For visualization, timeline graphs can be plotted to show where V8 is spending time. This can be used to find bottlenecks and spot things that are unexpected (for example, too much time spent in unoptimized code). Data for the plot are gathered by both sampling and instrumentation. Since the latter distorts the performance, the plot script attempts to undistort the logged timestamps.
$ make native -j32
Close all Chrome instances (both Canary and Stable).
To profile a web site in chrome, pass the same flags to V8 using
$ ./chrome --no-sandbox --js-flags="--prof --log-timer-events" mail.google.com & sleep 10; kill $!
Given that Gmail is already logged into, this profiles the first 10 seconds after starting Chrome and loading Gmail, before Chrome is killed.
This will create a v8.log file next to the Chrome binary folder.
If the plotting script fails stating that the logfile is inconsistent, retry while adding --logfile=v8-%p.log to the --js-flags, which will create a separate file for each process suffixed with the process' pid.
Since Android has additional security sandboxing, the renderer process will not be able to write to a file in the application's working directory (even when --no-sandbox is enabled). You must therefore create a directory which can be written by any user and instruct V8 to log it's output there.
$ adb shell mkdir /data/local/tmp/v8-logs/ $ adb shell chmod 777 /data/local/tmp/v8-logs/ [choose adb_content_shell_command_line or adb_chromium_testshell_command_line below depending upon your target] $ ./build/android/adb_content_shell_command_line --no-sandbox --js-flags=\"--prof --log-timer-events --logfile=/data/local/tmp/v8-logs/v8.log\"
Now, after restarting the content shell / chromium test shell, the logs will be available in /data/local/tmp/v8-logs/v8.log and can be retrieved using adb pull.
To profile a script in d8, run d8 with the appropriate flags
$ out/native/d8 --prof --log-timer-events imaging-darkroom.js
The log entries will be written into
Make sure you have the
$ tools/plot-timer-events [location of v8.log]
Some optional environment variables are available:
Colors and layout details are constants in
This technique was described by Peter Flynn (Adobe) in his JSConf talk: https://github.com/peterflynn/jsconf-2013 (jump to 22:07):
You can hack the plot-timer-events script to generate an intermediate .plt file.
This .plt is drawing commands. Delete first line to open in a gnuplot GUI viewer.
In the GUI viewer:
Let's profile Octane's pdfjs benchmark. Due to the nature of the benchmark, having many accesses to typed arrays, we expect a considerable amount of time being spent in external callbacks (which implement typed arrays).
Figure 1: Using only
Figure 2: Adding the option
Figure 3: Using
Figure 4: Having the distortion parameter automatically calibrated (plot range is manually set for easier comparison), we can see that due to the instrumentation overhead, the benchmark run with instrumentation only executed a fraction of what would have been without instrumentation. The background to this is that Octane benchmarks are repeated until a minimum length of run time has been reached. The un-instrumented run manages to complete more iterations of the benchmark than the instrumented run, in the same length of run time.
Now that the plot has been undistorted, it almost completely agrees with previous plots in figures 1 and 2.
Figure 5: Zooming into the interesting part of the undistorted plot.