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Profiling Bytewax

So you think Bytewax is slow? Let's do some profiling to produce CPU flame graphs.

Instructions are Linux only as of now.

0. Installation

Install perf on Ubuntu with:

sudo apt install linux-tools-common

We'll also be using Hotspot to visualize the flame graphs.

sudo apt install hotspot

Python must be compiled with the appropriate options to be able to reconstruct call stacks. This is easiest done via pyenv.

curl https://pyenv.run | bash

1. Compile Correctly

Python

First let's compile Python with the appropriate flags.

PYTHON_CONFIGURE_OPTS="--enable-optimizations --with-lto" PYTHON_CFLAGS="-gdwarf -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer" pyenv install 3.12.0

  • --enable-optimizations and --with-lto give us a "release" build. This makes the compile quite long; you can leave this out but it will change the timings of the resulting dataflow slightly.

  • -gdwarf adds in debugging symbols.

  • -fno-omit-frame-pointer and -mno-omit-leaf-frame-pointer helps call stack recreation.

I encourage using Python 3.12+ because it includes features to see Python functions in the call stack.

Bytewax

The performance characteristics of the library changes drastically when compiled in release mode, so before doing any profiling / benchmarking, we need to ensure we compile in that mode.

RUSTFLAGS="-C debuginfo=1 -C force-frame-pointers" maturin develop --release

  • -C debuginfo=1 adds enough debug info to name functions.

  • -C force-frame-pointers helps call stack recreation.

2. Profile Execution

We can now use perf to profile an execution of a dataflow:

perf record -F max --sample-cpu --call-graph=dwarf --aio -- python -X perf -m bytewax.run example_dataflow

This will write ./perf.data for later analysis.

  • -F max says to sample the call stack very fast.

  • --sample-cpu says to record which CPU was running each thread during sampling.

  • --call-graph dwarf says to use debug information to determine the call graph. This is crucial.

  • --aio to improve performance while tracing.

  • You can also use a -p $PID flag to profile an already running process.

  • -X perf enables the support for perf trampolines so you can see (some of the) Python call stack in the flamegraphs. This only works with Python 3.12+, so leave it out if you're profiling and older version. It appears that Python function calls performed directly from PyO3 are sometimes missing, so don't expect perfect vision, but this still helps.

3. Generate Flame Graphs

The best tool I've found for constructing flame graphs is Hotspot, a GUI visualizer.

Specifically, it seems to do two things better than using perf report:

  1. It seems to be better at finding debug symbols where they actually get installed.

  2. It uses a custom perf.data parser that can use DWARF data to reconstruct call stacks when it is available, but will fall back to using frame pointers when not. My understanding is that since some of the code being linked in does not have debug symbols, the frame pointers aid in call stack reconstruction.

Load the perf.data that was generate from your run.

It'll help to select the actual Timely worker threads for analysis. It'll be the one that has the most solid orange bar in the timeline at the bottom of the window since almost all the work is occurring in it. Right click it and choose "Filter In On Thread".