Analyzing UI Performance with Systrace

谷歌官方关于systrace 的介绍.

Analyzing UI Performance with Systrace
In this document
Overview
Generating a Trace
Analyzing a Trace
Inspecting Frames
Investigating Alerts
Tracing Application Code
See also
Systrace
While developing your application, you should check that user interactions are buttery smooth, running at a consistent 60 frames per second. If something goes wrong, and a frame gets dropped, the first step in fixing the problem is understanding what the system is doing.

The Systrace tool allows you to collect and inspect timing information across an entire Android device, which is called a trace. It shows where time and CPU cycles are being spent, displaying what each thread and process is doing at any given time. It also inpects the captured tracing information to highlight problems that it observes, from list item recycling to rendering content, and provide recommendations about how to fix them. This document explains how to navigate the trace files produced by the tool, and use them to analyze the performance of an application’s user interface (UI).

Overview
Systrace helps you analyze how the execution of your application fits into the many running systems on an Android device. It puts together system and application thread execution on a common timeline. In order to analyze your app with Systrace, you first collect a trace log of your app, and the system activity. The generated trace allows you to view highly detailed, interactive reports showing everything happening the system for the traced duration.

Systrace example overview
Figure 1. An example Systrace, showing 5 seconds of scrolling an app when it is not performing well.

Figure 1. shows a trace captured while scrolling an app that is not rendering smoothly. By default, a zoomed out view of the traced duration is shown. The horizontal axis is time, and trace events are grouped by process, and then by thread on the vertical axis.

The groupings are in the order Kernel, SurfaceFlinger (the android compositor process), followed by apps, each labeled by package name. Each app process contains all of the tracing signals from each thread it contains, including a hierarchy of high level tracing events based on the enabled tracing categories.

Generating a Trace
In order to create a trace of your application, you must perform a few setup steps. First, you must have a device running Android 4.1 (API 16) or higher. Set up the device for debugging, connect it to your development system, and install your application. Some types of trace information, specifically disk activity and kernel work queues, require that you have root access to the device. However, most Systrace log data only requires that the device be enabled for developer debugging.

Systrace traces can be run either from a command line or from a graphical user interface. This guide focuses on using the command line options.

Tracing on Android 4.3 and higher

To run a trace on Android 4.3 and higher devices:

Make sure the device is connected through a USB cable and is enabled for debugging.
Run the trace with the options you want, for example:
cdandroid−sdk/platform−tools/systrace python systrace.py –time=10 -o mynewtrace.html sched gfx view wm
On the device, execute any user actions you want be included in the trace.
For more information on the available options for running Systrace, see the Systrace help page.

Tracing on Android 4.2 and lower

To use Systrace effectively with devices running Android 4.2 and lower, you must configure the types of processes you want to trace before running a trace. The tool can gather the following types of process information:

General system processes such as graphics, audio and input processes (selected using trace category tags).
Low level system information such as CPU, kernel and disk activity (selected using options).
To set trace tags for Systrace using the command-line:

Use the –set-tags option:
cdandroid−sdk/platform−tools/systrace python systrace.py –set-tags=gfx,view,wm
Stop and restart the adb shell to enable tracing of these processes.
adbshellstop adb shell start
To set trace tags for Systrace using the device user interface:

On the device connected for tracing, navigate to: Settings > Developer options > Monitoring > Enable traces.
Select the categories of processes to be traced and click OK.
Note: The adb shell does not have to be stopped and restarted when selecting trace tags using this method.

After you have configured the category tags for your trace, you can start collecting information for analysis.

To run a trace using the current trace tag settings:

Make sure the device is connected through a USB cable and is enabled for debugging.
Run the trace with the low-level system trace options and limits you want, for example:
$ python systrace.py –cpu-freq –cpu-load –time=10 -o mytracefile.html
On the device, execute any user actions you want be included in the trace.
For more information on the available options for running Systrace, see the Systrace help page.

Analyzing a Trace
After you have generated a trace, open the output html file using a web browser. This section explains how to analyze and interpret the information that the tool produces to find and fix UI performance problems.

Inspecting Frames

Each app that is rendering frames shows a row of frame circles, which are typically colored green. Circles that are colored yellow or red, exceeding the 16.6 millisecond run time limit required to maintain a stable 60 frames per second. Zoom in using the ‘w’ key to see the frames of your application, and look for long-running frames getting in the way of smoothness.

Note: Hit the ‘?’ key, or the button in the top right for help navigating the trace.

Zoomed in view of a frame
Figure 2. Systrace display after zooming in on a long-running frame.

Clicking on one such frame highlights it, focusing only on the work done by the system for that frame. On devices running Android 5.0 (API level 21) or higher, this work is split between the UI Thread and RenderThread. On prior versions, all work in creating a frame is done on the UI Thread.

Click on individual components of the frame to see how long they took to run. Some events, such as performTraversals, describe what the system is doing in that method when you select it. Selecting a frame displays any alerts present in that frame.

Investigating Alerts

Systrace does automatic analysis of the events in the trace, and highlights many performance problems as alerts, suggesting what to do next.

Problematic frame selected
Figure 3. Selecting the problematic frame, an alert is shown identifying a problem.

After you select a slow frame such as the one shown in Figure 3, an alert may be displayed. In the case above, it calls out that the primary problem with the frame is too much work being done inside ListView recycling and rebinding. There are links to the relevant events in the trace, which can be followed to explain more about what the system is doing during this time.

If you see too much work being done on the UI thread, as in this case with this ListView work, you can use Traceview, the app code profiling tool, to investigate exactly what is taking so much time.

Note that you can also find about every alert in the trace by clicking the Alerts tab to the far right of the window. Doing so expands the Alerts panel, where you can see every alert that the tool discovered in your trace, along with an occurrence count.

Alert tab shown
Figure 4. Clicking the Alert button to the right reveals the alert tab.

The Alerts panel helps you see which problems occur in the trace, and how often they contribute to jank. Think of the alerts panel as a list of bugs to be fixed, often a tiny change or improvement in one area can eliminate an entire class of alerts from your application!

Tracing Application Code
The tracing signals defined by the framework do not have visibility into everything your application is doing, so you may want to add your own. In Android 4.3 (API level 18) and higher, you can use the methods of the Trace class to add signals to your code. This technique can help you see what work your application’s threads are doing at any given time. Tracing begin and end events do add overhead while a trace is being captured, a few microseconds each, but sprinkling in a few per frame, or per worker thread task can go a long way to adding context to a trace of your app.

The following code example shows how to use the Trace class to track execution of an application method, including two nested code blocks within that method.

public void ProcessPeople() {
Trace.beginSection(“ProcessPeople”);
try {
Trace.beginSection(“Processing Jane”);
try {
// code for Jane task…
} finally {
Trace.endSection(); // ends “Processing Jane”
}

    Trace.beginSection("Processing John");
    try {
        // code for John task...
    } finally {
        Trace.endSection(); // ends "Processing John"
    }
} finally {
    Trace.endSection(); // ends "ProcessPeople"
}

}
Note: When you nest trace calls within each other, the endSection() method ends the most recently called beginSection(String) method. This means that a trace started within another trace cannot extend beyond the end of the enclosing trace, so make sure your beginning and ending method calls are properly matched to measure your applications processing.

Note: Traces must begin and end on the same thread. Do not call beginSection(String) on one thread of execution and then attempt to end the trace with a call to endSection() on another thread.

When using application-level tracing with Systrace, you must specify the package name of your application in the user interface or specify the -a or –app= options on the command line. For more information, see the Systrace usage guide.

You should enable app level tracing when profiling your app, even if you have not added signals yourself. Library code can include very useful tracing signals when you enable application-level tracing. The RecyclerView class is a great example of this, providing information about several important stages of work it executes.

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