docs/linux-perf.md

title: ‘V8’s Linux `perf` integration’ description: ‘This document explains how to analyze the performance of V8’s JITted code with the Linux `perf` tool.’

V8 has built-in support for the Linux perf tool. It is enabled by the --perf-prof command-line options. V8 writes out performance data during execution into a file that can be used to analyze the performance of V8’s JITted code (including JS-function names) with the Linux perf tool.

Requirements

linux-perf version 5 or higher (previous versions don't have JIT support).
Build V8/Chrome with enable_profiling=true for better symbolized C++ code.

Building V8

To use V8’s integration with Linux perf you need to build it with the enable_profiling = true gn flag:

echo 'enable_profiling = true' >> out/x64.release/args.gn
autoninja -C out/x64.release

Profiling `d8` with `linux-perf-d8.py`

After building d8, you can start using linux perf:

tools/profiling/linux-perf-d8.py out/x64.release/d8 path/to/test.js;

A more complete example:

echo '(function f() {
    var s = 0; for (var i = 0; i < 1000000000; i++) { s += i; } return s;
  })();' > test.js;

# Use custom V8 flags and a separate output dir for less clutter:
mkdir perf_results
tools/profiling/linux-perf-d8.py --perf-data-dir=perf_results \
    out/x64.release/d8 --expose-gc --allow-natives-syntax test.js;

# Fancy UI (`-flame` is googler-only, use `-web` as a public alternative):
pprof -flame perf_results/XXX_perf.data.jitted;
# Terminal-based tool:
perf report -i perf_results/XXX_perf.data.jitted;

Check linux-perf-d8.py --help for more details. Note that you can use all d8 flags after the d8 binary argument.

Profiling Chrome or content_shell with crossbench

We recommend using crossbench for profiling Chrome on all platforms (which uses linux-perf on linux).

$CHROME_CHECKOUT/src/tools/perf/cb loading \
  --browser=out/x64.release/chrome \
  --probe=profiling \
  --url=http://test.com

Profiling Chrome or content_shell on Linux with linux-perf-chrome.py

You can use the linux-perf-chrome.py script to profile Chrome. Make sure to add the required chrome gn flags to get proper C++ symbols.

Once your build is ready, you can profile a website with both full symbols for C++ and JS code.

mkdir perf_results;
tools/profiling/linux-perf-chrome.py out/x64.release/chrome \
    --perf-data-dir=perf_results --timeout=30

Navigate to your website and then close the browser (or wait for the --timeout to complete).

After quitting the browser, linux-perf-chrome.py will post-process the files and show a list with a result file for each renderer process:

chrome_renderer_1583105_3.perf.data.jitted      19.79MiB
chrome_renderer_1583105_2.perf.data.jitted       8.59MiB
chrome_renderer_1583105_4.perf.data.jitted       0.18MiB
chrome_renderer_1583105_1.perf.data.jitted       0.16MiB

Exploring linux-perf results

Finally, you can use the Linux perf tool to explore the profile of a d8 or Chrome renderer process:

perf report -i perf_results/XXX_perf.data.jitted

You can also use pprof to generate more visualizations:

# Note: `-flame` is google-only, use `-web` as a public alternative:
pprof -flame perf_results/XXX_perf.data.jitted;

V8 linux-perf Flags

--perf-prof is used on the V8 command-line to record performance samples in JIT code.

--interpreted-frames-native-stack is used to create different entry points (copied versions of InterpreterEntryTrampoline) for interpreted functions so they can be distinguished by perf based on the address alone. Since the InterpreterEntryTrampoline has to be copied, this comes at a slight performance and memory regression.

title: ‘V8’s Linux perf integration’ description: ‘This document explains how to analyze the performance of V8’s JITted code with the Linux perf tool.’