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Something you should know about Event Loop

· 2 min read
apriil15

Something you should know about Event Loop!

Concept​

  • all JavaScript, V8, and the event loop run in one thread, called the main thread
  • Node.js has two types of threads: one event loop and worker threads
  • Libuv creates a pool with 4 threads (default) that is only used if no asynchronous API is available
  • event loop is a set of phases with dedicated data structures for each phase

Micro-Task​

  • nextTick, promise are not part of Libuv, and are represented as an event loop on their own
  • micro-task (nextTick, promise) uses the concept of a job queue
  • micro-tasks’ callback will be executed immediately after the current phase of the event loop

micro-task (order: nextTick → promise)

Libuv’s Event Loop Phase​

  • data structure: timer (minHeap), immediate (queue)

Libuv’s event loop phase

Examples​

In this case, setTimeout and setImmediate will be random order

console.log('start')

setTimeout(() => {
  console.log('setTimeout')
}, 0)

setImmediate(() => {
  console.log('setImmediate')
})

console.log('end')

// start
// end
// setImmediate (random order)
// setTimeout (random order)

In this case, because after the IO phase, the next phase is setImmediate. So, setImmediate must before setTimeout

import { readFile } from 'fs'

console.log('start')

readFile('src/test.txt', (err, data) => {
  setTimeout(() => {
    console.log('setTimeout')
  }, 0)

  setImmediate(() => {
    console.log('setImmediate')
  })

  if (!err) {
    console.log(data.toString())
  }
})

console.log('end')

// start
// end
// data
// setImmediate
// setTimeout

Libuv​

  • Libuv will delegate the work to the operating system kernel whenever possible

    The only performance limitation is the network

    • TCP/UDP servers and clients (network request)
    • Pipes
    • dns.resolve - Child processes

  • use libuv’s thread pool

    • fs module (async function)
    • dns.lookup
    • Pipes (edge cases)

Thread Pool​

  • os concept: os has scheduler whose job is to allow certain processes to run on this 4-core system

Libuv’s thread pools (default = 4)

  1. tasks enqueue
  2. worker handles
  3. os scheduler handles
  4. core do something (parallel)

Reference​