shoney.arickathil
0b618ace19
databasev2 4 part A, task 6. Mostly documentation, plus one real fix the
full battery caught.
THE FIX. The drain held EVERY DB reply until the barrier — including
reads, which stage nothing and have no stake in durability. That parked
readers behind an fsync for no reason: durable.sN.mixread.p99 rose from
~1043us to 4057us. Only a statement that actually staged a record now has
its reply held. Caught by the gate, not by review.
THE TRADE, recorded rather than smoothed over. What remains is inherent: a
barrier blocks the owner shard LONGER (more records per fsync) though LESS
OFTEN, so anything queued behind one waits. Three full runs of the same
build gave durable.sN.mixread.p99 of 1043 / 2318 / 4147us and wmix.p99 of
8758 / 20000us — a 2-4x spread with the box near idle. So part A buys ~3x
write throughput at the cost of a longer, noisier tail on the owner shard,
and that is the strongest argument for part B (submit and keep serving).
- durable.sN.*.p99us tolerance widened to 100% WITH the reason in the
code: a 2-4x-variable tail gated at 50% gates the disk, not the engine.
The floor is the real guard and is not slack — mixread's (4172us) came
within 25us of tripping on the worst run. Baseline refreshed; a fresh
full run then passed 106 checks 0 failures
EXIT STATUS MOVED 3 -> 74 (sysexits EX_IOERR). 3 and 4 are already used by
SAMPLES for their own meanings — db-bench's own `verify` exits 3 on a
checksum mismatch, and it is the gate that exercises durability, so a
durability abort exiting 3 would have been indistinguishable from the
mismatch it should help diagnose. The low range belongs to programs.
Docs:
- story: progress, the payoff measured two ways, the cost side, criteria
split met/outstanding, and a "part B — its premise changed" section:
it was justified by "close the 66x gap", but that gap is two problems
and only the concurrent one was a batching problem
- board: standup entry in the six-question shape; both databasev2 4 rows
rewritten. They had said "close the 66x gap" — recorded as MIS-STATED
rather than quietly renumbered
- 00-wob-format.md and 04-db-binding.md: the normative failure contract
("a failed WAL commit traps WO_T_IO after un-applying the row") was
false; corrected, along with the tick-scoped group commit that never
happened
- database/src/CODE-LOGIC.md: where the barrier runs and why there, why
replies are held, why the inline path is asymmetric, the one failure
rule, and how to measure it
- db-bench README: the wmix mode, the env knobs, and the tmpfs warning
Battery: wovm-test 36 suites 0 fail, woc-test, oop-e2e 119/0,
db-bench 106/0, linkcheck clean.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
83 lines
5.1 KiB
Markdown
83 lines
5.1 KiB
Markdown
# db-bench — iteration 22's load generator
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The measurement backbone (spec:
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`docs/superpowers/specs/2026-08-21-db-bench-design.md`). Pure `.wo`;
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every measured mode prints one machine-parsable line per operation
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class:
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<op> <count> <ops/sec> <p50us> <p99us>
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Timing is per-operation via `time.ticks` (CLOCK_MONOTONIC µs).
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Percentiles come from a 1µs-bucket histogram clamped at 20000µs — exact
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to the microsecond below the clamp; a p99 AT 20000 means "clamp or
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worse". (A histogram, not the spec's reservoir: the language has no
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container element-write or sort, and the histogram's tail fidelity is
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strictly better. Recorded as a plan deviation.)
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## Modes
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| mode | what it prices |
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| --- | --- |
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| `all N` | the throughput campaign in ONE process: seed N, read N/2, query N/10, write N/2. Without `WO_DATA` the store is RAM and dies with the process, so the measured modes must share the seeding run. |
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| `seed N` | timed inserts: one parent per 100 children (FK probe each insert, unique-index maintenance per parent), k non-unique (10 rows/key), deterministic v. Writes `Meta` expectation rows. |
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| `read N` | indexed take-1 point lookups, LCG-spread keys. |
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| `query N` | full equality probes on the k index (≈10 rows each), materialized and counted. |
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| `write N` | alternating inserts (disjoint k range 2e6+) and updates through query results. Corrupts the checksum by design — durability legs run on a fresh store. |
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| `wal N` | the crash battery's vehicle: insert-only (k range 1e6+), `acked <i>` printed AFTER each insert returns — the return IS the ack (RAM applied, WAL record staged, ONE commit done). |
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| `wmix N C` | **databasev2 4:** every op a durable write (update through a query result), C at once. Exists because `mix` writes on one op in ten with C=4 — 20 writes in a quick run, measured mean batch **1.01** — so no existing leg could show whether group commit engages. Histogram kind 2, because a replayed store still holds the seeding run's kind-0/1 `Hist` rows. Seed first. |
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| `verify` | store vs its own Meta rows: count, checksum, one unique probe. Exit 3 on mismatch. |
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| `verify-acked M` | after kill -9 mid-`wal`: rows 1..M exist with the right v; rows beyond M allowed (acked after the last print flushed). Exit 3 on mismatch. |
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## Env knobs
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| var | effect |
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| --- | --- |
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| `WO_DATA=<dir>` | durability on: replay `<dir>/shard-0.wal` at boot, log every write. Without it the store is RAM-only |
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| `WO_SHARDS=<n>` | shard count. **`1` means every statement runs inline on shard 0 and group commit cannot engage** — batches form only where writes queue from other shards |
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| `WO_WAL_STATS=1` | **databasev2 4:** print one line at exit — `walstats batches=… records=… peak_batch=… peak_staged=…`. Opt-in so it does not pollute every durable program's output. Mean batch is `records/batches`; **mean 1.0 means group commit is not engaging**, which is expected for a serial writer or `WO_SHARDS=1` and a bug anywhere else |
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**Do not put `WO_DATA` on `/tmp`.** It is `tmpfs` on the reference machine,
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where `fdatasync` is free: the same `wmix` run measured **195 000 ops/s at p50
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1 µs** there against **2200 ops/s at p50 7200 µs** on ext4. There is no
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durability barrier to price on a memory filesystem. The driver keeps its stores
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under `bench/` for exactly this reason.
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## Coordination idiom (this side of iteration 31)
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There is no request/response surface yet: concurrent modes drive
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completion the db-actor way — actors write rows, main polls the store
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until the expected count, then settles. Retired when 31 lands.
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## Standing finding (2026-08-21, first run)
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A hand-built `multi Bucket` of insert results SEGVs on drop: the
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compiler classifies the elements OWNED while table refs are scalar ids.
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Query-built multis are runtime-typed and safe. Worked around here
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(single ref local, bucket-major seeding); the compiler fix is its own
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slice.
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## Reference-machine numbers (first campaign, 2026-08-21)
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`bench/baseline.json` is the contract; headline readings:
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- ram seed 245–290k inserts/s; **durable seed ≈4.5k/s** (fsync-per-commit
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≈220µs each — the gap iteration 23 exists to close).
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- reads/queries ≈1.1–1.3M ops/s at p50 1µs since the read-path index
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slice (2026-08-22, engine `wo_idx_probe` + emitter index selection) —
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up from ≈1.5k/s at p50 600µs when point lookups walked every slab
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(~×850). mixread 89k ops/s single-shard, ~1.9k multi-shard (was
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1,280 / 21): the RPC round-trip is now the visible cost, as designed.
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- msgrate ≈13M msgs/s same-heap vs ≈2.4M cross-shard (the mutex-inbox
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number, stage-2 deviation 4).
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- Tolerance policy lives in the DRIVER (`tolerance_for`), not hand-edits
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— a baseline refresh regenerates it: mix*/sN/read/query 50%
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(scheduling + µs-scale jitter), rest 15%; latency floors
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`max(4×value, 100µs)` — the tripwire means "µs became ms".
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## The gate must bite (proven 2026-08-21)
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`scripts/db-bench.py --check <results.json>` evaluates a recorded run:
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the real results pass 74/0; a doctored copy FAILS on exactly the
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doctored metrics — use a 15%-class metric (seed) halved plus a
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50%-class metric (read) quartered, so both tolerance classes prove they
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bite. Re-run the smoke after any gate or policy change.
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