rspack-sftrace

name	rspack-sftrace
description	Use sftrace, which is based on LLVM Xray instrumentation, to trace all Rust function calls. This can be used for performance analysis and troubleshooting.

Rspack Sftrace

Overview

Use sftrace (LLVM XRay) to trace rspack's Rust function calls and convert them to perfetto protobuf format for performance analysis and troubleshooting.

Default workflow: run inside the target example directory (for example examples/react) and store all trace artifacts in that directory (not /tmp).

Workflow

1) Build sftrace tools

git clone https://github.com/quininer/sftrace
cd sftrace
cargo build --release
mkdir "$(./target/release/sftrace record --print-solib-install-dir)"
cp ./target/release/libsftrace.so "$(./target/release/sftrace record --print-solib-install-dir)/"

2) Build sftrace-enabled profiling binding (once per code change)

SFTRACE=1 pnpm build:binding:profiling

3) Optional: Generate a filter file from symbols

sftrace filter works on function symbols from an object file (for rspack, the binding .node file).

# Enter the target example directory first
cd examples/react

# Prefer the locally built profiling binding from the monorepo
BINDING_NODE="$(realpath ../../crates/node_binding/rspack.linux-x64-gnu.node)"

# Regex mode
sftrace filter -p "$BINDING_NODE" -r 'finish_modules|FlagDependencyExportsPlugin' -o sftrace.filter

# List mode (one regex per line)
# sftrace filter -p "$BINDING_NODE" --list symbols.list -o sftrace.filter

If your binding file name differs by platform, replace the .node path accordingly.

4) Record sftrace (example: build in examples/react)

Run from the target example directory and keep outputs local to that example.

cd examples/react

TRACE_DIR="sftrace-$(date +%Y%m%d-%H%M%S)"
mkdir -p "$TRACE_DIR"

# Full trace
sftrace record -o "$TRACE_DIR/sf.log" -- pnpm build

# Filtered trace (requires sftrace.filter from step 3)
sftrace record -f sftrace.filter -o "$TRACE_DIR/sf.filtered.log" -- pnpm build

5) Optional: Analyze sf.log by polars

Convert sftrace log to pola dataframe.

cd examples/react
TRACE_DIR="sftrace-YYYYMMDD-HHMMSS" # replace with your run directory
sftrace convert --type pola "$TRACE_DIR/sf.log" -o "$TRACE_DIR/sf.pola"

This will generate two files, whose schema format is as follows:

1. sf.pola

This records all events from sftrace log.

name	type	description
frame_id	uint64	a unique id for each frame. a function's entry and exit have same frame id
parent	uint64	point to previous frame id. zero means non-existent
tid	uint32	thread id
func_id	uint64	function unique id
time	nanoseconds	time elapsed since program started
kind	uint32	event type, 1 is entry, 2 is exit, 3 is tail call

2. sf.pola.symtab

This records the function symbol name and file path of func_id.

name	type	description
func_id	uint64	function unique id
name	string	function symbol name (demangled)
path	string	the file path and line number of function

You can use python-polars to perform data analysis on sf.pola.

import polars as pl

sf = pl.scan_parquet("./sf.pola")
symtab = pl.scan_parquet("./sf.pola.symtab")

# Query the functions that appear most frequently
(
  sf
    .filter(pl.col("kind").eq(1))
    .group_by("func_id")
    .agg(pl.len().alias("func_count"))
    .top_k(10, by="func_count")
    .join(symtab, on="func_id")
    .collect()
)

# Query the leaf frame of longest execution time
(
  sf
    .filter(~pl.col("frame_id").is_in(pl.col("parent").implode()))
    .group_by("frame_id")
    .agg([
      pl.col("func_id").first(),
      pl.col("time").filter(pl.col("kind").eq(1)).first().alias("entry_time"),
      pl.col("time").filter(pl.col("kind").is_in([2, 3])).last().alias("exit_time"),
    ])
    .filter(pl.col("exit_time").is_not_null())
    .with_columns(pl.col("exit_time").sub("entry_time").alias("duration"))
    .top_k(10, by="duration")
    .join(symtab, on="func_id")
    .collect()
)

6) Optional: Visualization sf.log

Convert sftrace log to perfetto protobuf format.

cd examples/react
TRACE_DIR="sftrace-YYYYMMDD-HHMMSS" # replace with your run directory
sftrace convert "$TRACE_DIR/sf.log" -o "$TRACE_DIR/sf.pb.gz"

Visualization using viztracer

vizviewer --use_external_processor "$TRACE_DIR/sf.pb.gz"

Use this only for visualization.

Filtering Notes

• sftrace filter matches function symbols by regex/list. It is not a first-class crate-path/module-path filter.
• Filtering does not automatically keep all descendants. If a child function symbol does not match your filter, it may disappear from the trace.
• Cross-thread relationships (for example via rayon) are not reconstructed as a single uninterrupted call chain.
• For complete call stacks, record without filter (or with a broad filter) and narrow down during analysis.