worker: add eventLoopUtilization()

Allow calling eventLoopUtilization() directly on a worker thread:

    const worker = new Worker('./foo.js');
    const elu = worker.performance.eventLoopUtilization();
    setTimeout(() => {
      worker.performance.eventLoopUtilization(elu);
    }, 10);

Add a new performance object on the Worker instance that will hopefully
one day hold all the other performance metrics, such as nodeTiming.

Include benchmarks and tests.

PR-URL: #35664
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

Backport-PR-URL: #37165

Author: trevnorris

Diff for: benchmark/worker/bench-eventlooputil.js

@@ -0,0 +1,61 @@
+'use strict';
+
+const common = require('../common.js');
+const { Worker, parentPort } = require('worker_threads');
+
+if (process.argv[2] === 'idle cats') {
+  return parentPort.once('message', () => {});
+}
+
+const bench = common.createBenchmark(main, {
+  n: [1e6],
+  method: [
+    'ELU_simple',
+    'ELU_passed',
+  ],
+});
+
+function main({ method, n }) {
+  switch (method) {
+    case 'ELU_simple':
+      benchELUSimple(n);
+      break;
+    case 'ELU_passed':
+      benchELUPassed(n);
+      break;
+    default:
+      throw new Error(`Unsupported method ${method}`);
+  }
+}
+
+function benchELUSimple(n) {
+  const worker = new Worker(__filename, { argv: ['idle cats'] });
+
+  spinUntilIdle(worker, () => {
+    bench.start();
+    for (let i = 0; i < n; i++)
+      worker.performance.eventLoopUtilization();
+    bench.end(n);
+    worker.postMessage('bye');
+  });
+}
+
+function benchELUPassed(n) {
+  const worker = new Worker(__filename, { argv: ['idle cats'] });
+
+  spinUntilIdle(worker, () => {
+    let elu = worker.performance.eventLoopUtilization();
+    bench.start();
+    for (let i = 0; i < n; i++)
+      elu = worker.performance.eventLoopUtilization(elu);
+    bench.end(n);
+    worker.postMessage('bye');
+  });
+}
+
+function spinUntilIdle(w, cb) {
+  const t = w.performance.eventLoopUtilization();
+  if (t.idle + t.active > 0)
+    return process.nextTick(cb);
+  setTimeout(() => spinUntilIdle(w, cb), 1);
+}

Diff for: doc/api/perf_hooks.md

@@ -53,6 +53,68 @@ added: v8.5.0
 If `name` is not provided, removes all `PerformanceMark` objects from the
 Performance Timeline. If `name` is provided, removes only the named mark.
 
+### `performance.eventLoopUtilization([utilization1[, utilization2]])`
+<!-- YAML
+added:
+ - v14.10.0
+ - v12.19.0
+-->
+
+* `utilization1` {Object} The result of a previous call to
+    `eventLoopUtilization()`.
+* `utilization2` {Object} The result of a previous call to
+    `eventLoopUtilization()` prior to `utilization1`.
+* Returns {Object}
+  * `idle` {number}
+  * `active` {number}
+  * `utilization` {number}
+
+The `eventLoopUtilization()` method returns an object that contains the
+cumulative duration of time the event loop has been both idle and active as a
+high resolution milliseconds timer. The `utilization` value is the calculated
+Event Loop Utilization (ELU).
+
+If bootstrapping has not yet finished on the main thread the properties have
+the value of `0`. The ELU is immediately available on [Worker threads][] since
+bootstrap happens within the event loop.
+
+Both `utilization1` and `utilization2` are optional parameters.
+
+If `utilization1` is passed, then the delta between the current call's `active`
+and `idle` times, as well as the corresponding `utilization` value are
+calculated and returned (similar to [`process.hrtime()`][]).
+
+If `utilization1` and `utilization2` are both passed, then the delta is
+calculated between the two arguments. This is a convenience option because,
+unlike [`process.hrtime()`][], calculating the ELU is more complex than a
+single subtraction.
+
+ELU is similar to CPU utilization, except that it only measures event loop
+statistics and not CPU usage. It represents the percentage of time the event
+loop has spent outside the event loop's event provider (e.g. `epoll_wait`).
+No other CPU idle time is taken into consideration. The following is an example
+of how a mostly idle process will have a high ELU.
+
+```js
+'use strict';
+const { eventLoopUtilization } = require('perf_hooks').performance;
+const { spawnSync } = require('child_process');
+
+setImmediate(() => {
+  const elu = eventLoopUtilization();
+  spawnSync('sleep', ['5']);
+  console.log(eventLoopUtilization(elu).utilization);
+});
+```
+
+Although the CPU is mostly idle while running this script, the value of
+`utilization` is `1`. This is because the call to
+[`child_process.spawnSync()`][] blocks the event loop from proceeding.
+
+Passing in a user-defined object instead of the result of a previous call to
+`eventLoopUtilization()` will lead to undefined behavior. The return values
+are not guaranteed to reflect any correct state of the event loop.
+
 ### `performance.mark([name])`
 <!-- YAML
 added: v8.5.0
@@ -760,3 +822,4 @@ require('some-module');
 [Performance Timeline]: https://w3c.github.io/performance-timeline/
 [Web Performance APIs]: https://w3c.github.io/perf-timing-primer/
 [User Timing]: https://www.w3.org/TR/user-timing/
+[Worker threads]: worker_threads.md#worker_threads_worker_threads

Diff for: doc/api/worker_threads.md

@@ -767,6 +767,65 @@ If the Worker thread is no longer running, which may occur before the
 [`'exit'` event][] is emitted, the returned `Promise` will be rejected
 immediately with an [`ERR_WORKER_NOT_RUNNING`][] error.
 
+### `worker.performance`
+<!-- YAML
+added: REPLACEME
+-->
+
+An object that can be used to query performance information from a worker
+instance. Similar to [`perf_hooks.performance`][].
+
+#### `performance.eventLoopUtilization([utilization1[, utilization2]])`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `utilization1` {Object} The result of a previous call to
+    `eventLoopUtilization()`.
+* `utilization2` {Object} The result of a previous call to
+    `eventLoopUtilization()` prior to `utilization1`.
+* Returns {Object}
+  * `idle` {number}
+  * `active` {number}
+  * `utilization` {number}
+
+The same call as [`perf_hooks` `eventLoopUtilization()`][], except the values
+of the worker instance are returned.
+
+One difference is that, unlike the main thread, bootstrapping within a worker
+is done within the event loop. So the event loop utilization will be
+immediately available once the worker's script begins execution.
+
+An `idle` time that does not increase does not indicate that the worker is
+stuck in bootstrap. The following examples shows how the worker's entire
+lifetime will never accumulate any `idle` time, but is still be able to process
+messages.
+
+```js
+const { Worker, isMainThread, parentPort } = require('worker_threads');
+
+if (isMainThread) {
+  const worker = new Worker(__filename);
+  setInterval(() => {
+    worker.postMessage('hi');
+    console.log(worker.performance.eventLoopUtilization());
+  }, 100).unref();
+  return;
+}
+
+parentPort.on('message', () => console.log('msg')).unref();
+(function r(n) {
+  if (--n < 0) return;
+  const t = Date.now();
+  while (Date.now() - t < 300);
+  setImmediate(r, n);
+})(10);
+```
+
+The event loop utilization of a worker is available only after the [`'online'`
+event][] emitted, and if called before this, or after the [`'exit'`
+event][], then all properties have the value of `0`.
+
 ### `worker.postMessage(value[, transferList])`
 <!-- YAML
 added: v10.5.0
@@ -880,6 +939,7 @@ active handle in the event system. If the worker is already `unref()`ed calling
 
 [`'close'` event]: #worker_threads_event_close
 [`'exit'` event]: #worker_threads_event_exit
+[`'online'` event]: #worker_threads_event_online
 [`ArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer
 [`AsyncResource`]: async_hooks.html#async_hooks_class_asyncresource
 [`Buffer`]: buffer.html
@@ -899,6 +959,8 @@ active handle in the event system. If the worker is already `unref()`ed calling
 [`fs.open()`]: fs.html#fs_fs_open_path_flags_mode_callback
 [`fs.close()`]: fs.html#fs_fs_close_fd_callback
 [`markAsUntransferable()`]: #worker_threads_worker_markasuntransferable_object
+[`perf_hooks.performance`]: #perf_hooks.md#perf_hooks_perf_hooks_performance
+[`perf_hooks` `eventLoopUtilization()`]: perf_hooks.md#perf_hooks_performance_eventlooputilization_utilization1_utilization2
 [`port.on('message')`]: #worker_threads_event_message
 [`port.onmessage()`]: https://developer.mozilla.org/en-US/docs/Web/API/MessagePort/onmessage
 [`port.postMessage()`]: #worker_threads_port_postmessage_value_transferlist

Diff for: lib/internal/worker.js

@@ -17,6 +17,7 @@ const {
 const EventEmitter = require('events');
 const assert = require('internal/assert');
 const path = require('path');
+const { timeOrigin } = internalBinding('performance');
 
 const errorCodes = require('internal/errors').codes;
 const {
@@ -67,6 +68,8 @@ const kOnMessage = Symbol('kOnMessage');
 const kOnCouldNotSerializeErr = Symbol('kOnCouldNotSerializeErr');
 const kOnErrorMessage = Symbol('kOnErrorMessage');
 const kParentSideStdio = Symbol('kParentSideStdio');
+const kLoopStartTime = Symbol('kLoopStartTime');
+const kIsOnline = Symbol('kIsOnline');
 
 const SHARE_ENV = SymbolFor('nodejs.worker_threads.SHARE_ENV');
 let debug = require('internal/util/debuglog').debuglog('worker', (fn) => {
@@ -214,6 +217,12 @@ class Worker extends EventEmitter {
         null,
       hasStdin: !!options.stdin
     }, transferList);
+    // Use this to cache the Worker's loopStart value once available.
+    this[kLoopStartTime] = -1;
+    this[kIsOnline] = false;
+    this.performance = {
+      eventLoopUtilization: eventLoopUtilization.bind(this),
+    };
     // Actually start the new thread now that everything is in place.
     this[kHandle].startThread();
   }
@@ -245,6 +254,7 @@ class Worker extends EventEmitter {
   [kOnMessage](message) {
     switch (message.type) {
       case messageTypes.UP_AND_RUNNING:
+        this[kIsOnline] = true;
         return this.emit('online');
       case messageTypes.COULD_NOT_SERIALIZE_ERROR:
         return this[kOnCouldNotSerializeErr]();
@@ -406,6 +416,52 @@ function makeResourceLimits(float64arr) {
   };
 }
 
+function eventLoopUtilization(util1, util2) {
+  // TODO(trevnorris): Works to solve the thread-safe read/write issue of
+  // loopTime, but has the drawback that it can't be set until the event loop
+  // has had a chance to turn. So it will be impossible to read the ELU of
+  // a worker thread immediately after it's been created.
+  if (!this[kIsOnline] || !this[kHandle]) {
+    return { idle: 0, active: 0, utilization: 0 };
+  }
+
+  // Cache loopStart, since it's only written to once.
+  if (this[kLoopStartTime] === -1) {
+    this[kLoopStartTime] = this[kHandle].loopStartTime();
+    if (this[kLoopStartTime] === -1)
+      return { idle: 0, active: 0, utilization: 0 };
+  }
+
+  if (util2) {
+    const idle = util1.idle - util2.idle;
+    const active = util1.active - util2.active;
+    return { idle, active, utilization: active / (idle + active) };
+  }
+
+  const idle = this[kHandle].loopIdleTime();
+
+  // Using performance.now() here is fine since it's always the time from
+  // the beginning of the process, and is why it needs to be offset by the
+  // loopStart time (which is also calculated from the beginning of the
+  // process).
+  const active = now() - this[kLoopStartTime] - idle;
+
+  if (!util1) {
+    return { idle, active, utilization: active / (idle + active) };
+  }
+
+  const idle_delta = idle - util1.idle;
+  const active_delta = active - util1.active;
+  const utilization = active_delta / (idle_delta + active_delta);
+  return { idle: idle_delta, active: active_delta, utilization };
+}
+
+// Duplicate code from performance.now() so don't need to require perf_hooks.
+function now() {
+  const hr = process.hrtime();
+  return (hr[0] * 1000 + hr[1] / 1e6) - timeOrigin;
+}
+
 module.exports = {
   ownsProcessState,
   isMainThread,

Diff for: src/node_worker.cc

@@ -765,6 +765,39 @@ void Worker::TakeHeapSnapshot(const FunctionCallbackInfo<Value>& args) {
   args.GetReturnValue().Set(scheduled ? taker->object() : Local<Object>());
 }
 
+void Worker::LoopIdleTime(const FunctionCallbackInfo<Value>& args) {
+  Worker* w;
+  ASSIGN_OR_RETURN_UNWRAP(&w, args.This());
+
+  Mutex::ScopedLock lock(w->mutex_);
+  // Using w->is_stopped() here leads to a deadlock, and checking is_stopped()
+  // before locking the mutex is a race condition. So manually do the same
+  // check.
+  if (w->stopped_ || w->env_ == nullptr)
+    return args.GetReturnValue().Set(-1);
+
+  uint64_t idle_time = uv_metrics_idle_time(w->env_->event_loop());
+  args.GetReturnValue().Set(1.0 * idle_time / 1e6);
+}
+
+void Worker::LoopStartTime(const FunctionCallbackInfo<Value>& args) {
+  Worker* w;
+  ASSIGN_OR_RETURN_UNWRAP(&w, args.This());
+
+  Mutex::ScopedLock lock(w->mutex_);
+  // Using w->is_stopped() here leads to a deadlock, and checking is_stopped()
+  // before locking the mutex is a race condition. So manually do the same
+  // check.
+  if (w->stopped_ || w->env_ == nullptr)
+    return args.GetReturnValue().Set(-1);
+
+  double loop_start_time = w->env_->performance_state()->milestones[
+      node::performance::NODE_PERFORMANCE_MILESTONE_LOOP_START];
+  CHECK_GE(loop_start_time, 0);
+  args.GetReturnValue().Set(
+      (loop_start_time - node::performance::timeOrigin) / 1e6);
+}
+
 namespace {
 
 // Return the MessagePort that is global for this Environment and communicates
@@ -798,6 +831,8 @@ void InitWorker(Local<Object> target,
     env->SetProtoMethod(w, "unref", Worker::Unref);
     env->SetProtoMethod(w, "getResourceLimits", Worker::GetResourceLimits);
     env->SetProtoMethod(w, "takeHeapSnapshot", Worker::TakeHeapSnapshot);
+    env->SetProtoMethod(w, "loopIdleTime", Worker::LoopIdleTime);
+    env->SetProtoMethod(w, "loopStartTime", Worker::LoopStartTime);
 
     Local<String> workerString =
         FIXED_ONE_BYTE_STRING(env->isolate(), "Worker");

Diff for: src/node_worker.h

@@ -66,6 +66,8 @@ class Worker : public AsyncWrap {
       const v8::FunctionCallbackInfo<v8::Value>& args);
   v8::Local<v8::Float64Array> GetResourceLimits(v8::Isolate* isolate) const;
   static void TakeHeapSnapshot(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static void LoopIdleTime(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static void LoopStartTime(const v8::FunctionCallbackInfo<v8::Value>& args);
 
  private:
   void CreateEnvMessagePort(Environment* env);

Diff for: test/parallel/test-performance-eventlooputil.js

@@ -1,8 +1,9 @@
 'use strict';
 
-require('../common');
+const { mustCall } = require('../common');
 
-const TIMEOUT = 50;
+const TIMEOUT = 10;
+const SPIN_DUR = 50;
 
 const assert = require('assert');
 const { performance } = require('perf_hooks');
@@ -21,13 +22,15 @@ if (nodeTiming.loopStart === -1) {
                          { idle: 0, active: 0, utilization: 0 });
 }
 
-// Place in setTimeout() to make sure there is some idle time, but not going to
-// assert this since it could make the test flaky.
-setTimeout(() => {
+setTimeout(mustCall(function r() {
   const t = Date.now();
   const elu1 = eventLoopUtilization();
 
-  while (Date.now() - t < 50) { }
+  // Force idle time to accumulate before allowing test to continue.
+  if (elu1.idle <= 0)
+    return setTimeout(mustCall(r), 5);
+
+  while (Date.now() - t < SPIN_DUR) { }
 
   const elu2 = eventLoopUtilization();
   const elu3 = eventLoopUtilization(elu1);
@@ -38,12 +41,13 @@ setTimeout(() => {
   assert.strictEqual(elu3.utilization, 1);
   assert.strictEqual(elu4.utilization, 1);
   assert.strictEqual(elu2.active - elu1.active, elu4.active);
-  assert.ok(elu2.active > elu3.active);
-  assert.ok(elu2.active > elu4.active);
-  assert.ok(elu3.active > elu4.active);
+  assert.ok(elu3.active > SPIN_DUR - 10, `${elu3.active} <= ${SPIN_DUR - 10}`);
+  assert.ok(elu3.active > elu4.active, `${elu3.active} <= ${elu4.active}`);
+  assert.ok(elu2.active > elu3.active, `${elu2.active} <= ${elu3.active}`);
+  assert.ok(elu2.active > elu4.active, `${elu2.active} <= ${elu4.active}`);
 
-  setTimeout(runIdleTimeTest, TIMEOUT);
-}, 5);
+  setTimeout(mustCall(runIdleTimeTest), TIMEOUT);
+}), 5);
 
 function runIdleTimeTest() {
   const idleTime = nodeTiming.idleTime;
@@ -55,7 +59,7 @@ function runIdleTimeTest() {
   assert.strictEqual(elu1.idle, idleTime);
   assert.strictEqual(elu1.utilization, elu1.active / sum);
 
-  setTimeout(runCalcTest, TIMEOUT, elu1);
+  setTimeout(mustCall(runCalcTest), TIMEOUT, elu1);
 }
 
 function runCalcTest(elu1) {
@@ -65,18 +69,20 @@ function runCalcTest(elu1) {
   const active_delta = elu2.active - elu1.active;
   const idle_delta = elu2.idle - elu1.idle;
 
-  assert.ok(elu2.idle >= 0);
-  assert.ok(elu2.active >= 0);
-  assert.ok(elu3.idle >= 0);
-  assert.ok(elu3.active >= 0);
-  assert.ok(elu2.idle + elu2.active > elu1.idle + elu2.active);
-  assert.ok(elu2.idle + elu2.active >= now - nodeTiming.loopStart);
+  assert.ok(elu2.idle >= 0, `${elu2.idle} < 0`);
+  assert.ok(elu2.active >= 0, `${elu2.active} < 0`);
+  assert.ok(elu3.idle >= 0, `${elu3.idle} < 0`);
+  assert.ok(elu3.active >= 0, `${elu3.active} < 0`);
+  assert.ok(elu2.idle + elu2.active > elu1.idle + elu1.active,
+            `${elu2.idle + elu2.active} <= ${elu1.idle + elu1.active}`);
+  assert.ok(elu2.idle + elu2.active >= now - nodeTiming.loopStart,
+            `${elu2.idle + elu2.active} < ${now - nodeTiming.loopStart}`);
   assert.strictEqual(elu3.active, elu2.active - elu1.active);
   assert.strictEqual(elu3.idle, elu2.idle - elu1.idle);
   assert.strictEqual(elu3.utilization,
                      active_delta / (idle_delta + active_delta));
 
-  setImmediate(runWorkerTest);
+  setImmediate(mustCall(runWorkerTest));
 }
 
 function runWorkerTest() {
@@ -90,10 +96,11 @@ function runWorkerTest() {
   const elu1 = eventLoopUtilization();
   const worker = new Worker(__filename, { argv: [ 'iamalive' ] });
 
-  worker.on('message', (msg) => {
+  worker.on('message', mustCall((msg) => {
     const elu2 = eventLoopUtilization(elu1);
     const data = JSON.parse(msg);
 
-    assert.ok(elu2.active + elu2.idle > data.active + data.idle);
-  });
+    assert.ok(elu2.active + elu2.idle > data.active + data.idle,
+              `${elu2.active + elu2.idle} <= ${data.active + data.idle}`);
+  }));
 }

Diff for: test/parallel/test-worker-eventlooputil.js

@@ -0,0 +1,116 @@
+'use strict';
+
+const { mustCall, mustCallAtLeast } = require('../common');
+
+const assert = require('assert');
+const {
+  Worker,
+  MessageChannel,
+  MessagePort,
+  parentPort,
+} = require('worker_threads');
+const { eventLoopUtilization, now } = require('perf_hooks').performance;
+
+// Use argv to detect whether we're running as a Worker called by this test vs.
+// this test also being called as a Worker.
+if (process.argv[2] === 'iamalive') {
+  const iaElu = idleActive(eventLoopUtilization());
+  // Checks that the worker bootstrap is running after the event loop started.
+  assert.ok(iaElu > 0, `${iaElu} <= 0`);
+  parentPort.once('message', mustCall((msg) => {
+    assert.ok(msg.metricsCh instanceof MessagePort);
+    msg.metricsCh.on('message', mustCallAtLeast(workerOnMetricsMsg, 1));
+  }));
+  return;
+}
+
+function workerOnMetricsMsg(msg) {
+  if (msg.cmd === 'close') {
+    return this.close();
+  }
+
+  if (msg.cmd === 'elu') {
+    return this.postMessage(eventLoopUtilization());
+  }
+
+  if (msg.cmd === 'spin') {
+    const t = now();
+    while (now() - t < msg.dur);
+    return this.postMessage(eventLoopUtilization());
+  }
+}
+
+let worker;
+let metricsCh;
+let mainElu;
+let workerELU;
+
+(function r() {
+  // Force some idle time to accumulate before proceeding with test.
+  if (eventLoopUtilization().idle <= 0)
+    return setTimeout(mustCall(r), 5);
+
+  worker = new Worker(__filename, { argv: [ 'iamalive' ] });
+  metricsCh = new MessageChannel();
+  worker.postMessage({ metricsCh: metricsCh.port1 }, [ metricsCh.port1 ]);
+
+  workerELU = worker.performance.eventLoopUtilization;
+  mainElu = eventLoopUtilization();
+  metricsCh.port2.once('message', mustCall(checkWorkerIdle));
+  metricsCh.port2.postMessage({ cmd: 'elu' });
+  // Make sure it's still safe to call eventLoopUtilization() after the worker
+  // hass been closed.
+  worker.on('exit', mustCall(() => {
+    assert.deepStrictEqual(worker.performance.eventLoopUtilization(),
+                           { idle: 0, active: 0, utilization: 0 });
+  }));
+})();
+
+
+function checkWorkerIdle(wElu) {
+  const tmpMainElu = eventLoopUtilization(mainElu);
+  const perfWorkerElu = workerELU();
+  const eluDiff = eventLoopUtilization(perfWorkerElu, mainElu);
+
+  assert.strictEqual(idleActive(eluDiff),
+                     (perfWorkerElu.active - mainElu.active) +
+                       (perfWorkerElu.idle - mainElu.idle));
+  assert.ok(idleActive(wElu) > 0, `${idleActive(wElu)} <= 0`);
+  assert.ok(idleActive(workerELU(wElu)) > 0,
+            `${idleActive(workerELU(wElu))} <= 0`);
+  assert.ok(idleActive(perfWorkerElu) > idleActive(wElu),
+            `${idleActive(perfWorkerElu)} <= ${idleActive(wElu)}`);
+  assert.ok(idleActive(tmpMainElu) > idleActive(perfWorkerElu),
+            `${idleActive(tmpMainElu)} <= ${idleActive(perfWorkerElu)}`);
+
+  wElu = workerELU();
+  setTimeout(mustCall(() => {
+    wElu = workerELU(wElu);
+    // Some clocks fire early. Removing a few milliseconds to cover that.
+    assert.ok(idleActive(wElu) >= 45, `${idleActive(wElu)} < 45`);
+    // Cutting the idle time in half since it's possible that the call took a
+    // lot of resources to process?
+    assert.ok(wElu.idle >= 25, `${wElu.idle} < 25`);
+
+    checkWorkerActive();
+  }), 50);
+}
+
+function checkWorkerActive() {
+  const w = workerELU();
+
+  metricsCh.port2.postMessage({ cmd: 'spin', dur: 50 });
+  metricsCh.port2.once('message', (wElu) => {
+    const w2 = workerELU(w);
+
+    assert.ok(w2.active >= 50, `${w2.active} < 50`);
+    assert.ok(idleActive(wElu) > idleActive(w2),
+              `${idleActive(wElu)} <= ${idleActive(w2)}`);
+
+    metricsCh.port2.postMessage({ cmd: 'close' });
+  });
+}
+
+function idleActive(elu) {
+  return elu.idle + elu.active;
+}