microtasks.md

Tasks and micro tasks

This document takes a closer look at both tasks and microtasks.

Tasks

As mentioned in the introduction a task is something that is placed on the task/callback queue and will be picked up by the runtime and placed onto the execution stack. For example, if a function calls setTimeout that will place the function to be executed on the task/callback queue once the timer has expired.

Let's take a look at a concrete example using node.js

$ env NODE_DEBUG=timer node lib/task.js 
main...
TIMER 3924762: no 1 list was found in insert, creating a new one
main...done
TIMER 3924762: process timer lists 145
TIMER 3924762: timeout callback 1
something
TIMER 3924762: 1 list empty

In node.js setTimeout is implemented using libuv's uv_timer_start

Microtasks

The introduction has some background information about microtasks. This page is going to look into the internals.

There is an example of running a microtask in microtask_test.

The Isolate class exposes a number of functions related to enqueueing and running microtasks:

class V8_EXPORT Isolate {                                                       
 public:     
 ...
  V8_DEPRECATE_SOON("Use PerformMicrotaskCheckpoint.")                          
  void RunMicrotasks() { PerformMicrotaskCheckpoint(); }  
  void PerformMicrotaskCheckpoint();
  void EnqueueMicrotask(Local<Function> microtask);
  void EnqueueMicrotask(MicrotaskCallback callback, void* data = nullptr);
};

Lets take a look at what happens when we enqueue a microtask function:

$ lldb -- ./test/microtask_test
(lldb) br s -f microtask_test.cc -l 33
(lldb) r

Isolate::EnqueueMicrotask can be found in src/api/api.cc The actual call will end up in microtask-queue.cc which can be found in src/execution/microtask-queue.cc:

void MicrotaskQueue::EnqueueMicrotask(v8::Isolate* v8_isolate,
                                      v8::Local<Function> function) {
  Isolate* isolate = reinterpret_cast<Isolate*>(v8_isolate);
  HandleScope scope(isolate);
  Handle<CallableTask> microtask = isolate->factory()->NewCallableTask(
      Utils::OpenHandle(*function), isolate->native_context());
  EnqueueMicrotask(*microtask);
}

NewCallableTask can be found in src/heap/factory.cc.