playing with the event loop

Author: givanse

.gitignore

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+node_modules

index.js

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+#!/usr/bin/env node
+
+// https://medium.com/the-node-js-collection/what-you-should-know-to-really-understand-the-node-js-event-loop-and-its-metrics-c4907b19da4c
+// https://blog.risingstack.com/node-js-at-scale-understanding-node-js-event-loop/#event-loop
+// https://html.spec.whatwg.org/multipage/webappapis.html#task-queue
+
+// The event loop as a process is a set of phases with specific tasks that are processed in a round-robin manner.
+
+const chalk = require('chalk');
+const fs = require('fs');
+
+const logger = {
+  // timers
+  t: function() {
+    console.log(chalk.blue('     timers phase queue'), ...arguments);
+  },
+  // poll phase (IO Callbacks)
+  // it has a hard maximum (system dependent) before it stops polling for more events.
+  p: function() {
+    console.log(chalk.yellow('  poll phase queue'), ...arguments);
+  },
+  // pending callbacks (some system operations)
+  pc: function() {
+    console.log(chalk.gray('pending callbacks'), ...arguments);
+  },
+  /**
+   * nextTick is not really the next tick, its just `immediate` and then
+   * setImmediate is not really immediate at all, its more like the next tick 
+   *  - Bert Belder
+   */
+  // check 
+  // runs immediately after the poll phase completed or has become idle
+  // fires on the following iteration or 'tick' of the event loop
+  immediate: function() {
+    console.log(chalk.magenta('   check phase'), ...arguments);
+  },
+  // Tick - Will resolve before the event loop continues.
+  // It allows you to "starve" your I/O by making recursive process.nextTick() calls
+  // Alternative phrasing:
+  //   Allow the script to run to completion before executing the callback.
+  tick: function() {
+    console.log(chalk.cyan('    tick'), ...arguments);
+  },
+  // close events
+  ce: function() {
+    console.log(chalk.gray('close events'), ...arguments);
+  },
+};
+
+function shuffle(array) {
+  for (let i = array.length - 1; i > 0; i--) {
+      const j = Math.floor(Math.random() * (i + 1));
+      [array[i], array[j]] = [array[j], array[i]];
+  }
+}
+
+function blockQueue(ms) {
+  const startCallback = Date.now();
+  while (Date.now() - startCallback < ms) {
+    // do nothing
+  }
+}
+
+const arr = [
+
+  () => {
+    const m = 0;
+    const scheduled = Date.now();
+    setTimeout(function() {
+      const delay = Date.now() - scheduled;
+      logger.t(`timeout ${m} (delay ${delay})`);
+    }, m);
+    console.log(`setTimeout(${m}) scheduled`);
+  },
+
+  () => {
+    const m = 1000;
+    const scheduled = Date.now();
+    setTimeout(function() {
+      const delay = Date.now() - scheduled - m;
+      logger.t(`timeout ${m} (delay ${delay})`);
+    }, m);
+    console.log(`setTimeout(${m}) scheduled`);
+  },
+
+  () => {
+    const m = 0;
+    const scheduled = Date.now();
+    const id = setInterval(function() {
+      const delay = Date.now() - scheduled;
+      logger.t(`interval ${m} (delay ${delay})`);
+      clearInterval(id);
+    }, m);
+    console.log(`setInterval(${m}) scheduled`);
+  },
+
+  () => {
+    const total = 2;
+    let count = 1;
+    const m = 1000;
+    const scheduled = Date.now();
+    const id = setInterval(function() {
+      let delay = Date.now() - scheduled;
+      delay -= m * count;
+      logger.t(`interval ${m} ${count}/${total} (delay ${delay})`);
+      count++;
+      if (count > total) clearInterval(id);
+    }, m);
+    console.log(`setInterval(${m}) scheduled`);
+  },
+
+  /*() =>  {
+    const m = 1000;
+    fs.readFile('index.js', function(err/*, data/) {
+      if (err) throw err;
+      blockQueue(m);
+      logger.p(`read index.js (blocking ${m})`);
+    });
+    console.log(`readFile (block ${m}) scheduled`);
+  },*/
+
+  () =>  {
+    const m = 2000;
+    fs.readFile('index.js', function(err/*, data*/) {
+      if (err) throw err;
+      blockQueue(m);
+      logger.p(`read index.js (blocking ${m})`);
+
+      global.setImmediate(function() {
+        logger.immediate('setImmediate');
+      });
+    });
+    console.log(`readFile (block ${m}) scheduled`);
+
+  },
+
+  () => {
+    // setImmediate() is actually a special timer that runs in a separate phase of the event loop.
+    // It uses a libuv API that schedules callbacks to execute after the poll phase has completed.
+    /**
+     * The main advantage to using setImmediate() over setTimeout() is setImmediate() will always be executed
+     * before any timers if scheduled within an I/O cycle, independently of how many timers are present.
+     */
+    global.setImmediate(function() {
+      logger.immediate('setImmediate');
+    });
+    console.log(`setImmediate scheduled`);
+  },
+
+  () => {
+    process.nextTick(function() {
+      logger.tick('nextTick');
+    });
+    console.log(`nextTick scheduled`);
+  },
+
+];
+
+function run() {
+  shuffle(arr);
+
+  console.log(chalk.white('start scheduling'));
+  for (const fn of arr) {
+    fn();
+  }
+  console.log(chalk.white('done scheduling\n'));
+}
+
+
+global.setImmediate(function() { console.log('on main: immediate'); });
+global.setTimeout(function() { console.log('on main: timeout'); }, 0);
+process.nextTick(function() { console.log('on main: next tick'); });
+
+run();
+
+console.log('on main: event loop for main complete');

package.json

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+{
+  "name": "event-loop",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "author": "givanse",
+  "license": "MIT",
+  "dependencies": {
+    "chalk": "^2.4.2"
+  }
+}

timeout-vs-immediate.js

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+#!/usr/bin/env node
+
+function test() {
+  setImmediate(() => {
+    console.log('immediate');
+  });
+
+  setTimeout(() => {
+    console.log('timeout');
+  }, 0);
+}
+
+function testAsync() {
+  const p2 = new Promise(function(resolve) {
+    setImmediate(() => {
+      resolve('immediate');
+    });
+  });
+
+  const p1 = new Promise(function(resolve) {
+    setTimeout(() => {
+      resolve('timeout');
+    }, 0);
+  });
+
+  return Promise.all([p1, p2]);
+}
+
+async function run() {
+  for (let i = 0; i < 5; i++) {
+    test();
+  }
+
+  for (let i = 0; i < 5; i++) {
+    const str = await testAsync();
+    console.log(str.join(''));
+  }
+}
+
+run();
+

yarn.lock

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+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+
+ansi-styles@^3.2.1:
+  version "3.2.1"
+  resolved "https://artifacts.riotgames.com:443/artifactory/api/npm/npm/ansi-styles/-/ansi-styles-3.2.1.tgz#41fbb20243e50b12be0f04b8dedbf07520ce841d"
+  integrity sha1-QfuyAkPlCxK+DwS43tvwdSDOhB0=
+  dependencies:
+    color-convert "^1.9.0"
+
+chalk@^2.4.2:
+  version "2.4.2"
+  resolved "https://artifacts.riotgames.com:443/artifactory/api/npm/npm/chalk/-/chalk-2.4.2.tgz#cd42541677a54333cf541a49108c1432b44c9424"
+  integrity sha1-zUJUFnelQzPPVBpJEIwUMrRMlCQ=
+  dependencies:
+    ansi-styles "^3.2.1"
+    escape-string-regexp "^1.0.5"
+    supports-color "^5.3.0"
+
+color-convert@^1.9.0:
+  version "1.9.3"
+  resolved "https://artifacts.riotgames.com:443/artifactory/api/npm/npm/color-convert/-/color-convert-1.9.3.tgz#bb71850690e1f136567de629d2d5471deda4c1e8"
+  integrity sha1-u3GFBpDh8TZWfeYp0tVHHe2kweg=
+  dependencies:
+    color-name "1.1.3"
+
+color-name@1.1.3:
+  version "1.1.3"
+  resolved "https://artifacts.riotgames.com:443/artifactory/api/npm/npm/color-name/-/color-name-1.1.3.tgz#a7d0558bd89c42f795dd42328f740831ca53bc25"
+  integrity sha1-p9BVi9icQveV3UIyj3QIMcpTvCU=
+
+escape-string-regexp@^1.0.5:
+  version "1.0.5"
+  resolved "https://artifacts.riotgames.com:443/artifactory/api/npm/npm/escape-string-regexp/-/escape-string-regexp-1.0.5.tgz#1b61c0562190a8dff6ae3bb2cf0200ca130b86d4"
+  integrity sha1-G2HAViGQqN/2rjuyzwIAyhMLhtQ=
+
+has-flag@^3.0.0:
+  version "3.0.0"
+  resolved "https://artifacts.riotgames.com:443/artifactory/api/npm/npm/has-flag/-/has-flag-3.0.0.tgz#b5d454dc2199ae225699f3467e5a07f3b955bafd"
+  integrity sha1-tdRU3CGZriJWmfNGfloH87lVuv0=
+
+supports-color@^5.3.0:
+  version "5.5.0"
+  resolved "https://artifacts.riotgames.com:443/artifactory/api/npm/npm/supports-color/-/supports-color-5.5.0.tgz#e2e69a44ac8772f78a1ec0b35b689df6530efc8f"
+  integrity sha1-4uaaRKyHcveKHsCzW2id9lMO/I8=
+  dependencies:
+    has-flag "^3.0.0"