Managing power and thermals

When the HoloLens 2 is running in warm environments or with heavy performance requirements (CPU/GPU usage, peripheral usage, etc.), it might get hot enough that it takes actions automatically to keep itself from overheating. These actions would include things like:

• Adjusting charging performance
• Providing user feedback
• Closing applications

... and in worst-case scenarios:

• Shutting down the HoloLens 2

If your application demands high peripheral performance, consider using the PowerThermalNotification Software Development Kit (SDK) to subscribe to notification events and implement your own custom actions. Doing so can allow the device to operate longer in situations when otherwise an application may be terminated by the system.

This article describes the PowerThermalNotification SDK and its basic usage to get you started.

Where do I get the SDK?

The PowerThermalNotification SDK is downloadable via the Mixed Reality Feature Tool.

PowerThermalNotification SDK supports language projections for C# and C++, allowing developers to develop applications for Win32 or UWP platforms.

Conceptual overview

The power consumed by the HoloLens 2 is dissipated in heat. A traditional PC device would have a fan to address this, but a wearable device must be lightweight. Because of this, the cooling solution is more complex. HoloLens 2 has built-in hardware and software safety features to ensure that the headset doesn't get too hot for the user, but these features must be balanced with user experience as well. For example, if we know which part of the HoloLens 2 is heating up, we can choose to throttle the peripherals responsible for this heat. As a last resort, we might close an application that's thought to be responsible for the power that led to this heat.

HoloLens 2 handles heat issues by using temperature sensors. A thermal framework ties groups of sensors to different peripherals on the device. The sensors are grouped because it may be impossible to determine which peripheral in a physical area is responsible for the power draw that heats up the HoloLens 2.

The PowerThermalNotification SDK exposes the APIs necessary to monitor these groups of sensors. SDK events fire when a peripheral being used by the application is showing signs that a mitigation may be required. The application can then adapt its customer experience to reduce the thermal impact. Reducing impact means less risk of system action such as application or device shutdown.

A simple example would be an application that's using the CPU to process a large amount of video data. The application could subscribe to a performance notification for the CPU component. When the application receives a notification, it can reduce the CPU workload. If another event is received that indicates no further mitigation is necessary, the CPU workload can be restored.

Platform response

The following table is a breakdown of system actions by peripheral. Actions described below can be suppressed using the SDK. See Suppressing Default System Mitigations

Suggestions for application response

The following is a breakdown of suggested mitigations an application can take based on which peripheral(s) need mitigation. It's up to the application developer to determine which of these actions may have a more significant effect on each peripheral since every application is different. Developers should prioritize actions they take based on the impact to the end user.

Suggested mitigations by peripheral

CPU

• Adjust per-frame workload
  • Physics operations
  • Minimize background CPU cycles

GPU

• Reduce render resolution
  • App can then reduce Field of View (FOV) to offset content blurriness
• Reduce scene complexity(number of triangles and texture)
• Reduce photo/video camera processing
  • Reduce camera resolution
  • Reduce camera frame rate
  • Reduce app post-processing of camera images
• Stop using the photo/video camera

DRAM

• Texture complexity
• Reduce render resolution

Network

• Reduce bandwidth
• Reduce video call frame rate
• Stop background network activity (for example, application telemetry)

Battery

• Move to a cooler environment
• Use device without a charger
• Avoid running on a charger with a below-50% charge

Display

• Increase the number of black pixels in the scene
• Use low-power colors (for example, green)
• Dim the display

Photo/video camera

• Overview
• Reduce camera resolution
• Reduce camera frame rate
• Reduce app post-processing of camera images
• Stop using the photo/video camera

Implementation use cases

The SDK is designed to support two standard use cases to get information:

• Event-based
• Polling-based

Event-based notification will provide the quickest feedback path to the application in case it needs to take action. However, in some cases it may be more convenient for the developer to use polling.

Event-based API usage

Registering for events

To get notifications, there are three requirements:

• System support for the API including the specified peripheral
• A non-empty PeripheralsOfInterest property
• A non-empty PowerThermalMitigationLevelChanged event handler or a non-empty PowerThermalThermalScoreChanged event handler

You won't receive events if your application doesn't meet these requirements.

The first item can be checked using the IsSupported function. If the system supports notifications for at least one of the peripherals in the mask, the function will return true. You can choose not to check support using this function as long as your application doesn't explicitly depend on PowerThermalNotification SDK events.

Once you meet the three requirements above, you'll receive initial notifications for all supported PeripheralsOfInterest. If you later change PeripheralsOfInterest or either of the event handlers, you'll receive another set of notifications based on current status.

Here's a code snippet for grabbing the PowerThermalNotification class instance and configuring it for notifications for both PowerThermalPeripheralFlags.Cpu and PowerThermalPeripheralFlags.PhotoVideoCamera:

using Microsoft.MixedReality.PowerThermalNotification;

private void NotificationHandler(object sender, PowerThermalEventArgs args)
{
    //  Notification handling can be done here using information contained in args
}

private void InitializeThermalNotifications()
{
    PowerThermalNotification p = PowerThermalNotification.GetForCurrentProcess();
    
    PowerThermalPeripheralFlags requestedFlags = PowerThermalPeripheralFlags.Cpu | PowerThermalPeripheralFlags.PhotoVideoCamera;
     if (PowerThermalNotification.IsSupported(requestedFlags))
    {
        //At least one of these peripherals is supported by the system
        p.PeripheralsOfInterest = requestedFlags;
        p.PowerThermalMitigationLevelChanged += NotificationHandler;
    }  
}

Handling events

When the PowerThermalMitigationLevelChanged event fires, it comes with PowerThermalEventArgs. These should be used to understand the event.

Similarly, when the PowerThermalThermalScoreChanged event fires, it comes with PowerThermalScoreArgs.

When an event is received, the event handler should inspect args.ImpactedPeripherals, which identifies which peripheral(s) are impacted (there may be more than one).

For PowerThermalMitigationLevelChanged events, the args.MitigationLevel indicates how severe of a mitigation is recommended for the specified peripherals. If the args.MitigationLevel is PowerThermalMitigationLevel.NoUserImpact then any mitigations associated with the specified peripherals should be removed.

For PowerThermalThermalScoreChanged events, the args.ThermalScore indicates a score from 100 to 0 reflecting a linear scale approaching an application shutdown event (zero). The Thermal Score range begins outside of the mitigation reporting range to allow earlier notification to the application when approaching the need for mitigations.

Here's an example handler:

bool doCpuThrottle = false;

private void NotificationHandler(object sender, PowerThermalEventArgs args)
{
    if (args.ImpactedPeripherals.HasFlag(PowerThermalPeripheralFlags.Cpu))
    {
        if(args.MitigationLevel = PowerThermalMitigationLevel.NoUserImpact)
        {
            doCpuThrottle = false;
        }
        else if(args.MitigationLevel >= PowerThermalMitigationLevel.MinimumUserImpact)
        {
            // Note that this only kicks in at MinimumUserImpact and does not get released until NoUserImpact
            doCpuThrottle = true;
        }
    }

    if (args.ImpactedPeripherals.HasFlag(PowerThermalPeripheralFlags.PhotoVideoCamera))
    {
        SetMitigationStatus(PhotoVideoCameraStatusText, PhotoVideoRectangle, args.MitigationLevel);
    }
}

Putting it together (event-based model)

Here's a simple example of a set of scripts that can be used in Unity to enable this functionality. The NotificationComponent class can be added to any game object and that game object can track the mitigation level of the assigned peripheral. The NotificationManager class deals with the SDK managing subscriptions through the single instance of the PowerThermalNotification class.

Here's the NotificationManager class:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

using Microsoft.MixedReality.PowerThermalNotification;

public class NotificationManager
{
    private static readonly object listLock = new object();
    private static List<NotificationComponent> components = new List<NotificationComponent>();
    private static PowerThermalNotification p = PowerThermalNotification.GetForCurrentProcess();
    private static bool FirstTime = true;

    private static void NotificationHandler(object sender, PowerThermalEventArgs args)
    {
        lock (listLock)
        {
            foreach (NotificationComponent c in components)
            {
                UnityEngine.WSA.Application.InvokeOnAppThread(() =>
                {
                    c.SetMitigationLevel(args.ImpactedPeripherals, args.MitigationLevel);
                }, false);
            }
        } 
    }

    public static void ChangeSuppression(PowerThermalPeripheralFlags peripherals, bool suppress)
    {
        p.SuppressPlatformMitigation(peripherals, suppress);
    }

    public static void AddNotification(NotificationComponent component, PowerThermalPeripheralFlags peripheralsOfInterest)
    {
        if (FirstTime)
        {
            p.PowerThermalMitigationLevelChanged += NotificationHandler;
            FirstTime = false;
        }
        
        if (PowerThermalNotification.IsSupported(peripheralsOfInterest))
        {
            lock (listLock)
            {
                component.SetMitigationLevel(peripheralsOfInterest, (PowerThermalMitigationLevel)0);
                components.Add(component);
            }
            p.PeripheralsOfInterest |= peripheralsOfInterest;
        }
    }
}

Here is the NotificationComponent class:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

using Microsoft.MixedReality.PowerThermalNotification;

public class NotificationComponent : MonoBehaviour
{
    //Note that this could be multiple peripherals, just need to make sure to look at impactedPeripherals in the handler
    public PowerThermalPeripheralFlags monitoredPeripheral = (PowerThermalPeripheralFlags) 0;
    public bool isSuppressed = false;

    public void SetMitigationLevel(PowerThermalMitigationLevel level)
    {
        Color newColor = Color.white;

        if (level == PowerThermalMitigationLevel.NoUserImpact)
        {
            newColor = Color.green;
        }
        else if (level == PowerThermalMitigationLevel.MinimumUserImpact)
        {
            newColor = Color.yellow;
        }
        else if (level == PowerThermalMitigationLevel.MediumUserImpact)
        {
            newColor = new Color32(255, 127, 37, 255);//Orange
        }
        else
        {
            newColor = Color.red;
        }

        MaterialPropertyBlock props = new MaterialPropertyBlock();
        props.SetColor("_Color", newColor);
        GetComponent<Renderer>().SetPropertyBlock(props);
    }

    public void SetMitigationLevel(PowerThermalPeripheralFlags impactedPeripherals, PowerThermalMitigationLevel level)
    {
        if (impactedPeripherals.HasFlag(monitoredPeripheral))
        {
            SetMitigationLevel(level);
        }
    }

    void Start()
    {
        NotificationManager.AddNotification(this, monitoredPeripheral);
        NotificationManager.ChangeSuppression(monitoredPeripheral, isSuppressed);
    }

}

Polling-based API usage

Updating peripherals of interest

Similar to the event-based usage, setting the PeripheralsOfInterest property is required to poll a given peripheral.

Calling the polling APIs

Once PeripheralsOfInterest has the peripheral bit(s) set that you wish to poll, you can call into GetLastPeripheralState.

The returned PowerThermalPeripheralState contains the latest values for Thermal Score and Mitigation Level for the given peripheral.

See Handling Events for details on handling of the Thermal Score and MitigationLevel returned by PowerThermalPeripheralState.

Here's a small snippet showing polling:

private async void timerCallback(object state)
{
    await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () =>
    {
        PowerThermalNotification p = PowerThermalNotification.GetForCurrentProcess();

        PowerThermalPeripheralState CpuState = p.GetLatestPeripheralState(PowerThermalPeripheralFlags.Cpu);
        PowerThermalPeripheralState PhotoVideoCameraState = p.GetLatestPeripheralState(PowerThermalPeripheralFlags.PhotoVideoCamera);
        
        CpuScoreText.Text = CpuState.ThermalScore.ToString();
        PhotoVideoScoreText.Text = PhotoVideoCameraState.ThermalScore.ToString();
    });
}

private void InitializeThermalNotifications()
{
    PowerThermalNotification p = PowerThermalNotification.GetForCurrentProcess();

    PowerThermalPeripheralFlags requestedFlags = PowerThermalPeripheralFlags.Cpu | PowerThermalPeripheralFlags.PhotoVideoCamera;
    p.SuppressedPlatformMitigationForPeripherals = requestedFlags;//Suppress any platform mitigation on CPU or PhotoVideoCamera

    if (PowerThermalNotification.IsSupported(requestedFlags))
    {
        p.PeripheralsOfInterest = requestedFlags;

        Timer timer = new Timer(timerCallback, null, 0, 3000);
    }
    else
    {
        TitleLabel.Text = "Not Supported";
    }
}

Suppressing default system mitigations

If you don't want the system to attempt to mitigate certain peripherals, you can suppress them. To do this, just update the SuppressedPlatformMitigationForPeripherals property, or call the SuppressPlatformMitigation function.

Here's a small snippet:

PowerThermalNotification p = PowerThermalNotification.GetForCurrentProcess();
PowerThermalPeripheralFlags requestedFlags = PowerThermalPeripheralFlags.Cpu | PowerThermalPeripheralFlags.PhotoVideoCamera;

//You can do this to set the property explicitly
p.SuppressedPlatformMitigationForPeripherals = requestedFlags;

//Or you can do this to manipulate the property mask. 
//This specific example clears the CPU, leaving the PhotoVideoCamera suppressed
p.SuppressPlatformMitigation(PowerThermalPeripheralFlags.Cpu, false);

Testing

Once you've integrated the SDK into your application, you'll want to test it. For HoloLens 2 operating systems that support the SDK, a developer page will be available in Device Portal. From this page, you can control the mitigation levels and thermal scores for each peripheral. You can also monitor which peripherals have mitigations being actively suppressed.

You can also leverage the REST APIs to monitor/test mitigation levels and thermal scores from another device. More information can be found on the Device Portal API Reference