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id perf-profiling
title perf Profiling

This document describes how to profile the performance of Static Hermes.

Profiling JIT

Currently, we rely on Linux perf tool for profiling JIT'ed code, and we only enable JIT on ARM64 platform. So you need to have a Linux machine with ARM64.

How To Profile

  1. Recording the trace. -Xperf-prof is to enable perf profiling, -Xperf-prof-dir is to specify the output directory (by default it's /tmp so if you don't want to look into those generated files, you can skip this flag). -k mono means we use CLOCK_MONOTONIC for clock id, as it's used in jitdump. Here we only sample user space events, but you could also sample kernel events and also specify different flags (e.g., -g for call graph).
  2. Injecting the jitdump into the perf.data. This will create an ELF binary for each JIT'ed function.
  3. Reporting the trace.
perf record -g -k mono -e cycles:u ./bin/hermes ~/js/test.js -Xjit=force \
  -Xperf-prof -Xperf-prof-dir ./
perf inject -j -i perf.data -o perf.data.jitted
perf report -i perf.data.jitted --call-graph=fractal,callee --children