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## Context This PR addresses `AztecProtocol/barretenberg#1656` by making the `MSM::batch_multi_scalar_mul(...)` phase breakdown measurable and by adding a benchmark case that targets the exact regime called out in the issue (`2^16` points with `256` threads). The goal is to answer: is the single-threaded final reduction (`accumulate_results`) actually a bottleneck relative to the full MSM? ## What Changed ### 1) Phase Breakdown (`BB_BENCH`) inside `batch_multi_scalar_mul` Added `BB_BENCH` scopes for: - `MSM::batch_multi_scalar_mul/evaluate_work_units` - `MSM::batch_multi_scalar_mul/accumulate_results` - `MSM::batch_multi_scalar_mul/batch_normalize` - `MSM::batch_multi_scalar_mul/scalars_to_montgomery` These are surfaced in google-benchmark output via `GOOGLE_BB_BENCH_REPORTER(state)`. ### 2) New Benchmark Case: `BatchMSM_1656` Added a dedicated benchmark: - `PippengerBench/BatchMSM_1656/256/{msm_size}` - Single MSM (`num_polys = 1`) - Sizes: - `msm_size ∈ {2^16, 2^20}` This uses `bb::set_parallel_for_concurrency(256)` to force the intended partitioning and restores the original value after the benchmark finishes. ## Results (Local) Machine: 4 vCPU laptop (so `256 threads` is intentionally oversubscribed; the key point is the *absolute* reduction overhead). Key takeaways (from `BB_BENCH` counters; time counters are nanoseconds): - `2^16, 256 threads`: - `accumulate_results ≈ 139k ns` (~0.139 ms) - total `≈ 521 ms` - reduction fraction `~0.027%` - `2^20, 256 threads`: - `accumulate_results ≈ 141k ns` (~0.141 ms) - total `≈ 3457 ms` - reduction fraction `~0.004%` So the final reduction appears negligible in these regimes on my setup, and the benchmark now makes it easy to validate on a 64+ core machine where the original concern is most relevant. ## Notes / Background - `MSM` here is a Pippenger-style MSM. For background reading on multiexponentiation / multi-product methods: - Nicholas Pippenger, *On the evaluation of powers and related problems* (1976). - Jurjen Bos and Matthijs Coster, *Addition Chain Heuristics* (CRYPTO 1989). - Ryan Henry, *Pippenger's Multiproduct and Multiexponentiation Algorithms* (2010). ## How To Run ``` cmake --build <build-dir> --target pippenger_bench CRS_PATH=<path-to-bn254_g1.dat> <build-dir>/bin/pippenger_bench \ --benchmark_filter='BatchMSM_1656' \ --benchmark_min_time=0.1s \ --benchmark_counters_tabular=true ``` Fixes AztecProtocol/barretenberg#1656 --------- Co-authored-by: Peter <peter@users.noreply.github.com> Co-authored-by: Jonathan Hao <jonathan@aztec-labs.com>
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chore: bench phase breakdown + thread sweep for MSM reduction (#21885)
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