docs+fix(db): T6 closeout — and reads no longer wait for the barrier
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>
This commit is contained in:
parent
6183a67dfc
commit
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|
||||
"value": 29930
|
||||
},
|
||||
"ram.sN.mixread.p50us": {
|
||||
"dir": "lower",
|
||||
"floor": 320,
|
||||
"floor": 272,
|
||||
"tolerance_pct": 50,
|
||||
"value": 80
|
||||
"value": 68
|
||||
},
|
||||
"ram.sN.mixread.p99us": {
|
||||
"dir": "lower",
|
||||
"floor": 624,
|
||||
"floor": 372,
|
||||
"tolerance_pct": 50,
|
||||
"value": 156
|
||||
"value": 93
|
||||
},
|
||||
"ram.sN.mixwrite.ops_sec": {
|
||||
"dir": "higher",
|
||||
"floor": 831,
|
||||
"tolerance_pct": 50,
|
||||
"value": 3324
|
||||
"value": 3325
|
||||
},
|
||||
"ram.sN.mixwrite.p50us": {
|
||||
"dir": "lower",
|
||||
"floor": 348,
|
||||
"floor": 296,
|
||||
"tolerance_pct": 50,
|
||||
"value": 87
|
||||
"value": 74
|
||||
},
|
||||
"ram.sN.mixwrite.p99us": {
|
||||
"dir": "lower",
|
||||
"floor": 608,
|
||||
"floor": 420,
|
||||
"tolerance_pct": 50,
|
||||
"value": 152
|
||||
"value": 105
|
||||
},
|
||||
"ram.sN.msgrate.msgs_sec": {
|
||||
"dir": "higher",
|
||||
"floor": 248897,
|
||||
"floor": 307283,
|
||||
"tolerance_pct": 50,
|
||||
"value": 1991179
|
||||
"value": 2458270
|
||||
},
|
||||
"ram.sN.query.ops_sec": {
|
||||
"dir": "higher",
|
||||
"floor": 227790,
|
||||
"floor": 246669,
|
||||
"tolerance_pct": 50,
|
||||
"value": 911161
|
||||
"value": 986679
|
||||
},
|
||||
"ram.sN.query.p50us": {
|
||||
"dir": "lower",
|
||||
|
|
@ -466,13 +466,13 @@
|
|||
"dir": "lower",
|
||||
"floor": 100,
|
||||
"tolerance_pct": 50,
|
||||
"value": 2
|
||||
"value": 1
|
||||
},
|
||||
"ram.sN.read.ops_sec": {
|
||||
"dir": "higher",
|
||||
"floor": 216919,
|
||||
"floor": 262357,
|
||||
"tolerance_pct": 50,
|
||||
"value": 867678
|
||||
"value": 1049428
|
||||
},
|
||||
"ram.sN.read.p50us": {
|
||||
"dir": "lower",
|
||||
|
|
@ -484,13 +484,13 @@
|
|||
"dir": "lower",
|
||||
"floor": 100,
|
||||
"tolerance_pct": 50,
|
||||
"value": 2
|
||||
"value": 1
|
||||
},
|
||||
"ram.sN.seed.ops_sec": {
|
||||
"dir": "higher",
|
||||
"floor": 60469,
|
||||
"floor": 63510,
|
||||
"tolerance_pct": 50,
|
||||
"value": 241878
|
||||
"value": 254042
|
||||
},
|
||||
"ram.sN.seed.p50us": {
|
||||
"dir": "lower",
|
||||
|
|
@ -502,13 +502,13 @@
|
|||
"dir": "lower",
|
||||
"floor": 100,
|
||||
"tolerance_pct": 50,
|
||||
"value": 10
|
||||
"value": 8
|
||||
},
|
||||
"ram.sN.write.ops_sec": {
|
||||
"dir": "higher",
|
||||
"floor": 41677,
|
||||
"floor": 47959,
|
||||
"tolerance_pct": 50,
|
||||
"value": 166708
|
||||
"value": 191839
|
||||
},
|
||||
"ram.sN.write.p50us": {
|
||||
"dir": "lower",
|
||||
|
|
@ -520,6 +520,6 @@
|
|||
"dir": "lower",
|
||||
"floor": 100,
|
||||
"tolerance_pct": 50,
|
||||
"value": 15
|
||||
"value": 12
|
||||
}
|
||||
}
|
||||
|
|
@ -104,3 +104,62 @@ rather than acknowledging what disk never got.
|
|||
columns excluded (engine raw-eq is narrower than VM float-eq, and a
|
||||
probe miss cannot be resurrected by a recheck). Pinned by
|
||||
`tests/corpus/run/query-index-probe`.
|
||||
|
||||
## Group commit: one barrier per drain (databasev2 4 part A, 2026-08-28)
|
||||
|
||||
**What changed:** the engine used to commit per *statement*. `db.c` called
|
||||
`wo_wal_commit` immediately after every append, at all six sites, so each row
|
||||
change bought its own `pwrite` and its own `fdatasync`. Now the barrier belongs
|
||||
to the drain, not to the statement.
|
||||
|
||||
**Where the barrier runs, and why there.** A statement on a worker shard has no
|
||||
WAL to write — the runtime asserts workers hold neither `db` nor `wal` — so it
|
||||
marshals to shard 0 and parks. Shard 0 executes those requests in its envelope
|
||||
drain (`wo_vm_adopt`), and the drain now **holds each reply** instead of pushing
|
||||
it as the statement finishes. When the queue empties it issues one barrier, then
|
||||
releases every held reply.
|
||||
|
||||
Holding the reply is the whole mechanism. Pushing it early would unpark the
|
||||
requester before its record was durable; holding it means each writer is
|
||||
acknowledged after the barrier that carried *its own* record. That was always
|
||||
the intended contract — it was simply true by accident before, because every
|
||||
batch had exactly one member.
|
||||
|
||||
**Why the queue is the boundary.** Not a tick, and not a timer. A queue of one
|
||||
gives a batch of one, so a lone writer pays exactly what it paid before; the
|
||||
batch grows only when writes genuinely contend. A tick boundary would have
|
||||
added latency even with nothing to batch against, which is taxing an idle
|
||||
system to serve a busy one. There is nothing to tune, which is the point.
|
||||
|
||||
**Why the inline path is asymmetric.** A statement already on shard 0 stages and
|
||||
commits before returning, batch size one. It cannot hold a reply because there
|
||||
is nobody to reply to — it returns into its own fiber. Batching it would mean
|
||||
parking that fiber on the barrier, which is part B's machinery. Two consequences
|
||||
worth keeping in mind: single-shard configurations get no batching at all, by
|
||||
design; and the inline commit is only safe because the drain commits
|
||||
*unconditionally* whenever anything is staged, so the buffer is empty when an
|
||||
inline statement runs. If that ever stops holding, the inline path would make
|
||||
another statement's record durable early and acknowledge it to the wrong writer.
|
||||
|
||||
**One rule for failure: once a statement has mutated RAM, the outcomes are
|
||||
durable or process death.** It replaced three behaviours that disagreed —
|
||||
`insert` un-applied itself, while `update` and `delete` returned a catchable
|
||||
trap and left RAM ahead of disk, which their own comments said out loud.
|
||||
Batching would have multiplied that from one row to a whole batch. So a failed
|
||||
stage or a failed barrier now prints one diagnostic (operation, log path,
|
||||
`errno`, record count) and exits 3; `WO_T_IO` is unreachable from a write.
|
||||
Retrying is not offered because it is unsound: on Linux a failed `fsync` may
|
||||
already have discarded the dirty pages, so a second call can report success
|
||||
having written nothing. Replay is the recovery that works.
|
||||
|
||||
**Measuring it.** `WO_WAL_STATS=1` makes the runtime print one line at exit —
|
||||
batches, records, peak batch, peak staged bytes. Opt-in, because it would
|
||||
otherwise pollute every durable program's output. The counters live in `wo_wal`
|
||||
rather than behind a builtin: they are diagnostic, not part of the language.
|
||||
`db-bench`'s `wmix N C` leg exists to exercise this at all — `mix` writes on one
|
||||
op in ten with C=4, which produced a measured mean batch of 1.01, so it could
|
||||
never have shown whether batching worked.
|
||||
|
||||
**If you are looking at this because writes got slower**, check the mean batch
|
||||
first. Mean 1.0 means the mechanism is not engaging, which is expected for a
|
||||
serial writer or a single-shard configuration and a bug anywhere else.
|
||||
|
|
|
|||
|
|
@ -90,9 +90,15 @@ int wo_wal_append_update(wo_wal *w, wo_db *db, uint32_t class_id, uint64_t id);
|
|||
#define WO_WAL_ERR_WRITE (-1)
|
||||
#define WO_WAL_ERR_SYNC (-2)
|
||||
|
||||
/* The process exit status for a durability failure. 1 is a trap and 2 is a
|
||||
* refusal, so this takes a third of its own. */
|
||||
#define WO_EXIT_DURABILITY 3
|
||||
/* The process exit status for a durability failure.
|
||||
*
|
||||
* 74 is sysexits' EX_IOERR, chosen deliberately over a small number: 1 is a
|
||||
* trap and 2 is a loader refusal, but 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 is supposed to help
|
||||
* diagnose. The low range belongs to programs; the runtime takes a high one. */
|
||||
#define WO_EXIT_DURABILITY 74
|
||||
|
||||
/* Write the staged batch and fdatasync — the ack line. Empty batch = ok,
|
||||
* no syscall. 0 ok, WO_WAL_ERR_WRITE / WO_WAL_ERR_SYNC on failure (the
|
||||
|
|
|
|||
|
|
@ -24,9 +24,24 @@ strictly better. Recorded as a plan deviation.)
|
|||
| `query N` | full equality probes on the k index (≈10 rows each), materialized and counted. |
|
||||
| `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. |
|
||||
| `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). |
|
||||
| `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. |
|
||||
| `verify` | store vs its own Meta rows: count, checksum, one unique probe. Exit 3 on mismatch. |
|
||||
| `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. |
|
||||
|
||||
## Env knobs
|
||||
|
||||
| var | effect |
|
||||
| --- | --- |
|
||||
| `WO_DATA=<dir>` | durability on: replay `<dir>/shard-0.wal` at boot, log every write. Without it the store is RAM-only |
|
||||
| `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 |
|
||||
| `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 |
|
||||
|
||||
**Do not put `WO_DATA` on `/tmp`.** It is `tmpfs` on the reference machine,
|
||||
where `fdatasync` is free: the same `wmix` run measured **195 000 ops/s at p50
|
||||
1 µs** there against **2200 ops/s at p50 7200 µs** on ext4. There is no
|
||||
durability barrier to price on a memory filesystem. The driver keeps its stores
|
||||
under `bench/` for exactly this reason.
|
||||
|
||||
## Coordination idiom (this side of iteration 31)
|
||||
|
||||
There is no request/response surface yet: concurrent modes drive
|
||||
|
|
|
|||
|
|
@ -65,7 +65,7 @@ The metadata exists for exactly one reason: `json.encode`/`json.decode` are runt
|
|||
- **the OS half** — fs.exists/list/stat/read_all/read_at/append, time.sleep/local/iso, env.get/stopping, net.listen/accept/read/write/close, proc.run. Ids 40–56; `runtime/src/sysio.c`. A member that returns a record takes that record's **class id as its last argument**, so the VM allocates what it fills without knowing any source type name.
|
||||
- **json** — encode (value + the value's static kind), decode (text + the class id to build). Ids 57–58; `runtime/src/json.c`. Decode yields the zero word on malformed input rather than trapping, which is what makes `json.decode(t) as T` a checked decode.
|
||||
- **59 `map_get_opt`** (`m[k]`'s optional read), **60 `text_copy`** (Text's ownership-boundary copy — Task 1 of the executable plan).
|
||||
- **database** — **61 `db_insert`** (iteration 9, Task 3): window is R[B] = class id, R[B+1..] = one slot per **declared** field in declaration order; result R[A] = the new row's id. The loader validates the class-id slot statically (variable window: the field slots are validated at runtime by the engine against the class table). Engine failure traps `WO_T_DB`; a failed WAL commit traps `WO_T_IO` after un-applying the row. `database/src/db.c`.
|
||||
- **database** — **61 `db_insert`** (iteration 9, Task 3): window is R[B] = class id, R[B+1..] = one slot per **declared** field in declaration order; result R[A] = the new row's id. The loader validates the class-id slot statically (variable window: the field slots are validated at runtime by the engine against the class table). Engine failure traps `WO_T_DB`. **A failed WAL commit no longer traps (databasev2 4, 2026-08-28): it ENDS THE PROCESS** with exit status 74 and a diagnostic naming the failing operation, the log path, `errno` and the batch size. `WO_T_IO` is unreachable from any DB write. The reason is that only `insert` could ever un-apply itself — `update` and `delete` never could, and their own comments admitted they left RAM ahead of disk — so continuing after a durability failure meant serving state that would not survive a restart. Retrying is not offered either: on Linux a failed `fsync` may already have discarded the dirty pages, so a second call can report success having written nothing. Replay is the recovery that works. `database/src/db.c`, `database/src/wal.c`.
|
||||
|
||||
**`?T` and nil.** A heap-shaped optional (`?Text`, `?Rec`, `?multi`, `?map`, `?@gc`) stores what `T` stores and spells nil as the **zero word** — every per-kind drop plan already ignores a zero slot, so `?T`'s field kind is `T`'s. A **nullable scalar** (`?Int`, `?Bool`, `?Timestamp`, `?Id`) cannot: `0` is a perfectly good `Int`, and real programs store it in a `?Int`. Its nil is therefore `WO_NIL_SCALAR` = −2^62 (not `INT64_MIN`: the compiler's own integers are 63-bit, so that value is not expressible on the emitting side). Such a field is marked `WOB_FIELD_NIL_SCALAR` in `field_class[i]`, which is how the runtime knows to write that word where it must produce absence itself — today only `json.decode` leaving a key absent, and `parse_int` on unparseable input.
|
||||
|
||||
|
|
|
|||
|
|
@ -118,11 +118,29 @@ R[B+1..] = one slot per declared field in declaration order (the literal's
|
|||
order is irrelevant — slots are the class table's).
|
||||
|
||||
Execution: `wo_row_insert` (RAM, engine copies every value), then — when
|
||||
durability is on — stage + **commit before the builtin returns**: the
|
||||
builtin's return IS the acknowledgment, so ack-after-fsync holds at
|
||||
statement granularity until iteration 8 brings tick-scoped group commit. A
|
||||
failed commit un-applies the row and traps `WO_T_IO`; engine failures trap
|
||||
`WO_T_DB`. Durability is opt-in: `WO_DATA=<dir>` makes the CLI replay
|
||||
durability is on — stage, then a barrier before the acknowledgment. **Updated
|
||||
2026-08-28 (databasev2 4 part A): group commit landed, and the barrier's
|
||||
location now depends on which path the statement takes.**
|
||||
|
||||
A statement arriving from a worker shard marshals to shard 0 and parks; shard 0
|
||||
stages every such request, issues **one** barrier when its queue empties, and
|
||||
only then releases the held replies — so each writer is acknowledged after the
|
||||
barrier that carried *its* record. A statement already running on shard 0 takes
|
||||
the inline path and still commits before the builtin returns, because it has no
|
||||
reply to hold: it returns into its own fiber, and batching it would require
|
||||
parking that fiber on the barrier (deferred to part B). The boundary is the
|
||||
queue draining, **not** the tick this document previously anticipated — a tick
|
||||
would add latency to a lone writer, taxing an idle system to serve a busy one.
|
||||
|
||||
Measured: ~2.9× durable write throughput and ~2.1× lower p50 on a
|
||||
write-concurrent workload; unchanged for a serial writer, which has nothing to
|
||||
batch with.
|
||||
|
||||
A failed commit **no longer traps — it ends the process** (exit 74, with a
|
||||
diagnostic naming the operation, log path, `errno` and batch size). So does a
|
||||
failed staging. `WO_T_IO` is unreachable from a DB write. One rule: once a
|
||||
statement has mutated RAM, the outcomes are durable or death. Engine failures
|
||||
still trap `WO_T_DB`. Durability is opt-in: `WO_DATA=<dir>` makes the CLI replay
|
||||
`<dir>/shard-0.wal` before the entry runs and commit every insert; without
|
||||
it the engine is RAM-only (every corpus fixture runs that way).
|
||||
|
||||
|
|
|
|||
|
|
@ -138,3 +138,31 @@ durable inserts. They arrive as an end-of-run burst, which is batch-friendly,
|
|||
so `mean_batch` is not purely update-driven. Peak staged bytes stayed small
|
||||
(2793 B at C=64), which is what settled the decision to ship **no batch cap**:
|
||||
the request queue's existing upstream bound is sufficient.
|
||||
|
||||
### The cost side: tail latency on the owner shard
|
||||
|
||||
Group commit is a trade, and the full battery made the other side of it visible.
|
||||
|
||||
**A bug first, caught by `durable.sN.mixread.p99`.** The drain initially 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
|
||||
and pushed read p99 from ~1043 µs to **4057 µs**. Reads are now released
|
||||
immediately; only a statement that actually staged a record has its reply held.
|
||||
|
||||
**What remains is inherent, not a bug.** A barrier now blocks the owner shard
|
||||
**longer** (more records per fsync) even though it blocks **less often**, so
|
||||
anything arriving during a barrier — reads included — waits behind it. Measured
|
||||
across three full runs of the same build, `durable.sN.mixread.p99` came in at
|
||||
**1043 / 2318 / 4147 µs** and `wmix.p99` at **8758 / 20000 µs**, a 2–4× spread
|
||||
with the box near idle.
|
||||
|
||||
So the honest summary of part A on a single-threaded owner shard: **~3× write
|
||||
throughput, at the price of a longer and noisier tail for everything queued
|
||||
behind a barrier.** That is precisely what part B (async submission — submit the
|
||||
barrier and keep serving) would undo, and it is a better argument for part B than
|
||||
the "close the 66× gap" framing part B was originally given.
|
||||
|
||||
**Gating consequence.** `durable.sN.*.p99us` now carries a 100% tolerance,
|
||||
because a 2–4×-variable tail gated at 50% gates the disk rather than the engine.
|
||||
The **floor** is the real guard there, and it is not slack: `mixread`'s floor
|
||||
(4172 µs) came within 25 µs of tripping on the worst observed run.
|
||||
|
|
|
|||
|
|
@ -50,6 +50,60 @@ behind this board; live Obsidian Dataview views:
|
|||
|
||||
## ▶ NEXT PLAN
|
||||
|
||||
### Landed 2026-08-28 — databasev2 4 part A, WAL group commit
|
||||
|
||||
**Implemented last time (2026-08-28):** one durability barrier per drain
|
||||
instead of one per statement. Shard 0 stages every queued write request, holds
|
||||
each reply, commits once when its queue empties, then releases all — so a writer
|
||||
is acknowledged after the barrier that carried *its* record, which was the
|
||||
intended contract all along and was true before only because every batch had one
|
||||
member. Six tasks, brainstormed and spec'd first
|
||||
([spec](../superpowers/specs/2026-08-28-wal-group-commit-design.md) ·
|
||||
[plan](../superpowers/plans/2026-08-28-wal-group-commit.md)).
|
||||
|
||||
**Key findings (measured, not asserted):** **≈2.9× durable write throughput,
|
||||
≈2.1× lower p50** on a write-concurrent workload, confirmed a second way by the
|
||||
`s1`-vs-`sN` split within one build (1467 → 5117 ops/s, mean batch 1.0 → 5.43,
|
||||
peak 57) — 2.9× and 3.5× agreeing. Batching scales with contention: mean batch
|
||||
1.13 / 1.76 / 5.35 at C = 4 / 16 / 64. **The story's premise was wrong**: it
|
||||
said "fsync-per-commit" and the engine was fsync-per-**statement**, committing
|
||||
after every append at all six sites — so part A was closer to deleting calls
|
||||
than adding a mechanism.
|
||||
|
||||
**Learned — three things the measurement corrected, not the code:**
|
||||
(1) **`/tmp` is tmpfs here, where `fdatasync` is free.** The same run reported
|
||||
195 000 ops/s at p50 1 µs there against 2200 at 7200 µs on ext4. A group-commit
|
||||
measurement taken on a memory filesystem measures nothing; `db-bench` is right
|
||||
to keep its stores under `bench/`. (2) **No existing leg could exercise the
|
||||
feature** — `mix` writes on one op in ten with C=4, giving 20 writes and mean
|
||||
batch 1.01, so a `wmix` write-concurrent leg had to be added or the payoff was
|
||||
unevaluable either way. (3) **The before-p99 was off the instrument** —
|
||||
`hist_add` clamps at 20000 µs and both before-runs pinned there, so the gain is
|
||||
*at least* 2.3× and the true old p99 is unknown.
|
||||
|
||||
**Dependencies unblocked — and one dependency invalidated.** `WO_T_IO` is
|
||||
unreachable from a DB write: a failed stage or barrier now ends the process
|
||||
(exit 74, diagnosed), replacing three behaviours that disagreed — `insert`
|
||||
un-applied itself while `update` and `delete` returned a catchable trap and
|
||||
admitted in their own comments that they left RAM ahead of disk. **Part B's
|
||||
premise is invalidated**: it was justified by "close the 66× durable gap", but
|
||||
that gap is two problems. Concurrent fan-in was a batching problem and is now
|
||||
~3× better; a **serial** writer waiting on one barrier is a latency problem that
|
||||
batching cannot touch and io_uring does not obviously help either. Part B should
|
||||
be re-brainstormed, not started.
|
||||
|
||||
**Next steps:** either re-brainstorm part B against its corrected premise, or
|
||||
take chain 6 ([databasev2 3](databasev2/03-wal-checkpoint.md), WAL checkpoint),
|
||||
which now has the replay "before" it lacked. Two debts named rather than hidden:
|
||||
the abort path is not exercised (forcing a real `fdatasync` failure needs mount
|
||||
privileges), and single-shard concurrent batching needs the inline-path park —
|
||||
the same machinery part B would need.
|
||||
|
||||
**`.dev/reference` used:** none this slice. The sources were the engine's own
|
||||
code and the Linux `fsync`-failure semantics that make retrying unsound.
|
||||
|
||||
---
|
||||
|
||||
### Landed 2026-08-27 — iteration 24, chat + actor lifecycle (absorbing 31 + 34)
|
||||
|
||||
**Implemented last time (2026-08-27):** the slice closed and merged to master
|
||||
|
|
@ -248,6 +302,9 @@ both still literal holes in `wob.h`'s builtin enum; then T8 the chat
|
|||
sample, T9 its gate, T10 closeout setting 24/31/34 to `status: done`) → 23
|
||||
(io_uring group-commit — target: close the 4.5k→297k durable gap) →
|
||||
32 (WAL checkpoint). Held tail resumes on its own precedence notes.
|
||||
> (**Superseded 2026-08-28:** 24 landed, and 23's part A landed with it —
|
||||
> "close the 4.5k→297k durable gap" turned out to be the wrong target; see
|
||||
> the databasev2 4 row.)
|
||||
|
||||
**`.dev/reference` used:** none this slice (the LW_SOAK discipline and
|
||||
linkcheck.py precedent came from in-repo scripts).
|
||||
|
|
@ -407,7 +464,7 @@ that sequences its tasks. Read one, approve, then the next starts.
|
|||
| 22 | [Durability, throughput, scale](language-runtime-database/22-durability-throughput-scale.md) | ✅ **landed 2026-08-21** — db-bench + baseline.json (74 metrics) + restart/kill -9 proofs both shard counts; durable 4.5k vs ram 297k inserts/s, reads O(table), msgrate 13.4M/2.45M |
|
||||
| 31 | [Actor lifecycle](language-runtime-database/31-actor-lifecycle.md) | ✅ **LANDED 2026-08-27 inside 24** (directive 2026-08-23). All four mechanisms: `call`/reply with a typed scalar reply (`WO_B_CALL = 88`, WO-E226), bounded mailboxes (`WO_MAILBOX`, cap 1024, catchable `WO_T_ACTOR`), actor death that traps callers instead of hanging them, **`monitor` (89)** and **`time.after` (90)** — the reserved holes in `wob.h` are filled. A fifth mechanism it did not anticipate came out of proving the gate: the shutdown drain guarantee, [40](language-runtime-database/40-shutdown-drain-guarantee.md). Supervision trees stay out of v1 |
|
||||
| 24 | [chat: WebSocket workload](language-runtime-database/24-chat-websocket-workload.md) | ✅ **LANDED 2026-08-27** (absorbing 31 + 34) — all ten tasks; merged to master `ed5334d`. `just chat` **11 checks, 0 failures** at the full 1000-client soak: handshake, functional matrix on both `WO_IO` backends and on one shard, the soak, the fd invariant, the SIGTERM drain, `WO_MAILBOX=8` backpressure, ASan clean. Finishing its gate found a real runtime bug, split out as [40](language-runtime-database/40-shutdown-drain-guarantee.md) |
|
||||
| 23 | [io_uring group-commit](databasev2/04-io-uring-commit.md) | ⬜ fifth in chain, after stage 3 + 22 |
|
||||
| 23 | [io_uring group-commit](databasev2/04-io-uring-commit.md) | ✅ **part A LANDED 2026-08-28 — group commit**, one barrier per drain instead of one per statement (the engine was fsync-per-STATEMENT, not per commit; the story's premise was wrong). Shard 0 holds each reply, commits once when its queue empties, releases all — so a writer is acked after the barrier carrying ITS record. **≈2.9× durable write throughput, ≈2.1× lower p50**, two measurement methods agreeing (2.9× controlled, 3.5× s1-vs-sN); mean batch 5.43, peak 57. A durability failure is now **fatal (exit 74), not a catchable `WO_T_IO`** — replacing three behaviours that disagreed, two of which admitted leaving RAM ahead of disk. **What it did NOT do:** `durable.sN.mixwrite` 480→492 (unchanged — that workload does 20 writes at C=4, mean batch 1.01) and `seed` unchanged (serial writers have nothing to batch with). **This row used to say "close the 66× gap"; that target was mis-stated** — the gap is two problems and part A fixes only the concurrent one. ⬜ part B (io_uring) **needs re-brainstorming**, not starting on the old premise |
|
||||
| 32 | [WAL checkpoint](databasev2/03-wal-checkpoint.md) | ⬜ last in chain, after 23 — disk reclamation + bounded replay (story written 2026-08-21) |
|
||||
| 33 | [Single-file store](databasev2/07-single-file-db.md) | ⬜ off-chain, small — `WO_DATA=<path>.db` file form; driver-only (story written 2026-08-22) |
|
||||
| 34 | [Crypto builtins](language-runtime-database/34-crypto-builtins.md) | 🔄 **code landed** as 24's T1 (`d14fa9f`): `sha1`/`sha256`/`hmac_sha256`, ids 85–87 in `wob.h`, `runtime/src/crypto.c`, RFC/FIPS vectors 18/0, corpus pin. The 24 gate that once needed it is cleared. Frontmatter keeps `status: refine` only until 24's T10 closeout sets it to `done` |
|
||||
|
|
@ -682,7 +739,7 @@ the language arc as v1 history.
|
|||
| 1 | [RAM ceiling: measure the breaking point](databasev2/01-ram-ceiling-measurement.md) | ⬜ **first, and startable today** — nobody here can say what happens at 90% RAM. Curve not cliff: swap onset, latency departure, the three exits (checked trap / swap thrash / OOM killer), and `kill -9` durability *at exhaustion*. Output is `perf-targets.md` + baseline rows, not prose |
|
||||
| 2 | [`@table` storage modes](databasev2/02-table-storage-modes.md) | ⬜ **the language enrichment** — `mode: ram \| durable \| cold` per table, replacing the global switch. `durable` defaults so nothing changes silently; the compiler refuses a `durable` row holding a `ref` into a `ram` table. `.wob` format change. Grammar is small (`Ast.table_cfg` gains a key); semantics are the iteration |
|
||||
| 3 | [WAL checkpoint](databasev2/03-wal-checkpoint.md) *(was 32)* | ⬜ snapshot + truncate: disk reclaimed, replay bounded |
|
||||
| 4 | [io_uring group commit](databasev2/04-io-uring-commit.md) *(was 23)* | ⬜ close the 66× gap iteration 22 measured (durable 4.5k vs ram 297k inserts/s) |
|
||||
| 4 | [io_uring group commit](databasev2/04-io-uring-commit.md) *(was 23)* | ✅ **part A LANDED 2026-08-28 — group commit**, one barrier per drain instead of one per statement (the engine was fsync-per-STATEMENT, not per commit; the story's premise was wrong). Shard 0 holds each reply, commits once when its queue empties, releases all — so a writer is acked after the barrier carrying ITS record. **≈2.9× durable write throughput, ≈2.1× lower p50**, two measurement methods agreeing (2.9× controlled, 3.5× s1-vs-sN); mean batch 5.43, peak 57. A durability failure is now **fatal (exit 74), not a catchable `WO_T_IO`** — replacing three behaviours that disagreed, two of which admitted leaving RAM ahead of disk. **What it did NOT do:** `durable.sN.mixwrite` 480→492 (unchanged — that workload does 20 writes at C=4, mean batch 1.01) and `seed` unchanged (serial writers have nothing to batch with). **This row used to say "close the 66× gap"; that target was mis-stated** — the gap is two problems and part A fixes only the concurrent one. ⬜ part B (io_uring) **needs re-brainstorming**, not starting on the old premise |
|
||||
| 5 | [Bounded tables and eviction](databasev2/05-bounded-tables-eviction.md) | ⬜ a declared capacity + refuse/evict/back-pressure, and a process-level pressure signal that sheds **before** the allocator or OS gets involved — turning the invisible failure into a managed one |
|
||||
| 6 | [Cold tiering](databasev2/06-cold-tiering.md) | ⬜ the iteration that raises the ceiling, and the riskiest. Mostly forks: which shape, whether the index itself fits, whether the *language* surfaces the fault cost, and whether `@unique` on a cold table is refused outright. A paged B-tree stays rejected — if tiering needs one, reject tiering |
|
||||
| 7 | [Single-file store](databasev2/07-single-file-db.md) *(was 33)* | ⬜ `WO_DATA=<path>.db`; driver-only, independent |
|
||||
|
|
|
|||
|
|
@ -95,6 +95,65 @@ chain: 5
|
|||
> approved; the plan is next. (The `readiness` axis that would say this
|
||||
> precisely lives on the unmerged `db-residency-doctrine`.)
|
||||
|
||||
## Progress — part A landed 2026-08-28
|
||||
|
||||
| # | Task | State |
|
||||
| --- | --- | --- |
|
||||
| 1 | a failed barrier is detected, and fatal | ✅ `d3ff03e` |
|
||||
| 2 | one barrier per drain; replies held | ✅ `b9b8a45` |
|
||||
| 3 | the inline path takes the fatal rule, asymmetry documented | ✅ `a6ccdbe` |
|
||||
| 4 | prove batches form — the `wmix` write-concurrent leg | ✅ `40d029c` |
|
||||
| 5 | measure the payoff, gate it, record it | ✅ `d52ea8a` |
|
||||
| 6 | closeout | ✅ this change |
|
||||
| — | **part B — io_uring submission** | ⬜ **not started; its premise changed, see below** |
|
||||
|
||||
### The payoff, measured two ways
|
||||
|
||||
| Measurement | Before | After |
|
||||
| --- | --- | --- |
|
||||
| controlled (same build, only `db.c`/`vm.c` swapped; `wmix 4000 32`) | 2213 · 2177 ops/s, p50 7183 · 7251 µs | **6216 · 6525 ops/s, p50 3458 · 3444 µs** |
|
||||
| committed baseline: `s1` inline vs `sN` batched | 1467 ops/s, mean batch 1.0 | **5117 ops/s, mean batch 5.43, peak 57** |
|
||||
|
||||
**≈2.9× throughput, ≈2.1× lower p50**, and the two methods agree (2.9× and
|
||||
3.5×). Batching scales with contention: mean batch **1.13 / 1.76 / 5.35** at
|
||||
C = 4 / 16 / 64.
|
||||
|
||||
### The cost side, and a bug the battery caught
|
||||
|
||||
**Reads were being held behind the barrier.** The drain first held *every* DB
|
||||
reply until the commit — including reads, which stage nothing. `mixread` p99 rose
|
||||
from ~1043 µs to **4057 µs** until only staging statements had their replies
|
||||
held. Caught by the gate, not by review.
|
||||
|
||||
**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 /
|
||||
4147 µs** — a 2–4× spread near idle. So part A buys ~3× write throughput at the
|
||||
cost of a longer, noisier tail on the owner shard. `durable.sN.*.p99us` was
|
||||
re-baselined at 100% tolerance for that reason, with the floor as the real guard
|
||||
(`mixread`'s came within 25 µs of tripping).
|
||||
|
||||
**This is the strongest argument for part B** — submitting the barrier and
|
||||
continuing to serve is exactly what removes this cost.
|
||||
|
||||
### What did NOT improve — and it was predicted
|
||||
|
||||
- **`durable.sN.mixwrite`: 480 → 492 ops/s, i.e. unchanged.** This was the
|
||||
spec's *original* payoff metric, and correcting it was part of the brainstorm:
|
||||
`mix` writes on one op in ten with C=4, so a quick run performs **20 writes**
|
||||
and measured mean batch **1.01**. A workload that never has two writes in
|
||||
flight cannot be helped by batching them.
|
||||
- **`durable.*.seed`: unchanged.** A serial single writer has nothing to batch
|
||||
with, under any scheme.
|
||||
- **This board's stated target was mis-stated.** It read "close the 66× gap
|
||||
iteration 22 measured (durable 4.5k vs ram 297k inserts/s)". Part A does not
|
||||
close that gap and structurally cannot: `seed` is serial, and one writer
|
||||
waiting on one barrier is a **latency** problem, not a batching one. Recorded
|
||||
rather than quietly renumbered.
|
||||
- **The before-p99 is not a measurement.** `hist_add` clamps at 20000 µs and
|
||||
both before-runs pinned exactly there, so the true value is ≥20 ms and
|
||||
unknown. The gain is *at least* 2.3×.
|
||||
|
||||
## Goals
|
||||
|
||||
- **Replace fsync-per-commit with io_uring group-commit** on the WAL write
|
||||
|
|
@ -113,26 +172,50 @@ chain: 5
|
|||
|
||||
## Acceptance Criteria
|
||||
|
||||
- What to achieve?
|
||||
- **Given** the io_uring write path under the iteration-22 crash battery
|
||||
(concurrent writers, kill -9 mid-stream, reboot, replay),
|
||||
- **when** it runs,
|
||||
- **then** every acknowledged write is present after replay and no
|
||||
unacknowledged partial write is ever visible — the exact result the
|
||||
fsync path gives, so durability is provably unchanged.
|
||||
- What to achieve?
|
||||
- **Given** the iteration-22 durable write benchmark,
|
||||
- **when** it is run on the fsync-per-commit path and then the io_uring
|
||||
group-commit path on the same machine,
|
||||
- **then** the io_uring path's write throughput is materially higher and
|
||||
its p99 commit latency lower, with the before/after numbers recorded —
|
||||
the payoff, measured, not asserted.
|
||||
- What to achieve?
|
||||
- **Given** a kernel without io_uring (old, or restricted by seccomp),
|
||||
- **when** the runtime starts,
|
||||
- **then** it falls back to the pwrite + fdatasync path automatically and
|
||||
correctly — io_uring is an accelerator, never a hard dependency, and a
|
||||
binary that runs everywhere is the whole project's premise.
|
||||
Met:
|
||||
|
||||
- **Given** the io_uring write path under iteration 22's crash battery, **when**
|
||||
it runs, **then** every acknowledged write is present after replay. ✅ — the
|
||||
criterion applies unchanged to part A's batching. `crash.sN` (the batched
|
||||
path) recovered every acked row after `kill -9`, `crash.s1` likewise, and both
|
||||
restart legs replay byte-true. This was the one thing batching could break.
|
||||
- **Given** the durable write benchmark before and after, **then** throughput is
|
||||
materially higher and p99 lower, recorded. ✅ ~2.9× and ~2.1× (p50); see
|
||||
`perf-targets.md` §6. **Scoped honestly:** on a write-concurrent workload
|
||||
only, and p99's "before" is at the histogram ceiling.
|
||||
- **Given** batching, **when** it runs, **then** it is proven to engage rather
|
||||
than assumed. ✅ mean batch 5.43, peak 57 on the gated leg, and the live
|
||||
assertion fails the suite if the mean drops to 1.
|
||||
- **Given** a durability failure, **when** it happens, **then** the engine does
|
||||
not continue with RAM ahead of disk. ✅ fatal, diagnosed, exit 74 — replacing
|
||||
three behaviours that disagreed.
|
||||
|
||||
Outstanding:
|
||||
|
||||
- **Given** a kernel without io_uring, **when** the runtime starts, **then** it
|
||||
falls back automatically. *(part B — part A adds no syscall interface, so
|
||||
nothing to fall back from yet.)*
|
||||
- **Single-shard concurrent batching.** A statement on shard 0 commits inline
|
||||
and cannot batch; doing so needs the inline path to park its fiber on the
|
||||
barrier — the same machinery part B needs. So `WO_SHARDS=1` gets no batching
|
||||
at all, by design and measured (mean batch 1.0).
|
||||
- **The abort path is not exercised.** Forcing a real `fdatasync` failure needs a
|
||||
full or read-only filesystem, which the gate cannot arrange without mount
|
||||
privileges. The unit test proves the error is *detected*; the exit three lines
|
||||
later is covered by inspection. Disclosed rather than papered over — iteration
|
||||
40 was exactly a fatal path nothing exercised.
|
||||
|
||||
## Part B — its premise changed
|
||||
|
||||
Part B was justified by "close the 66× durable gap". Part A shows that framing
|
||||
was wrong: the gap is **two** problems. Concurrent write fan-in was a batching
|
||||
problem and is now ~3× better. What remains is a **serial** writer waiting on a
|
||||
single barrier, which no amount of batching can help — and io_uring does not
|
||||
obviously help it either, since one writer still needs one durable barrier
|
||||
before its ack. Part B's real candidates are overlapping the barrier with other
|
||||
work on the shard, and the inline-path park that single-shard batching also
|
||||
needs. **It should be re-brainstormed against that, not started on the old
|
||||
premise.**
|
||||
|
||||
## Out Of Scope
|
||||
|
||||
|
|
|
|||
|
|
@ -189,13 +189,23 @@ static int wo_vm_adopt(wo_vm *vm) {
|
|||
if (re) {
|
||||
re->kind = 4;
|
||||
re->payload = e->payload;
|
||||
/* HELD, not pushed: pushing here would unpark the requester
|
||||
* before its record is durable, which is the ack contract
|
||||
* this iteration exists to make literally true. FIFO so the
|
||||
* first waiter is released first. */
|
||||
re->next = NULL;
|
||||
if (rtail) rtail->next = re; else rhead = re;
|
||||
rtail = re;
|
||||
if (dw && dw->len > before) {
|
||||
/* This statement STAGED a record, so its reply is HELD:
|
||||
* pushing it now would unpark the requester before its
|
||||
* record is durable, which is the ack contract this
|
||||
* iteration exists to make literally true. FIFO, so the
|
||||
* first waiter is released first. */
|
||||
if (rtail) rtail->next = re; else rhead = re;
|
||||
rtail = re;
|
||||
} else {
|
||||
/* A READ (or any statement that staged nothing) has no
|
||||
* durability to wait for. Holding it too was measurably
|
||||
* wrong: it parked readers behind an fsync they had no
|
||||
* stake in, and durable.sN.mixread p99 rose ~4x
|
||||
* (1043 -> 4057us) until this branch existed. */
|
||||
inbox_push_to(q->from_shard, re);
|
||||
}
|
||||
} /* OOM: the requester stays parked until stop — leak, not UB */
|
||||
break;
|
||||
}
|
||||
|
|
|
|||
|
|
@ -290,6 +290,18 @@ def tolerance_for(key):
|
|||
# blanket waiver here would have left the whole leg ungated.
|
||||
if key.endswith((".wmix.mean_batch", ".wmix.peak_batch", ".wmix.peak_staged")):
|
||||
return 100
|
||||
# databasev2 4: DURABLE multi-shard p99 is an fsync TAIL, and group commit
|
||||
# made it both noisier and legitimately higher. Measured across three full
|
||||
# runs of the same build, durable.sN.mixread.p99 was 1043 / 2318 / 4147 us
|
||||
# and wmix.p99 8758 / 20000 — a 2-4x spread with the box near idle, because
|
||||
# a barrier now blocks the owner shard LONGER (more records per fsync) even
|
||||
# though it blocks LESS OFTEN. That is the trade group commit makes on a
|
||||
# single-threaded owner, and part B (async submission) is what would undo
|
||||
# it. Gating a 2-4x-variable tail at 50% gates the disk, not the engine, so
|
||||
# the FLOOR is the real guard here — and it is not slack: mixread's floor
|
||||
# (4172us) came within 25us of tripping on the worst run.
|
||||
if key.startswith("durable.sN.") and key.endswith(".p99us"):
|
||||
return 100
|
||||
if ".mixread." in key or ".mixwrite." in key: return 50
|
||||
if ".sN." in key: return 50
|
||||
if ".read." in key or ".query." in key: return 50
|
||||
|
|
|
|||
Loading…
Reference in a new issue