Phase Sync is a frame timing management technique used to manage latency adaptively. It is available for native Oculus Quest and Oculus Quest 2 apps as an option in the latest Mobile SDK release.
Phase Sync provides Oculus Quest and Quest 2 apps an alternative to the traditional fixed-latency mode to manage frame timing. Fixed-latency mode means frames are composited as early as possible to avoid the missing the current frame and needing to reuse a stale frame. Stale frames can negatively impact the user experience.
In contrast to fixed-latency, Phase Sync handles frame timing adaptively according to the app’s workload. The goal of Phase Sync is to have a frame finish rendering just before the compositor needs the completed frame. This reduces rendering latency without missing frames.
The adaptive frame timing offered by Phase Sync should be enabled for apps that target both Quest and Quest 2. Note that Quest 2 has more CPU and GPU resources than Quest and can render frames too early, increasing latency, and Phase Sync helps reduce this latency.
The following image shows the difference between a fixed latency and when Phase Sync is enabled for a typical multi-threaded VR app.
Note the following when Phase Sync is enabled:
To enable Phase Sync, call
vrapi_BeginFrame each frame.
vrapi_WaitFramemarks a new frame’s start and automatically adjusts the frame start time to reduce pose prediction latency. In doing so,
vrapi_WaitFramemay block the application’s calling thread.
vrapi_WaitFramemust be called from the main (simulation) thread.
vrapi_WaitFramemay be called from the render thread before calling
vrapi_BeginFrameis called prior to the start of frame rendering and marks a new render frame’s start. Applications must guarantee that
vrapi_BeginFrameis only called after a corresponding call to
vrapi_BeginFrameshould be called from the same thread as
vrapi_SubmitFrame2should be called from the same thread as
vrapi_BeginFrame(render thread) as it indicates the render frame’s completion.
The following image shows the required frame API ordering for multi-threaded apps:
The following image shows the required frame API ordering for single-threaded apps:
Note: It is the app developer’s responsibility to do external synchronization and make sure these APIs are being called from the proper thread and with the correct order. Otherwise, the behavior is undefined.
After the correct frame API ordering is in place, Phase Sync can be successfully enabled. To do so, the app needs to set the VrApi mode flag
VRAPI_MODE_FLAG_PHASE_SYNC to enable Phase Sync explicitly, as in this example:
ovrModeParms parms = vrapi_DefaultModeParms(&app->Java); // Enable Phase Sync parms.Flags |= VRAPI_MODE_FLAG_PHASE_SYNC; app->Ovr = vrapi_EnterVrMode(&parms);
After enabling Phase Sync in your app, you can verify that it is active and see how much latency it has saved by examining the logcat logs.
adb logcat -s VrApi
The Prd value indicates the render latency as measured by the runtime. To calculate how much latency Phase Sync has saved, compare the Prd values between when Phase Sync is active and not active. For example, if the Prd with Phase Sync is 35ms and the Prd without is 45ms, you saved 10ms of latency with Phase Sync.
To more easily compare performance with and without Phase Sync, you can turn it on and off with an adb shell setprop. You must restart the app after changing the setprop for the change to have effect.
adb shell setprop debug.oculus.phaseSync 0
adb shell setprop debug.oculus.phaseSync 1
If you don’t see Lat=-1 after enabling Phase Sync, it is most likely you didn’t call
vrapi_SubmitFrame in the proper sequence. For example, if
vrapi_WaitFrame was missed in a certain frame, the runtime will turn off Phase Sync automatically in this app session, and logcat will print something like the following:
VrApi : Phase sync mode isn't allowed on app [your package name] since WaitFrame / BeginFrame APIs weren't invoked explicitly
For Rift Phase Sync: Oculus Connect 4 | Deep Dive into the Oculus Rift SDK
For extra latency : Understanding Gameplay Latency for Oculus Quest, Oculus Go and Gear VR
For VrApi log: OVR Metrics Tool + VrApi: What Do These Metrics Mean?