nodejs/node

setTimeout Calling Callback Too Early

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#26,578 opened on Mar 11, 2019

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Description

  • Version: v8.15.1
  • Platform: Darwin Charlies-MacBook-Pro.local 18.2.0 Darwin Kernel Version 18.2.0: Thu Dec 20 20:46:53 PST 2018; root:xnu-4903.241.1~1/RELEASE_X86_64 x86_64

I have the following code in Node.js.

const timeout = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

I'm trying to test this code with the following test:

it("Should wait for given time before resolving", async () => {
    const MS = 100;
    const start = process.hrtime();

    await timeout(MS);

    const diff = process.hrtime(start);
    expect(((diff[0] * NS_PER_SEC) + diff[1]) / 1000000).to.at.least(MS);
});

The problem is sometimes (rarely), this test fails:

Should wait for given time before resolving:

  AssertionError: expected 99.595337 to be at least 100
  + expected - actual

  -99.595337
  +100

Obviously this is some type of timing issue with Node.js or something. If anything I expect await timeout(MS); to take slightly longer than MS. In no case do I expect it to take less time.

What is it about the internals of JavaScript/Node.js that causes this to happen?

This occurred on macOS 10.14.3 running Node.js version 8.15.1.

Contributor guide

Tech stack
javascriptnodejs
Domain
backendperformance
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
understanding of Node.js event loopknowledge of setTimeout internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the Node.js timer implementation in lib/timers.js to understand why setTimeout can fire slightly before the specified delay. The issue is likely due to the timer's resolution and the fact that setTimeout uses a minimum delay of 1ms and may fire early due to how the event loop handles timers. Review the code at https://github.com/nodejs/node/blob/master/lib/timers.js and consider the influence of process.hrtime() precision. The test failure shows 99.6ms instead of 100ms, which is within the margin of error. Suggest exploring the use of process.hrtime.bigint() for higher precision, but note that the core issue is about timer accuracy. Also check if this is a known behavior by searching for similar issues in the Node.js repository.