused = false;
+    this._eventsSinceLastTick = 0;
+
+    // Public API
+    this.cycle = null;
+    this.testDurationMS = gDefaultTestDuration * 1000;
+
+    // Constants
+    this.CYCLE_STARTED = 1;
+    this.CYCLE_STOPPED = 2;
+    this.LOAD_ENDED = 4;
+    this.LOAD_STARTED = 8;
   }
 
   activeLoad() {
@@ -132,20 +185,58 @@ class AllocationLoadManager {
     }
   }
 
-  tick() {
+  tick(now = performance.now()) {
+    this.lastActive = this._active;
+    let events = this._eventsSinceLastTick;
+    this._eventsSinceLastTick = 0;
+
+    if (this.cycle) {
+      const prev = this.cycle.current;
+      this.cycle.tick(now);
+      if (this.cycle.current != prev) {
+        if (this.cycle.current) {
+          this.setActiveLoadByName(this.cycle.current);
+        } else {
+          this.deactivateLoad();
+        }
+        events |= this.LOAD_ENDED;
+        if (this.cycle.done()) {
+          events |= this.CYCLE_STOPPED;
+          this.cycle = null;
+        } else {
+          events |= this.LOAD_STARTED;
+        }
+      }
+    }
+
     if (this._active && !this._paused) {
       this._active.tick();
     }
+
+    return events;
+  }
+
+  startCycle(tests_to_run, now = performance.now()) {
+    this.cycle = new LoadCycle(tests_to_run, this.testDurationMS);
+    this.cycle.start(now);
+    this.setActiveLoadByName(this.cycle.current);
+    this._eventsSinceLastTick |= this.CYCLE_STARTED | this.LOAD_STARTED;
+  }
+
+  cycleStopped() {
+    return !this.cycle || this.cycle.done();
+  }
+
+  cycleCurrentLoadRemaining(now = performance.now()) {
+    return this.cycleStopped()
+      ? 0
+      : this.testDurationMS - this.cycle.currentLoadElapsed(now);
   }
 }
 
 // Current test state.
 var gLoadMgr = undefined;
 
-var testDuration = undefined; // ms
-var testStart = undefined; // ms
-var testQueue = [];
-
 function format_gcBytes(bytes) {
   if (bytes < 4000) {
     return `${bytes} bytes`;
@@ -171,7 +262,7 @@ function compute_test_score(histogram) {
   for (let [delay, count] of histogram) {
     score += Math.abs((delay - 1000 / 60) * count);
   }
-  score = score / (testDuration / 1000);
+  score = score / (gLoadMgr.testDurationMS / 1000);
   return Math.round(score * 1000) / 1000;
 }
 
diff --git a/js/src/devtools/gc-ubench/ui.js b/js/src/devtools/gc-ubench/ui.js
index 234845eaefdf..7fd3696c0f57 100644
--- a/js/src/devtools/gc-ubench/ui.js
+++ b/js/src/devtools/gc-ubench/ui.js
@@ -431,17 +431,20 @@ function handler(timestamp) {
     return;
   }
 
-  if (testState === "running" && timestamp - testStart > testDuration) {
-    end_test(timestamp);
+  const events = gLoadMgr.tick(timestamp);
+  if (events & gLoadMgr.LOAD_ENDED) {
+    end_test(timestamp, gLoadMgr.lastActive);
+    if (!gLoadMgr.cycleStopped()) {
+      start_test();
+    }
   }
 
   if (testState == "running") {
     document.getElementById("test-progress").textContent =
-      ((testDuration - (timestamp - testStart)) / 1000).toFixed(1) + " sec";
+      (gLoadMgr.cycleCurrentLoadRemaining(timestamp) / 1000).toFixed(1) +
+      " sec";
   }
 
-  gLoadMgr.tick();
-
   const delay = gFrameTimer.on_frame_finished(timestamp);
 
   update_histogram(gHistogram, delay);
@@ -592,15 +595,9 @@ function run_all_tests() {
 
 function start_test_cycle(tests_to_run) {
   // Convert from an iterable to an array for pop.
-  testQueue = [];
-  for (var key of tests_to_run) {
-    testQueue.push(key);
-  }
+  gLoadMgr.startCycle(tests_to_run);
   testState = "running";
-  testStart = performance.now();
   gHistogram.clear();
-
-  start_test(testQueue.shift());
   reset_draw_state();
 }
 
@@ -615,24 +612,19 @@ function update_load_state_indicator() {
   document.getElementById("load-running").textContent = loadState;
 }
 
-function start_test(testName) {
-  change_load(testName);
-  console.log(`Running test: ${testName}`);
-  document.getElementById("test-selection").value = testName;
+function start_test() {
+  console.log(`Running test: ${gLoadMgr.activeLoad().name}`);
+  document.getElementById("test-selection").value = gLoadMgr.activeLoad().name;
   update_load_state_indicator();
 }
 
-function end_test(timestamp) {
+function end_test(timestamp, load) {
   document.getElementById("test-progress").textContent = "(not running)";
-  report_test_result(gLoadMgr.activeLoad(), gHistogram);
+  report_test_result(load, gHistogram);
   gHistogram.clear();
-  console.log(`Ending test ${gLoadMgr.activeLoad().name}`);
-  if (testQueue.length) {
-    start_test(testQueue.shift());
-    testStart = timestamp;
-  } else {
+  console.log(`Ending test ${load.name}`);
+  if (gLoadMgr.cycleStopped()) {
     testState = "idle";
-    testStart = 0;
   }
   reset_draw_state();
 }
@@ -699,8 +691,10 @@ function change_load(new_load_name) {
 
 function duration_changed() {
   var durationInput = document.getElementById("test-duration");
-  testDuration = parseInt(durationInput.value) * 1000;
-  console.log(`Updated test duration to: ${testDuration / 1000} seconds`);
+  gLoadMgr.testDurationMS = parseInt(durationInput.value) * 1000;
+  console.log(
+    `Updated test duration to: ${gLoadMgr.testDurationMS / 1000} seconds`
+  );
 }
 
 function onLoadChange() {