writeonce/docs/plan/perf-targets.md

Performance targets — measured, named, waiting

A register like discarded.md/learnings.md: optimization candidates that exist because a NUMBER says so, not a hunch. Every row cites its measurement (the db-bench campaign, bench/baseline.json, or the go-sqlite comparison) and names an owner iteration when one exists. A target leaves this file by landing (delta recorded in the baseline) or by being rejected into discarded.md with its reason.

1. The write path — update-through-query re-probes, insert re-encodes

Measured 2026-08-22 (go-sqlite comparison, N=20k, same machine, ext4): ram mixed writes 195,465 ops/s vs SQLite's 380,069 (×1.9 behind); durable mixed writes 2,324 vs 3,257 (×1.4 behind) — while writeonce WINS durable seed ×1.4 and reads ×2.6–6.4. The write gap is specifically the UPDATE half of the mix.

Suspected costs, in probable order (attribute before optimizing — the C-API microbench the 22 spec reserves exists for exactly this):

1. Update-through-query runs a whole query statement per update: probe (now O(1)) + materialize an id multi (arena alloc) + DB_GET_FIELD/DB_UPDATE_FIELD builtin round-trips per touched field. SQLite's equivalent is one page write inside one statement.
2. wo_row_update_field walks every index three times (shadow unique check, old-entry removal, new-entry add — three touches-loops over t->indexes per update; see database/src/table.c).
3. Insert encodes per field with a malloc per text/owned value (db_val_encode) — visible as ram seed ×1.2 behind SQLite (245k vs 297k) even though the durable flavor wins.
4. A WAL update record re-encodes the whole row (wo_wal_append_update writes the row image, not a delta).

Owner: none yet. Sequence note: iteration 23 (io_uring group-commit) rewrites the durable write path's syscall story anyway — re-measure after 23 lands, then decide whether the RAM-side costs (1–3) earn their own slice. Acceptance shape: ram write ops/s closes on SQLite's number with reads unharmed; baseline refreshed with the delta recorded.

2. Cross-shard DB RPC halves concurrent read throughput

Measured 2026-08-22 (db-bench campaign): ram mixread 89,538 ops/s single-shard vs 44,918 at default cores; durable 9,211 vs 4,324. The DB actor serializes every statement on shard 0 and each op pays an envelope + park/unpark round-trip.

Owner: by design, priced deliberately (story 8's settled decision 1 — rejected alternatives: engine lock, partitioned tables "wait for a measured need"). THIS is the measured need's first data point; the recorded escalation path is partitioned/replicated read state, only if a real workload (iteration 24's chat) hurts. Not actionable before 24.

3. The mutex inbox costs ~6× on cross-shard message rate

Measured 2026-08-22: 16.7M msgs/s same-heap vs 2.85M cross-shard (msgrate). Owner: iteration 31 (mailbox/backpressure decisions consume this number) and stage-2 deviation 4 (lock-free rings arrive only if the mutex is the measured bottleneck — at 2.85M msgs/s it is not the limiting factor for any current workload).

4. Durable write throughput is fsync-bound at ~4.5k/s

Measured 2026-08-21: durable seed 4,460 inserts/s vs ram 245k — the ~55× gap is one fdatasync per statement (~220µs each). Owner: iteration 23 (io_uring group-commit) — its acceptance is literally this number moving while the crash battery stays green.