CPU Profiling Chrome

Introduction

These are instructions for collecting a CPU profile of chromium. All of the profiling methods described here produce output that can be view using the pprof tool. pprof is highly customizable; here's a screenshot of some example pprof output:

This doc is intended to be an authoritative one-stop resource for profiling chromium. At the time of writing, there are a number of existing docs with profiling instructions, in varying states of obsolescence:

• ./linux_profiling.md
• ./profiling_content_shell_on_android.md
• https://www.chromium.org/developers/profiling-chromium-and-webkit
• https://www.chromium.org/developers/telemetry/profiling

CPU profiling is not to be confused with tracing or task profiling:

• https://www.chromium.org/developers/how-tos/trace-event-profiling-tool
• https://www.chromium.org/developers/threaded-task-tracking

Profiling on Linux

Profiling support is built into tcmalloc and exposed in chromium, so any platform that uses tcmalloc should be able to generate profiling data without using external tools.

Preparing your checkout

Profiling should always be done on a Release build, which has very similiar performance characteristics to an official build. Make sure the following appears in your args.gn file:

is_debug = false
enable_profiling = true
enable_callgrind = true
blink_symbol_level = 2
symbol_level = 2

Preparing your environment

By default, the profiler will take a sample 100 times per second. You can adjust this rate by setting the CPUPROFILE_FREQUENCY environment variable before launching chromium:

$ export CPUPROFILE_FREQUENCY=1000

The maximum supported rate is 4000 samples per second.

Profiling a process over its entire lifetime

To profile the main browser process, add the following argument to your chrome invocation:

--enable-profiling --profiling-at-start

To profile, e.g., every renderer process, add the following argument to your chrome invocation:

--enable-profiling --profiling-at-start=renderer --no-sandbox

To profile the gpu process, add the following argument to your chrome invocation:

--enable-profiling --profiling-at-start=gpu-process --no-sandbox --profiling-flush

The gpu process does not shut down cleanly and so requires periodic flushing to write the profile to disk.

The --no-sandbox argument is required to allow the renderer process to write the profiling output to the file system.

When the process being profiled ends, you should see one or more chrome-profile-{process type}-{process ID} files in your $PWD. Run pprof to view the results, e.g.:

$ pprof -web chrome-profile-renderer-12345

pprof is packed with useful features for visualizing profiling data. Try pprof --help for more info.

Tip for Googlers: running prodaccess first will make pprof run faster, and eliminate some useless spew to the terminal.

Profiling a process or thread for a defined period of time using perf

First, make sure you have the linux-perf package installed:

$ sudo apt-get install linux-perf

After starting up the browser and loading the page you want to profile, press ‘Shift-Escape’ to bring up the task manager, and get the Process ID of the process you want to profile.

Run the perf tool like this:

$ perf record -g -p <Process ID> -o <output file>

perf does not honor the CPUPROFILE_FREQUENCY env var. To adjust the sampling frequency, use the -F argument, e.g., -F 1000.

To stop profiling, press Control-c in the terminal window where perf is running. Run pprof to view the results, providing the path to the browser executable; e.g.:

$ pprof -web src/out/Release/chrome <perf output file>

pprof is packed with useful features for visualizing profiling data. Try pprof --help for more info.

If you want to limit the profile to a single thread, run:

$ ps -T -p <Process ID> 

From the output, find the Thread ID (column header “SPID”) of the thread you want. Now run perf:

$ perf record -g -t <Thread ID> -o <output file>

Use the same pprof command as above to view the single-thread results.

Profiling the renderer process for a period defined in javascript

You can generate a highly-focused profile for any period that can be defined in javascript using the chrome.gpuBenchmarking javascript interface. First, adding the following command-line flags when you start chrome:

$ chrome --enable-gpu-benchmarking --no-sandbox [...]

Open devtools, and in the console, use chrome.gpuBenchmarking.startProfiling and chrome.gpuBenchmarking.stopProfiling to define a profiling period. e.g.:

> chrome.gpuBenchmarking.startProfiling('perf.data'); doSomething(); chrome.gpuBenchmarking.stopProfiling()

chrome.gpuBenchmarking has a number of useful methods for simulating user-gesture-initiated actions; for example, to profile scrolling:

> chrome.gpuBenchmarking.startProfiling('perf.data'); chrome.gpuBenchmarking.smoothScrollBy(1000, () => { chrome.gpuBenchmarking.stopProfiling() });

Profiling on Android

Android (Nougat and later) supports profiling using the simpleperf tool.

Follow the instructions for building and installing chromium on android. With chromium running on the device, run the following command to start profiling on the browser process (assuming your build is in src/out/Release):

$ src/out/Release/bin/chrome_public_apk profile
Profiler is running; press Enter to stop...

Once you stop the profiler, the profiling data will be copied off the device to the host machine and post-processed so it can be viewed in pprof, as described above.

To profile the renderer process, you must have just one tab open in chromium, and use a command like this:

$ src/out/Release/bin/chrome_public_apk profile --profile-process=renderer

To limit the profile to a single thread, use a command like this:

$ src/out/Release/bin/chrome_public_apk profile --profile-process=renderer --profile-thread=main

The --profile-process and --profile-thread arguments support most of the common process names (‘browser’, ‘gpu’, ‘renderer’) and thread names (‘main’, ‘io’, ‘compositor’, etc.). However, if you need finer control of the process and/or thread to profile, you can specify an explicit Process ID or Thread ID. Check out the usage message for more info:

$ src/out/Release/bin/chrome_public_apk help profile

Profiling on ChromeOS

Follow the simple chrome instructions, to build and deploy chrome to your chromeos device. These instructions will set up a build directory for you, so be sure to gn args out_${SDK_BOARD}/Release to edit them and add the gn args listed above.

The easiest way to get a profile is to ssh to your device, which here will be referred to as chromeos-box, but replace that with whatever ip or hostname your device is. ssh to your device, create a folder in /tmp (which usually has more space than /) and record performance for the entire device. When you're done, use scp to copy the perf.data back to your desk and use pprof as per normal on that perf.data file.

Here's an example:

$ ssh root@chromeos-box
localhost ~ # export CPUPROFILE_FREQUENCY=3000
localhost ~ # mkdir -p /tmp/perf
localhost ~ # cd /tmp/perf
localhost /tmp/perf # perf record -g -a -e cycles
^C
[ perf record: Woken up 402 times to write data ]
[ perf record: Captured and wrote 100.797 MB perf.data (489478 samples) ]
localhost /tmp/perf # exit
$ scp root@chromeos-box:/tmp/perf/perf.data .
$ pprof -web out_${SDK_BOARD}/Release/chrome perf.data

Note: this will complain about missing chromeos symbols. Even pointing PPROF_BINARY_PATH at the expanded debug-board.tgz file that came along with the chromeos image does not seem to work. If you can make this work, please update this doc!

Profiling during a perf benchmark run

The perf benchmark runner can generate a CPU profile over the course of running a perf test. Currently, this is supported only on Linux and Android. To get info about the relevant options, run:

$ src/tools/perf/run_benchmark help run

... and look for the --interval-profiling-* options. For example, to generate a profile of the main thread of the renderer process during the “page interactions” phase of a perf benchmark, you might run:

$ src/tools/perf/run_benchmark run <benchmark name> --interval-profiling-target=renderer:main --interval-profiling-period=interactions --interval-profiling-frequency=2000

The profiling data will be written into the artifacts/ sub-directory of your perf benchmark output directory (default is src/tools/perf), to files with the naming pattern *.profile.pb. You can use pprof to view the results, as described above.

Googlers Only

If you use pprof -proto chrome-profile-renderer-12345 to turn your perf data into a proto file, you can then use that resulting file with internal tools. See http://go/cprof/user#fs-profiles for instructions on how to go about this.