Monitor your web page's total memory usage with measureUserAgentSpecificMemory()

Learn how to measure memory usage of your web page in production to detect regressions.

Brendan Kenny

Ulan Degenbaev

Browsers manage the memory of web pages automatically. Whenever a web page creates an object, the browser allocates a chunk of memory "under the hood" to store the object. Since memory is a finite resource, the browser performs garbage collection to detect when an object is no longer needed and to free the underlying memory chunk.

The detection is not perfect though, and it was proven that perfect detection is an impossible task. Therefore browsers approximate the notion of "an object is needed" with the notion of "an object is reachable". If the web page cannot reach an object via its variables and the fields of other reachable objects, then the browser can safely reclaim the object. The difference between these two notions leads to memory leaks as illustrated by the following example.

const object = {a: new Array(1000), b: new Array(2000)};
setInterval(() => console.log(object.a), 1000);

Here the larger array b is no longer needed, but the browser does not reclaim it because it is still reachable via object.b in the callback. Thus the memory of the larger array is leaked.

Memory leaks are prevalent on the Web. It is easy to introduce one by forgetting to unregister an event listener, by accidentally capturing objects from an iframe, by not closing a worker, by accumulating objects in arrays, and so on. If a web page has memory leaks, then its memory usage grows over time and the web page appears slow and bloated to the users.

The first step in solving this problem is measuring it. The new performance.measureUserAgentSpecificMemory() API allows developers to measure memory usage of their web pages in production and thus detect memory leaks that slip through local testing.

How is `performance.measureUserAgentSpecificMemory()` different from the legacy `performance.memory` API?

If you are familiar with the existing non-standard performance.memory API, you might be wondering how the new API differs from it. The main difference is that the old API returns the size of the JavaScript heap whereas the new API estimates the memory used by the web page. This difference becomes important when Chrome shares the same heap with multiple web pages (or multiple instances of the same web page). In such cases, the result of the old API may be arbitrarily off. Since the old API is defined in implementation-specific terms such as "heap", standardizing it is hopeless.

Another difference is that the new API performs memory measurement during garbage collection. This reduces the noise in the results, but it may take a while until the results are produced. Note that other browsers may decide to implement the new API without relying on garbage collection.

Suggested use cases

Memory usage of a web page depends on the timing of events, user actions, and garbage collections. That is why the memory measurement API is intended for aggregating memory usage data from production. The results of individual calls are less useful. Example use cases:

Regression detection during rollout of a new version of the web page to catch new memory leaks.
A/B testing a new feature to evaluate its memory impact and detect memory leaks.
Correlating memory usage with session duration to verify presence or absence of memory leaks.
Correlating memory usage with user metrics to understand the overall impact of memory usage.

Browser compatibility

Browser Support

Source

Currently the API is supported only in Chromium-based browsers, starting in Chrome 89. The result of the API is highly implementation dependent because browsers have different ways of representing objects in memory and different ways of estimating memory usage. Browsers may exclude some memory regions from accounting if proper accounting is too expensive or infeasible. Thus, results cannot be compared across browsers. It is only meaningful to compare the results for the same browser.

Using `performance.measureUserAgentSpecificMemory()`

Feature detection

The performance.measureUserAgentSpecificMemory function will be unavailable or may fail with a SecurityError if the execution environment does not fulfil the security requirements for preventing cross-origin information leaks. It relies on cross-origin isolation, which a web page can activate by setting COOP+COEP headers.

Support can be detected at runtime:

if (!window.crossOriginIsolated) {
  console.log('performance.measureUserAgentSpecificMemory() is only available in cross-origin-isolated pages');
} else if (!performance.measureUserAgentSpecificMemory) {
  console.log('performance.measureUserAgentSpecificMemory() is not available in this browser');
} else {
  let result;
  try {
    result = await performance.measureUserAgentSpecificMemory();
  } catch (error) {
    if (error instanceof DOMException && error.name === 'SecurityError') {
      console.log('The context is not secure.');
    } else {
      throw error;
    }
  }
  console.log(result);
}

Local testing

Chrome performs the memory measurement during garbage collection, which means that the API does not resolve the result promise immediately and instead waits for the next garbage collection.

Calling the API forces a garbage collection after some timeout, which is currently set to 20 seconds, though may happen sooner. Starting Chrome with the --enable-blink-features='ForceEagerMeasureMemory' command-line flag reduces the timeout to zero and is useful for local debugging and testing.

Example

The recommended usage of the API is to define a global memory monitor that samples memory usage of the whole web page and sends the results to a server for aggregation and analysis. The simplest way is to sample periodically, for example every M minutes. However, that introduces bias to the data because memory peaks may occur between the samples.

The following example shows how to do unbiased memory measurements using a Poisson process, which guarantees that samples are equally likely to occur at any point in time (demo, source).

First, define a function that schedules the next memory measurement using setTimeout() with a randomized interval.

function scheduleMeasurement() {
  // Check measurement API is available.
  if (!window.crossOriginIsolated) {
    console.log('performance.measureUserAgentSpecificMemory() is only available in cross-origin-isolated pages');
    console.log('See https://web.dev/coop-coep/ to learn more')
    return;
  }
  if (!performance.measureUserAgentSpecificMemory) {
    console.log('performance.measureUserAgentSpecificMemory() is not available in this browser');
    return;
  }
  const interval = measurementInterval();
  console.log(`Running next memory measurement in ${Math.round(interval / 1000)} seconds`);
  setTimeout(performMeasurement, interval);
}

The measurementInterval() function computes a random interval in milliseconds such that on average there is one measurement every five minutes. See Exponential distribution if you are interested in the math behind the function.

function measurementInterval() {
  const MEAN_INTERVAL_IN_MS = 5 * 60 * 1000;
  return -Math.log(Math.random()) * MEAN_INTERVAL_IN_MS;
}

Finally, the async performMeasurement() function invokes the API, records the result, and schedules the next measurement.

async function performMeasurement() {
  // 1. Invoke performance.measureUserAgentSpecificMemory().
  let result;
  try {
    result = await performance.measureUserAgentSpecificMemory();
  } catch (error) {
    if (error instanceof DOMException && error.name === 'SecurityError') {
      console.log('The context is not secure.');
      return;
    }
    // Rethrow other errors.
    throw error;
  }
  // 2. Record the result.
  console.log('Memory usage:', result);
  // 3. Schedule the next measurement.
  scheduleMeasurement();
}

Finally, begin measuring.

// Start measurements.
scheduleMeasurement();

The result may look as follows:

// Console output:
{
  bytes: 60_100_000,
  breakdown: [
    {
      bytes: 40_000_000,
      attribution: [{
        url: 'https://example.com/',
        scope: 'Window',
      }],
      types: ['JavaScript']
    },

    {
      bytes: 20_000_000,
      attribution: [{
          url: 'https://example.com/iframe',
          container: {
            id: 'iframe-id-attribute',
            src: '/iframe',
          },
          scope: 'Window',
      }],
      types: ['JavaScript']
    },

    {
      bytes: 100_000,
      attribution: [],
      types: ['DOM']
    },
  ],
}

The total memory usage estimate is returned in the bytes field. This value is highly implementation-dependent and cannot be compared across browsers. It may even change between different versions of the same browser. The value includes JavaScript and DOM memory of all iframes, related windows, and web workers in the current process.

The breakdown list provides further information about the used memory. Each entry describes some portion of the memory and attributes it to a set of windows, iframes, and workers identified by URL. The types field lists the implementation-specific memory types associated with the memory.

It is important to treat all lists in a generic way and to not hardcode assumptions based on a particular browser. For example, some browsers may return an empty breakdown or an empty attribution. Other browsers may return multiple entries in attribution indicating they could not distinguish which of these entries owns the memory.

Feedback

The Web Performance Community Group and the Chrome team would love to hear about your thoughts and experiences with performance.measureUserAgentSpecificMemory().

Tell us about the API design

Is there something about the API that doesn't work as expected? Or are there missing properties that you need to implement your idea? File a spec issue on the performance.measureUserAgentSpecificMemory() GitHub repo or add your thoughts to a existing issue.

Report a problem with the implementation

Did you find a bug with Chrome's implementation? Or is the implementation different from the spec? File a bug at new.crbug.com. Be sure to include as much detail as you can, provide simple instructions for reproducing the bug, and have Components set to Blink>PerformanceAPIs. Glitch works great for sharing quick and easy repros.

Show support

Are you planning to use performance.measureUserAgentSpecificMemory()? Your public support helps the Chrome team prioritize features and shows other browser vendors how critical it is to support them. Send a tweet to @ChromiumDev and let us know where and how you're using it.

Helpful links

Acknowledgements

Big thanks to Domenic Denicola, Yoav Weiss, Mathias Bynens for API design reviews, and Dominik Inführ, Hannes Payer, Kentaro Hara, Michael Lippautz for code reviews in Chrome. I also thank Per Parker, Philipp Weis, Olga Belomestnykh, Matthew Bolohan, and Neil Mckay for providing valuable user feedback that greatly improved the API.

Hero image by Harrison Broadbent on Unsplash