perf_hooks: introduce monitorEventLoopIdleness

`perf_hooks.monitorEventLoopIdleness` tracks the time event loop has spent waiting for new IO events (i.e. difference between IO poll start time and IO poll end time). The API is identical to `monitorEventLoopDelay`. NOTE: Some of the trace event names had to be renamed for consistency.
indutny · Feb 27, 2020 · Feb 27, 2020 · Feb 28, 2020 · 27375989ee47b01c6df96954b225c05cb90de689
commit 27375989ee47b01c6df96954b225c05cb90de689
diff --git a/doc/api/perf_hooks.md b/doc/api/perf_hooks.md
@@ -463,6 +463,23 @@ console.log(h.percentile(50));
 console.log(h.percentile(99));
 ```
 
+## `perf_hooks.monitorEventLoopIdleness([options])`
+<!-- YAML
+added: REPLACEME
+-->
+
+* `options` {Object}
+  * `resolution` {number} The sampling rate in milliseconds. Must be greater
+    than zero. **Default:** `10`.
+* Returns: {Histogram}
+
+Creates a `Histogram` object that samples and reports the event loop idleness
+over time (time difference between IO poll start and poll end). The idleness
+will be reported in nanoseconds.
+
+The behavior of `monitorEventLoopIdleness` is identical to
+[`monitorEventLoopDelay`][] above.
+
 ### Class: `Histogram`
 <!-- YAML
 added: v11.10.0
@@ -643,3 +660,4 @@ require('some-module');
 [`timeOrigin`]: https://w3c.github.io/hr-time/#dom-performance-timeorigin
 [Async Hooks]: async_hooks.html
 [W3C Performance Timeline]: https://w3c.github.io/performance-timeline/
+[`monitorEventLoopDelay`]: #perf_hooks_perf_hooks_monitoreventloopdelay_options
diff --git a/lib/perf_hooks.js b/lib/perf_hooks.js
@@ -12,7 +12,8 @@ const {
 } = primordials;
 
 const {
-  ELDHistogram: _ELDHistogram,
+  ELDHistogram,
+  ELIHistogram,
   PerformanceEntry,
   mark: _mark,
   clearMark: _clearMark,
@@ -558,7 +559,7 @@ function sortedInsert(list, entry) {
   list.splice(location, 0, entry);
 }
 
-class ELDHistogram extends Histogram {
+class EventLoopHistogram extends Histogram {
   enable() { return this[kHandle].enable(); }
   disable() { return this[kHandle].disable(); }
 }
@@ -575,13 +576,29 @@ function monitorEventLoopDelay(options = {}) {
   if (resolution <= 0 || !NumberIsSafeInteger(resolution)) {
     throw new ERR_INVALID_OPT_VALUE.RangeError('resolution', resolution);
   }
-  return new ELDHistogram(new _ELDHistogram(resolution));
+  return new EventLoopHistogram(new ELDHistogram(resolution));
+}
+
+function monitorEventLoopIdleness(options = {}) {
+  if (typeof options !== 'object' || options === null) {
+    throw new ERR_INVALID_ARG_TYPE('options', 'Object', options);
+  }
+  const { resolution = 10 } = options;
+  if (typeof resolution !== 'number') {
+    throw new ERR_INVALID_ARG_TYPE('options.resolution',
+                                   'number', resolution);
+  }
+  if (resolution <= 0 || !NumberIsSafeInteger(resolution)) {
+    throw new ERR_INVALID_OPT_VALUE.RangeError('resolution', resolution);
+  }
+  return new EventLoopHistogram(new ELIHistogram(resolution));
 }
 
 module.exports = {
   performance,
   PerformanceObserver,
-  monitorEventLoopDelay
+  monitorEventLoopDelay,
+  monitorEventLoopIdleness
 };
 
 ObjectDefineProperty(module.exports, 'constants', {

diff --git a/src/async_wrap.h b/src/async_wrap.h
@@ -36,6 +36,7 @@ namespace node {
   V(DIRHANDLE)                                                                \
   V(DNSCHANNEL)                                                               \
   V(ELDHISTOGRAM)                                                             \
+  V(ELIHISTOGRAM)                                                             \
   V(FILEHANDLE)                                                               \
   V(FILEHANDLECLOSEREQ)                                                       \
   V(FSEVENTWRAP)                                                              \