Announcing the second Compute Pressure origin trial

Kenneth Christiansen

Arnaud (Arno) Mandy

Over the last year, Intel has been collaborating with Google and other parties on the Compute Pressure API. In Chrome 115, you can register for an origin trial to help test this new API, and this post explains the problems the API has been designed to solve, and shows how to use it.

The problem

The web is becoming a key application platform, with new capabilities making applications such as video conferencing not just possible, but a delightful experience for users. Web-based experiences load instantly, they are accessible from anywhere, and need no up-front installation.

Users want fast-loading and responsive applications. They also want to get as much as possible out of their battery life, and silent devices that are not hot to touch. These things can sometimes be hard to achieve when also creating advanced experiences as smooth animations and background video blurring use a lot of processing power, pushing hardware to its limits and draining batteries.

In addition, there are a huge variety of devices being used to access web applications. A five-year old laptop will have very different capabilities to a brand new desktop computer, even when running the same browser version.

Developers often opt to develop for the lowest common denominator, avoiding using some features that would tax older or less capable devices. However, if it were possible to optimize the experience for users who have capable equipment and are in the right environment to benefit from it, why not do it? As an example, when joining a video call from your phone, just seeing the current speaker is likely the best experience. On a desktop however, it would be nice to see everyone on the call, and the hardware is usually up to the task. To achieve this, you need live hardware telemetry, without sacrificing the users' privacy, that can be used for scheduling tasks and progressively turning on and off features to ensure a smooth user experience. This is where the Compute Pressure API can help.

What is the Compute Pressure API?

The Compute Pressure API offers high-level states that represent the pressure on the system. These high-level states ensure a good balance between privacy (not sharing too much specific information that could identify a user) and information that developers can easily reason about. Additionally, it allows the implementation to use the right underlying hardware metrics to ensure that users can take advantage of all the processing power available to them as long as the system is not under unmanageable stress.

Modern CPUs, for instance, are designed to run fine at 100% utilization in most situations, on a single core or across all cores, so an API that hardcodes 80% utilization as being critical could result in developers under-utilizing the hardware's capabilities and offering a suboptimal user experience. On the other hand, a system might not have proper cooling, or the ambient temperature might be very high as in the summer, and the system might be throttling even before reaching high CPU utilization. The current API works on global CPU pressure, but we plan to experiment with enabling CPU pressure per page across the main thread and workers.

Compute pressure has the following states:

Nominal: Current workloads are causing minimal pressure, allowing the system to run at a lower clock frequency to preserve power.
Fair: The system is doing fine; everything is smooth, and it can take on additional work without issues.
Serious: There is some serious pressure on the system, but it is manageable, and the system is doing well, but could be getting close to its limits:
- Clock speed (depending on AC or DC power) is consistently high.
- Thermals are high but still manageable and not causing throttling.

At this point if you add more work the system may move into a critical state.

Critical: The system is now about to reach its limits, but it hasn't reached the limit yet. Critical doesn't mean that the system is being actively throttled, but this state is not sustainable for the long run and might result in throttling if the workload remains the same. This signal is the last call for the web application to lighten its workload.

Enable the Compute Pressure API

By default, the Compute Pressure API is not enabled in Chrome, but it can be experimented with in Chrome 115 by explicitly enabling the functionality. You can activate it locally by enabling the enable-experimental-web-platform-features flag.

To enable it for all visitors to your app, an origin trial is currently underway and set to end in Chrome 118 ( July 18, 2023). To participate in the trial, sign up and include a meta element with the origin trial token in either the HTML or HTTP header. For more information, refer to the Get started with origin trials post.

Observe compute pressure

The following code snippet illustrates how to monitor and act on changes of compute pressure:

// The `records` parameter is a sequence of records between two
// consecutive callbacks. Currently it contains ten entries, but
// this is an implementation detail.
function callback(records) {
  const lastRecord = records.pop();
  console.log(`Current pressure ${lastRecord.state}`);
  if (lastRecord.state === 'critical') {
    // Reduce workers load by 4.
  } else if (lastRecord.state === 'serious') {
    // Reduce workers load by 2.
  } else {
    // Do not reduce.
  }
}

const observer = new PressureObserver(callback, {
  // Sample rate in Hertz.
  sampleRate: 1,
});
observer.observe('cpu');

The following code snippet illustrates how to use the Compute Pressure API from an iframe:

<iframe src="https://mysite.com/" allow="compute-pressure">
  <script>
    // Use Compute Pressure API.
  </script>
</iframe>

Platform support

The Compute Pressure API is available in Chrome 115 on Linux, ChromeOS, macOS, and Windows.

Demo

Try the demo embedded below to see how the compute pressure state changes based on some artificial pressure.

In case your browser doesn't support the API, the video below shows a recording of the demo.

Feedback

Developer feedback is really important at this stage, so please file issues on GitHub with suggestions and questions.

Useful links

Acknowledgements

The hero image was created by Robert Anasch on Unsplash. This article was reviewed by Rachel Andrew and Thomas Steiner.