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. Profiling in ChromeClose all Chrome instances (both Canary and Stable). --js-flags and with --no-sandbox to enable writing into v8.log . For example: $ ./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. Chrome on AndroidSince 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. For example: $ 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.Profiling in d8To profile a script in d8, run d8 with the appropriate flags--prof enables tick based profiling--log-internal-timer-events enables timing the runtime of V8's internal components such as the GC, parser, compiler, etc.--log-timer-events implies --log-internal-timer-events , but also times external callbacks.For example $ out/native/d8 --prof --log-timer-events imaging-darkroom.js The log entries will be written into PlottingFind the sure you have theisolate-***.log or v8.log file created during profiling. It was created in the folder you launched that command from.
Interpretation of the plot. A case study.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. Other resources |
For Developers >