V8 has built-in sample based profiling. Profiling is turned off by default, but can be enabled via the
Build the d8 shell following the instructions at [[Building with GN|Building with GN]].
To start profiling, use the
--prof option. When profiling, V8 generates a
v8.log file which contains profiling data.
build\Release\d8 --prof script.js
Other platforms (replace “ia32” with “x64” if you want to profile the x64 build):
out/ia32.release/d8 --prof script.js
Log file processing is done using JS scripts running by the d8 shell. For this to work, a
d8 binary (or symlink, or
d8.exe on Windows) must be in the root of your V8 checkout, or in the path specified by the environment variable
D8_PATH. Note: this binary is just used to process the log, but not for the actual profiling, so it doesn't matter which version etc. it is.
d8 used for analysis was not build with
Mac OS X:
Preprocess the log with
--preprocess (to resolve C++ symbols, etc).
$V8_PATH/tools/linux-tick-processor --preprocess > v8.json
Open the browser at tools/profview/index.html and select the
v8.json file there.
When a snapshot-based VM build is being used, code objects from a snapshot that don't correspond to functions are reported with generic names like “A builtin from the snapshot”, because their real names are not stored in the snapshot. To see the names the following steps must be taken:
--log-snapshot-positions flag must be passed to VM (along with
--prof); this way, for deserialized objects the
(memory address, snapshot offset) pairs are being emitted into profiler log;
--snapshot-log=<log file from mksnapshot> flag must be passed to the tick processor script; a log file from the
mksnapshot program (a snapshot log) contains address-offset pairs for serialized objects, and their names; using the snapshot log, names can be mapped onto deserialized objects during profiler log processing; the snapshot log file is called
snapshot.log and resides alongside with V8's compiled files.
An example of usage:
out/ia32.release/d8 --prof --log-snapshot-positions script.js tools/linux-tick-processor --snapshot-log=out/ia32.release/obj.target/v8_snapshot/geni/snapshot.log v8.log
The timeline plot visualizes 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 the garbage collector). Data for the plot are gathered by both sampling and instrumentation. Linux with gnuplot 4.6 is required.
To create a timeline plot, run V8 as described above, with the option
--log-timer-events additional to
out/ia32.release/d8 --prof --log-timer-events script.js
The output is then passed to a plot script, similar to the tick-processor:
timer-events.png in the working directory, which can be opened with most image viewers.
Since recording log output comes with a certain performance overhead, the script attempts to correct this using a distortion factor. If not specified, it tries to find out automatically. You can however also specify the distortion factor manually.
tools/plot-timer-events --distortion=4500 v8.log
You can also manually specify a certain range for which to create the plot or statistical profile, expressed in milliseconds:
tools/plot-timer-events --distortion=4500 --range=1000,2000 v8.log tools/linux-tick-processor --distortion=4500 --range=1000,2000 v8.log