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feat(snmalloc-tools): rate-report subcommand + snapshot-vs-streaming docs - #73
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…docs Adds a `rate-report` subcommand to snmalloc-tools that stream-parses a JSON-Lines streaming event log and emits per-site (alloc_count, dealloc_count, peak_live_bytes, alloc_rate_per_sec) rows as CSV or a fixed-width table. Reader is strictly streaming — 6M-event logs use O(distinct sites) memory. Defines the JSONL on-disk schema for snmalloc streaming sessions (kind / site / size / ts_ns). Adds a "When to use snapshot vs streaming" section to snmalloc-rs README documenting the tradeoff: snapshot biased toward long-lived state, streaming captures transient churn — use streaming + rate-report for hot-path alloc-rate optimisation. New integration tests cover library round-trip and CLI surface (--help, default CSV, --pretty, --top truncation) against a worked 8-event fixture.
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What: new
snmalloc-tools rate-reportsubcommand — stream-parses a JSON-Lines streaming event log and emits per-site rows(site, alloc_count, dealloc_count, peak_live_bytes, alloc_rate_per_sec)as CSV (default) or a fixed-width table (--pretty). Also adds a "When to use snapshot vs streaming" section tosnmalloc-rs/README.mdand documents the streaming-log on-disk schema insnmalloc-tools/README.md.Why:
snapshot()is biased toward long-lived state and misses transient churn — a hot allocate-and-free site disappears before the snapshot is taken. Operators optimising a hot path need per-site allocation rate, which only the streaming-log path can answer. This adds the offline analysis half (the producer half is the existingProfilingSession::startcallback).Cost: ~390 LOC in
snmalloc-tools/src/rate_report.rs+ ~30 LOC main.rs wiring + ~150 LOC tests + ~80 LOC README. No new crate deps (reuses serde/serde_json/anyhow/clap already in the manifest). Strictly streaming reader: 6M-event logs use O(distinct sites) memory, not O(events).Evidence:
cargo test -p snmalloc-tools— 26 unit tests + 6 existing integration tests + 8 newrate_report.rsintegration tests all pass.cargo run -p snmalloc-tools -- rate-report --helpshows the subcommand with--input,--top,--pretty.cargo build -p snmalloc-toolsclean. Local verification only (CI on fork is workflow_dispatch).