Accurately Measure an App’s Per-Frame GPU Cost
When optimizing and profiling an app, it is necessary to have an easy method for reporting the entire frame’s GPU performance. This topic aims to describe some available methods and offers a recommendation to use a method that combines logcat VrApi logs with a command that temporarily disables TimeWarp.
A common way to estimate has been to use FPS, with the logic that FPS will be higher when the GPU is faster. However, this is not a good way to judge performance in VR. FPS has never been a stable metric for VR apps due to frame synchronization. Even if your app is GPU bound and your GPU frame cost is never equal to the full frame interval time, there are always manual waits added here and there in some frames. The result isn’t linear or reliable, and fluctuates a lot. It’s best to use another method.
Tools like
Perfetto and
RenderDoc give very detailed performance information. They are a great way to perform app analysis and identify bottlenecks. However, these tools come with performance overhead, making them not ideal if you want your frame’s true cost.
If you want to know how much your GPU frame costs without the overhead of other tools, the recommended way is to use
VrApi logs with a command to disable TimeWarp.
Using VrApi Logs with TimeWarp Disabled to Measure Per-Frame GPU Cost
When you enter adb logcat -s VrApi, you see something similar to the following, where App= is supposed to show you the app’s GPU cost.
However, the compositor is running in the background. The compositor can preempt the app’s GPU frame, so the frame might be overlapping with a couple TimeWarp slices (depending on app frame length and timing). This means the App= value could actually include some TimeWarp cost. To get the exact value, you can adjust for this by using a command to disable TimeWarp temporarily when profiling. The following command will disable TimeWarp for 1 minute:
adb shell am broadcast -a com.oculus.vrruntimeservice.COMPOSITOR_SKIP_RENDERING --ei milliseconds 60000
When you look at the VrApi log again, you will find TW=0.0, which means TimeWarp is disabled. The App= will reflect your app’s true cost:
To remove noise, it is recommended that you look at multiple log lines and average them for each frame.
Additionally, when profiling, it is also recommended you remove the following dynamic performance factors before launching the app:
- Lock the CPU and GPU levels
adb shell setprop debug.oculus.cpuLevel 4adb shell setprop debug.oculus.gpuLevel 4
- Disable dynamic foveation
adb shell setprop debug.oculus.foveation.level 0adb shell setprop debug.oculus.foveation.dynamic 0
