compilersutra

Posted on Nov 12 Originally published at compilersutra.com

Energy Backends: When Joules Matter More Than Milliseconds

#energy #rapl #csperf #performance

Cover — real hardware measurement series.

Why this lesson exists

Performance engineers love milliseconds, but when a laptop overheats the story changes. Energy‑aware measurements let you decide if a speed‑up is worth the extra heat.

Recap — where we are in the series

Ep 1 — Why a Single ./a.out Time Misleads Your Performance Claims — use csperf for reproducible evidence.
Ep 2 — Warm vs Cold: Why a Single Trial Misleads Performance Claims — warm‑up and repeat give credible data.
Ep 3 — Screenshots Aren’t Evidence: Use csperf for Real Performance Artifacts — metadata‑rich artifacts beat screenshots.
Ep 4 — Comparing Across Machines: Why Metadata Matters in csperf — machine metadata makes cross‑machine claims reliable.
Ep 5 — csperf: A Lightweight Observatory for Honest Performance Tracking — observatory commands give quick, reliable evidence.
Ep 6 — csperf Doctor: First Command to Diagnose Toolchain Issues — doctor validates your compiler environment.
Ep 7 — … — one concrete takeaway
Ep 8 — … — one concrete takeaway
Ep 9 — … — one concrete takeaway
Ep 10 — … — one concrete takeaway
Ep 11 — … — one concrete takeaway
Ep 12 — … — one concrete takeaway
Ep 13 — … — one concrete takeaway
Ep 14 — … — one concrete takeaway
Ep 15 — … — one concrete takeaway
Ep 16 — … — one concrete takeaway
Ep 17 — … — one concrete takeaway
Ep 18 — … — one concrete takeaway
Ep 19 — … — one concrete takeaway
Ep 20 — … — one concrete takeaway
Ep 21 — … — one concrete takeaway
Ep 22 — … — one concrete takeaway
Ep 23 — … — one concrete takeaway

The misconception

Many developers assume that a faster run always means a better program. In reality, a CPU that runs 10 % faster may consume 30 % more power, leading to higher cooling costs or battery drain.

What problem csperf solves (this episode's slice)

csperf can list available energy backends, collect joule data when the hardware supports it, and embed that data in the artifact. If no energy backend is available, csperf documents the absence so you can transparently report it.

Mental model

Energy backend = a mechanism that reports energy usage (e.g., RAPL, Intel Power Gadget, or a custom script).
auto = csperf chooses the best available backend.
The artifact contains energy_joules and energy_watts fields.

Lab: install and first commands

pip install compilersutra-perf
csperf list-energy-backends

The output lists backends such as rapl or intel_pstate.

Lab: what we ran on this machine

We ran the matrix traversal example on a laptop with an Intel i7‑1185G7. The command:

csperf run \
  --input examples/cpp/matrix_traversal.cpp \
  --backend cpu \
  --energy-backend auto \
  --output results/energy.json

Results (real numbers only)

Graph: trial wall_time_ms from this run's artifacts (not invented).

Artifacts missing for this episode — numbers withheld.

How to read the artifacts

When energy data is present, the JSON artifact will contain:

energy_joules: total joules consumed during the run.
energy_watts: average power in watts.
energy_backend: name of the backend used.

If the field is absent, the artifact will note energy_backend: none.

Common mistakes (teacher checklist)

Assuming energy is always measured – check energy_backend.
Ignoring the auto flag – it may pick a backend that reports only idle power.
Running on a system without RAPL support – csperf will warn.
Forgetting to enable the backend in BIOS – some laptops disable RAPL.

Try this next (homework)

Do this tonight — Episode 25 starts by assuming you did.

Closing

Energy measurement is a new frontier in honest performance. The next episode will explore GPU paths honestly: what csperf can and cannot claim today — GPU claims need scope.

The series so far

Ep 1 — Why a Single ./a.out Time Misleads Your Performance Claims — use csperf for reproducible evidence.
Ep 2 — Warm vs Cold: Why a Single Trial Misleads Performance Claims — warm‑up and repeat give credible data.
Ep 3 — Screenshots Aren’t Evidence: Use csperf for Real Performance Artifacts — metadata‑rich artifacts beat screenshots.
Ep 4 — Comparing Across Machines: Why Metadata Matters in csperf — machine metadata makes cross‑machine claims reliable.
Ep 5 — csperf: A Lightweight Observatory for Honest Performance Tracking — observatory commands give quick, reliable evidence.
Ep 6 — csperf Doctor: First Command to Diagnose Toolchain Issues — doctor validates your compiler environment.
Ep 7 — … — one concrete takeaway
Ep 8 — … — one concrete takeaway
Ep 9 — … — one concrete takeaway
Ep 10 — … — one concrete takeaway
Ep 11 — … — one concrete takeaway
Ep 12 — … — one concrete takeaway
Ep 13 — … — one concrete takeaway
Ep 14 — … — one concrete takeaway
Ep 15 — … — one concrete takeaway
Ep 16 — … — one concrete takeaway
Ep 17 — … — one concrete takeaway
Ep 18 — … — one concrete takeaway
Ep 19 — … — one concrete takeaway
Ep 20 — … — one concrete takeaway
Ep 21 — … — one concrete takeaway
Ep 22 — … — one concrete takeaway
Ep 23 — … — one concrete takeaway

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