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Android Jank detection with FrameTimeline

NOTE: FrameTimeline requires Android 12(S) or higher

A frame is said to be janky if the time the frame was presented on screen does not match the predicted present time given by the scheduler.

A jank can cause:

Unstable frame rate
Increased latency

FrameTimeline is a module within SurfaceFlinger that detects janks and reports the source of the jank. SurfaceViews are currently not supported, but will be, in future.

UI

Two new tracks are added for every application that had at least one frame on screen.

Expected Timeline Each slice represents the time given to the app for rendering the frame. To avoid janks in the system, the app is expected to finish within this time frame.
Actual Timeline These slices represent the actual time an app took to complete the frame (including GPU work) and send it to SurfaceFlinger for composition. Note: The actual frame start of apps is not yet known to FrameTimeline. Expected start time is used instead. The end time of the slices here represent max(gpu time, post time). Post time is the time the app's frame was posted to SurfaceFlinger.

Similarly, SurfaceFlinger also gets these two new tracks representing the expected time it's supposed to finish within, and the actual time it took to finish compositing frames and presenting on-screen. Here, SurfaceFlinger's work represents everything underneath it in the display stack. This includes the Composer and the DisplayHAL. So, the slices represent SurfaceFlinger main thread's start to on-screen update.

The names of the slices represent the token received from choreographer. You can compare a slice in the actual timeline track to its corresponding slice in the expected timeline track to see how the app performed compared to the expectations. In addition, for debugging purposes, the token is added to the app's doFrame and RenderThread slices. For SurfaceFlinger, the same token is shown in onMessageReceived.

Selecting an actual timeline slice

The selection details provide more information on what happened with the frame. These include:

Present Type

Was the frame early, on time or late.

On time finish

Did the application finish its work for the frame on time?

Jank Type

Was there a jank observed with this frame? If yes, this shows what type of jank was observed. If not, the type would be None.

Prediction type

Did the prediction expire by the time this frame was received by FrameTimeline? If yes, this will say Expired Prediction. If not, Valid Prediction.

GPU Composition

Boolean that tells if the frame was composited by the GPU or not.

Layer Name

Name of the Layer/Surface to which the frame was presented. Some processes update frames to multiple surfaces. Here, multiple slices with the same token will be shown in the Actual Timeline. Layer Name can be a good way to disambiguate between these slices.

Is Buffer?

Boolean that tells if the frame corresponds to a buffer or an animation.

Flow events

Selecting an actual timeline slice in the app also draws a line back to the corresponding SurfaceFlinger timeline slice.

Since SurfaceFlinger can composite frames from multiple layers into a single frame-on-screen (called a DisplayFrame), selecting a DisplayFrame draws arrows to all the frames that were composited together. This can span over multiple processes.

Color codes

Color	Image	Description
Green		A good frame. No janks observed
Light Green		High latency state. The framerate is smooth but frames are presented late, resulting in an increased input latency.
Red		Janky frame. The process the slice belongs to, is the reason for the jank.
Yellow		Used only by the apps. The frame is janky but app wasn't the reason, SurfaceFlinger caused the jank.
Blue		Dropped frame. Not related to jank. The frame was dropped by SurfaceFlinger, preferring an updated frame over this.

Janks explained

The jank types are defined in JankInfo.h. Since each app is written differently, there is no common way to go into the internals of the apps and specify what the reason for the jank was. Our goal is not to do this but rather, provide a quick way to tell if app was janky or if SurfaceFlinger was janky.

None

All good. No jank with the frame. The ideal state that should be aimed for.

App janks

AppDeadlineMissed

The app ran longer than expected causing a jank. The total time taken by the app frame is calculated by using the choreographer wake-up as the start time and max(gpu, post time) as the end time. Post time is the time the frame was sent to SurfaceFlinger. Since the GPU usually runs in parallel, it could be that the gpu finished later than the post time.

BufferStuffing

This is more of a state than a jank. This happens if the app keeps sending new frames to SurfaceFlinger before the previous frame was even presented. The internal Buffer Queue is stuffed with buffers that are yet to be presented, hence the name, Buffer Stuffing. These extra buffers in the queue are presented only one after the other thus resulting in extra latency. This can also result in a stage where there are no more buffers for the app to use and it goes into a dequeue blocking wait. The actual duration of work performed by the app might still be within the deadline, but due to the stuffed nature, all the frames will be presented at least one vsync late no matter how quickly the app finishes its work. Frames will still be smooth in this state but there is an increased input latency associated with the late present.

SurfaceFlinger Janks

There are two ways SurfaceFlinger can composite frames.

Device Composition - uses a dedicated hardware
GPU/Client composition - uses GPU to composite

An important thing to note is that performing device composition happens as a blocking call on the main thread. However, GPU composition happens in parallel. SurfaceFlinger performs the necessary draw calls and then hands over the gpu fence to the display device. The display device then waits for the fence to be signaled, and then presents the frame.

SurfaceFlingerCpuDeadlineMissed

SurfaceFlinger is expected to finish within the given deadline. If the main thread ran for longer than that, the jank is then SurfaceFlingerCpuDeadlineMissed. SurfaceFlinger’s CPU time is the time spent on the main thread. This includes the entire composition time if device composition was used. If GPU composition was used, this includes the time to write the draw calls and handing over the frame to the GPU.

SurfaceFlingerGpuDeadlineMissed

The time taken by SurfaceFlinger’s main thread on the CPU + the GPU composition time together were longer than expected. Here, the CPU time would have still been within the deadline but since the work on the GPU wasn’t ready on time, the frame got pushed to the next vsync.

DisplayHAL

DisplayHAL jank refers to the case where SurfaceFlinger finished its work and sent the frame down to the HAL on time, but the frame wasn’t presented on the vsync. It was presented on the next vsync. It could be that SurfaceFlinger did not give enough time for the HAL’s work or it could be that there was a genuine delay in the HAL’s work.

PredictionError

SurfaceFlinger’s scheduler plans ahead the time to present the frames. However, this prediction sometimes drifts away from the actual hardware vsync time. For example, a frame might have predicted present time as 20ms. Due to a drift in estimation, the actual present time of the frame could be 23ms. This is called a Prediction Error in SurfaceFlinger’s scheduler. The scheduler corrects itself periodically, so this drift isn’t permanent. However, the frames that had a drift in prediction will still be classified as jank for tracking purposes.

Isolated prediction errors are not usually perceived by the user as the scheduler is quick to adapt and fix the drift.

Unknown jank

As the name suggests, the reason for the jank is unknown in this case. An example here would be that SurfaceFlinger or the App took longer than expected and missed the deadline but the frame was still presented early. The probability of such a jank happening is very low but not impossible.

SQL

At the SQL level, frametimeline data is available in two tables

select ts, dur, surface_frame_token as app_token, display_frame_token as sf_token, process.name
from expected_frame_timeline_slice left join process using(upid)

ts	dur	app_token	sf_token	name
60230453475	20500000	3135	3142	com.google.android.apps.nexuslauncher
60241677540	20500000	3137	3144	com.google.android.apps.nexuslauncher
60252895412	20500000	3139	3146	com.google.android.apps.nexuslauncher
60284614241	10500000	0	3144	/system/bin/surfaceflinger
60295858299	10500000	0	3146	/system/bin/surfaceflinger
60297798913	20500000	3147	3150	com.android.systemui
60307075728	10500000	0	3148	/system/bin/surfaceflinger
60318297746	10500000	0	3150	/system/bin/surfaceflinger
60320236468	20500000	3151	3154	com.android.systemui
60329511401	10500000	0	3152	/system/bin/surfaceflinger
60340732956	10500000	0	3154	/system/bin/surfaceflinger
60342673064	20500000	3155	3158	com.android.systemui

select ts, dur, surface_frame_token as app_token, display_frame_token, jank_type, on_time_finish, present_type, layer_name, process.name
from actual_frame_timeline_slice left join process using(upid)

ts	dur	app_token	sf_token	jank_type	on_time_finish	present_type	layer_name	name
60230453475	26526379	3135	3142	Buffer Stuffing	1	Late Present	TX - com.google.android.apps.nexuslauncher/com.google.android.apps.nexuslauncher.NexusLauncherActivity#0	com.google.android.apps.nexuslauncher
60241677540	28235805	3137	3144	Buffer Stuffing	1	Late Present	TX - com.google.android.apps.nexuslauncher/com.google.android.apps.nexuslauncher.NexusLauncherActivity#0	com.google.android.apps.nexuslauncher
60252895412	2546525	3139	3142	None	1	On-time Present	TX - NavigationBar0#0	com.android.systemui
60252895412	27945382	3139	3146	Buffer Stuffing	1	Late Present	TX - com.google.android.apps.nexuslauncher/com.google.android.apps.nexuslauncher.NexusLauncherActivity#0	com.google.android.apps.nexuslauncher
60284808190	10318230	0	3144	None	1	On-time Present	[NULL]	/system/bin/surfaceflinger
60296067722	10265574	0	3146	None	1	On-time Present	[NULL]	/system/bin/surfaceflinger
60297798913	5239227	3147	3150	None	1	On-time Present	TX - NavigationBar0#0	com.android.systemui
60307246161	10301772	0	3148	None	1	On-time Present	[NULL]	/system/bin/surfaceflinger
60318497204	10281199	0	3150	None	1	On-time Present	[NULL]	/system/bin/surfaceflinger
60320236468	2747559	3151	3154	None	1	On-time Present	TX - NavigationBar0#0	com.android.systemui

TraceConfig

Trace Protos: FrameTimelineEvent

Datasource:

data_sources {
    config {
        name: "android.surfaceflinger.frametimeline"
    }
}

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