Add a tool to generate Performance Impact sections for adding to PR descriptions - #2
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Problem: In a macOS 27 environment, Speedometer 2/3 wedged partway through, with every browser process idle, after 45/48 and 6/58 tests. StyleBenchConservative never reached its first test — so three of the ten suites couldn’t be run in that environment at all. Cause: The benchmark server is a single-threaded HTTPServer with the default listen backlog of 5, and it accepts connections one at a time. Speedometer burst opens 7–8 connections in 1ms (TodoMVC-WebComponents fetches 20 files in two such waves). So the queue overflows at the edge. Linux then queues the overflow, and the client retransmits 1s later; but macOS aborts it with a reset — which RequestServer reports as curl error 56, and LibWeb as a failed script load. Which fetch loses is a race: a lost speculative preload is harmless, since the script element fetches again on its own — but a lost script fetch leaves the app under test un- initialized. A test step throws, the runner’s promise never resolves, and nothing’s left pending — hence the idle browser. One Speedometer 2 run on this server had 14 fetches reset yet still completed, others never got Flight-TodoMVC up at all, and StyleBenchConservative lost its own runner scripts, tests.js and style-bench.js, and never started. Fix: Give the server a backlog of 128, which is the most macOS allows and more than any suite opens at once. With that one change, and none to the browser, Speedometer 2/3, and StyleBenchConservative all run to completion in the tested environment — without a single failed load.
📝 Walkthrough📝 WalkthroughWalkthroughThis change adds a complete branch benchmarking workflow. Sequence Diagram(s)sequenceDiagram
participant bench_pr
participant bench_plan
participant LadybirdWorktrees
participant run_py
participant bench_compare
bench_pr->>bench_plan: create rounds and run preflight
bench_pr->>LadybirdWorktrees: build baseline and head
bench_pr->>run_py: run interleaved suite pairs
run_py-->>bench_pr: return round results
bench_pr->>bench_compare: analyze collected rounds
bench_compare-->>bench_pr: return Markdown performance section
Priority: ➖ Normal 🚥 Pre-merge checks | ✅ 3✅ Passed checks (3 passed)
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Inline comments:
In `@bench_compare.py`:
- Around line 114-118: Reject configurations with fewer than two paired rounds
before statistical analysis, including validating args.rounds in the CLI parser
so --rounds 1 produces a user-facing argument error rather than misleading
q-values and resolution. Preserve the existing analysis behavior for rounds of
two or more.
In `@bench_plan.py`:
- Line 60: Update the default kept-round configuration and its test so the
default uses an even number of rounds, ensuring the alternating order in the
benchmark scheduling logic balances base-first and head-first runs. Verify the
test explicitly covers the default 7-round configuration or its updated
even-round replacement as required by the scheduling behavior.
- Around line 121-122: Update build_parity_problems() to compare normalized
compiler-argument sequences rather than sets, preserving argument order and
duplicates while excluding only documented worktree-specific path and output
operands. Replace test_flag_order_does_not_matter with tests covering only the
order-independent normalization guaranteed by the benchmark contract.
In `@bench_pr.py`:
- Line 96: Update the cache seeding command around sh so it uses the
clone-optimized recursive copy where supported and falls back to a plain
recursive copy when the clone flag is unavailable, ensuring the ccache directory
is populated on Linux. Preserve the existing destination Build/caches and
cache-seeding flow.
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review. Visit https://docs.coderabbit.ai/cli.
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Run ID: fd6210b1-f045-4318-88a1-4381fee90b99
📒 Files selected for processing (8)
MEASURING-A-BRANCH.mdREADME.mdbench_compare.pybench_plan.pybench_pr.pyrun.pytest_bench_compare.pytest_bench_plan.py
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bench_pr.py compiles a Ladybird branch and its merge-base into worktrees of its own, runs every suite against both, and outputs a report for use in a Performance Impact section of the body of a PR description: It either reports that the branch changes have neutral performance impact — or else that the changes had measurable impact, and the report gives the numbers. bench_compare.py does the analysis, separately — so an archived run can be re-analyzed, without (re)measuring everything all over again. Both binaries tested get built with -DENABLE_LTO_FOR_RELEASE=ON. If the binaries disagree on the compile line or kind of object file produced, the run stops before it measures anything. Reason: Testing an LTO binary against a non-LTO one — with no code changes — was found to spuriously report 209 tests moving by 3% to 100%, just due to the LTO difference.
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Every PR that touches the engine raises a question: Did this make anything slower?
bench_pr.pyanswers that — with measurements, rather than a guess. Point it at a branch, and it produces a Performance Impact section you can paste into the body of a PR description. It either reports that the PR-branch changes have neutral performance impact:…or else it reports that the changes had measurable impact — and gives the numbers.
bench_compare.pydoes the analysis separately — so, an archived run can be re-analyzed, without (re)measuring everything all over again.The
MEASURING-A-BRANCH.mddoc covers how to read the output, and provides more details about how it all works.