Performance.now() discrepancies in workers

performance.now() Discrepancies in Workers – A Detailed Explanation

Introduction

The performance.now() method provides high-resolution timestamps with better precision than Date.now(). However, when used in Web Workers, discrepancies may arise due to differences in how timestamps are recorded and reported. These discrepancies can affect performance measurements and event timing, leading to unexpected results in time-sensitive applications.

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Understanding performance.now()

The performance.now() method returns the current timestamp in milliseconds with sub-millisecond precision (typically microsecond accuracy). It measures time relative to the page load, making it useful for performance tracking and benchmarking.

console.log(performance.now()); // Example: 1234.567890

Advantages of performance.now()

• Higher precision than Date.now()
• Monotonic clock (not affected by system clock adjustments)
• Useful for measuring short intervals

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How performance.now() Works in Workers

In Web Workers, performance.now() functions similarly but operates within the worker’s own execution context. Since workers run in separate threads, their performance timestamps may differ from those in the main thread.

self.onmessage = function (e) {
  console.log("Worker Time: ", performance.now());
};

Key Differences Between Main Thread and Worker Context

1. Independent Time Origins:
   • performance.now() in a worker starts counting from when the worker script starts executing, not from the page load time.
   • This means performance.now() values in a worker will be different from those in the main thread.
2. Potential Inconsistencies Across Browsers:
   • Different browsers may implement timekeeping in slightly different ways, leading to small discrepancies.
3. Cross-Context Timing Challenges:
   • Comparing timestamps from the main thread and workers can be problematic since their time origins are independent.

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Example of Discrepancy in Workers

Scenario: Measuring Time Difference Between Main Thread and Worker

Main Thread Code

const worker = new Worker("worker.js");

console.log("Main Thread Time:", performance.now());

worker.postMessage("Start");

Worker Code (worker.js)

self.onmessage = function (e) {
  console.log("Worker Time:", performance.now());
};

Expected Output

Main Thread Time: 1023.456
Worker Time: 2.345  // Starts from worker initialization

Why This Happens?

• The worker’s performance.now() starts from its own load time, so it does not align with the main thread’s timestamp.
• The worker might take time to initialize before executing the onmessage function.

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How to Handle performance.now() Discrepancies in Workers?

Solution 1: Use Shared Time Origin

Instead of relying on performance.now(), use Date.now() to synchronize timing between threads.

Main Thread

const worker = new Worker("worker.js");
const mainStartTime = Date.now();

console.log("Main Thread Time:", mainStartTime);
worker.postMessage(mainStartTime);

Worker

self.onmessage = function (e) {
  const workerStartTime = Date.now();
  console.log("Worker Time:", workerStartTime);
  console.log("Time Difference:", workerStartTime - e.data, "ms");
};

Solution 2: Transfer performance.now() Value from Main Thread

Since the main thread’s performance.now() is accurate relative to the page load, you can transfer this value to the worker.

const worker = new Worker("worker.js");
const timeOrigin = performance.now();
worker.postMessage(timeOrigin);

self.onmessage = function (e) {
  console.log("Adjusted Worker Time:", performance.now() + e.data);
};

Solution 3: Use Performance API with performance.timeOrigin

Instead of performance.now(), use performance.timeOrigin, which provides a common reference point.

const worker = new Worker("worker.js");
worker.postMessage(performance.timeOrigin);

self.onmessage = function (e) {
  console.log("Worker Time Based on Main Origin:", e.data + performance.now());
};

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Best Practices for Accurate Time Measurement

1. Use Date.now() if synchronization across threads is required.
2. Use performance.now() only for local (within the same thread) high-precision measurements.
3. Use performance.timeOrigin to align timestamps between main thread and workers.
4. Minimize reliance on absolute timestamps across different execution contexts.
5. Test across multiple browsers to ensure consistency.

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