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bench: hermetic wall-time benches + deliberate profiling-overhead target - #4449
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Pin BAML_PROFILE=0 (caller-overridable) in runtime_benchmark and compiler_benchmark so wall-time numbers exclude the default-ON tracing pipeline and hot workloads cannot abort on ring overflow mid-suite (compute::fib32_recursive reproduced the 1 GiB-cap abort on bare metal). Add a profiling_overhead bench target: the same VM harness with profiling pinned ON (4 GiB ring headroom, temp-dir sink) over a fixed four-workload subset exported from the speedtest corpus by build.rs — per-call ring-pair cost, allocation-heavy loop, the ring-overflow reproducer, and a string baseline. Overhead ratio = prof_on median / runtime_benchmark median. Measured on this tree: pure_call_1m 278.5 ms off vs 308.8 ms on (~11%, ~30 ns/call-pair); fib32_recursive completes under tracing instead of aborting.
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⏭️ Performance benchmarks were skippedPerf benchmarks (CodSpeed) are opt-in on pull requests — they no longer run on every push. They always run automatically after merge to To run them on this PR, do any of the following, then push a commit (or re-run CI):
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🚧 Files skipped from review as they are similar to previous changes (1)
📝 WalkthroughWalkthroughThe PR adds a profiling-overhead benchmark target. The build script generates four profiling workload sources. The benchmark compiles and executes them with profiling enabled. Existing benchmarks keep profiling disabled by default. ChangesProfiling overhead benchmark
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The benchmark harness now disables profiling by default unless explicitly overridden and adds a separate profiling-overhead target; no actionable merge-blocking risk remains, so the PR is merge-ready after normal checks and review. Sequence Diagram(s)sequenceDiagram
participant BuildScript
participant profiling_overhead
participant compile_source
participant BEXEngine
participant Divan
BuildScript->>profiling_overhead: provide generated workload sources
profiling_overhead->>compile_source: compile non-empty BAML source
compile_source->>BEXEngine: create native VM engine
Divan->>profiling_overhead: run workload benchmark
profiling_overhead->>BEXEngine: invoke main()
BEXEngine-->>Divan: return execution result
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🧪 Generate unit tests (beta)
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Inline comments:
In `@baml_language/crates/baml_tests/benches/profiling_overhead.rs`:
- Around line 51-58: Update the benchmark setup around divan::main() to create
and retain a Drop-backed temporary-directory handle when BAML_PROFILE_DIR is
unset, use that handle’s path for the environment variable, and keep the handle
alive until divan::main() returns so the directory is cleaned up automatically;
encapsulate cleanup in the handle’s Drop implementation.
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⚙️ Run configuration
Configuration used: Repository UI
Review profile: CHILL
Plan: Pro Plus
Run ID: 59ddb9b2-8230-45ab-8550-f1de526a12d0
📒 Files selected for processing (5)
baml_language/crates/baml_tests/Cargo.tomlbaml_language/crates/baml_tests/benches/compiler_benchmark.rsbaml_language/crates/baml_tests/benches/profiling_overhead.rsbaml_language/crates/baml_tests/benches/runtime_benchmark.rsbaml_language/crates/baml_tests/build.rs
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Address review: the default profile sink is now a tempfile::TempDir held until divan::main() returns, so repeated runs leave no artifacts in the system temp directory. Caller-provided BAML_PROFILE_DIR is untouched.
Addressed: sink dir is now a Drop-backed tempfile::TempDir held until divan::main() returns.
What
Two changes to the divan bench harness, prompted by a wall-time A/B session where the runtime suite aborted mid-run:
Pin
BAML_PROFILE=0(caller-overridable) inruntime_benchmarkandcompiler_benchmark. BAML profiling ships default-ON, so every local wall-time number has silently included the tracing pipeline's cost — and worse, hot workloads could abort the whole suite when the event ring hit its 1 GiB overflow cap (compute::fib32_recursivereproduced this on bare metal: the consumer can't keep up with a tight recursive loop's event rate, and the ring aborts by design rather than dropping events). A benchmark whose completion races a background consumer thread is not hermetic. The pin follows the existingDIVAN_MAX_TIMEpattern: only applied when the caller didn't set a value, so CI can still override deliberately.New
profiling_overheadbench target — the deliberate counterpart. Same VM harness, profiling pinned ON with 4 GiB ring headroom and a temp-dir sink, over a fixed four-workload subset exported from the speedtest corpus bybuild.rs(same single source of truth as the main benches): per-call ring-pair cost (pure_call_1m), an allocation-heavy loop (array_build_sum_100k), the ring-overflow reproducer (fib32_recursive), and a string baseline. Overhead ratio =prof_on_*median ÷ the same workload'sruntime_benchmarkmedian.Measured (this branch's tree)
pure_call_1m: 278.5 ms pinned-off vs 308.8 ms profiling-on — ~11%, ≈30 ns of tracing cost per call-pair.fib32_recursive: completes under tracing (2.13 s) with the 4 GiB headroom instead of aborting at the 1 GiB default.string_concat_loop_10k: 1.93 ms off vs 1.96 ms on — few traced events, overhead in the noise, as expected.Notes
prof_on_*names deliberately sit outside thevm_CodSpeed filter; whether/how to track tracing overhead in CI is left as a reviewer decision.Summary by CodeRabbit
New Features
Bug Fixes