shoney.arickathil
3507aafea3
Audit found 4 of 10 iterations citing it1 and 4 carrying stale claims the
measurement contradicts.
- 05: framing was contradicted, not merely incomplete. Its goal expected a
gradient to detect ("back-pressure before the cliff"); there is no cliff
— SIGKILL with swap off, exit 0 with swap on, and read latency STEPS
(1us -> 487us) rather than departing. Heading and goal rewritten; the
measurement makes the goal stronger, not weaker
- 05: budget must be bytes — 3.3x footprint spread — with headroom for
index doublings, else it fires during a rehash
- 05: new goal — eviction policy QUALITY is decisive, since getting the
resident set wrong costs 273x, not a few percent
- 06: its revival question now has a reference point. 273x is the KERNEL
SWAP path; `resident: keys` preads via page cache and must beat it. This
file revives only if 5c/5d lands near 273x rather than well below
- 04: write path is not where pressure bites (append ~1%, read 273x), so
the io_uring question that matters is iteration 2's deferred read-path
one, not group-commit
- 00-story: problem statement asserted the store "refuses the insert
rather than dying". Corrected in place — a banner above it was not
enough, a skimmer never reaches it
- residency spec: "swap thrash and the OOM killer" named exits that were
not measured; replaced with silence-or-a-corpse
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
168 lines
9 KiB
Markdown
168 lines
9 KiB
Markdown
---
|
||
track: databasev2
|
||
iteration: "4"
|
||
was_language_iteration: "23"
|
||
status: pending
|
||
readiness: ready
|
||
chain: 5
|
||
---
|
||
|
||
# databasev2 4 — io_uring group-commit write path
|
||
|
||
> **Moved 2026-08-26** from the language track, where this was iteration 23.
|
||
> Part of [Story — databasev2: the database beyond RAM](00-story.md). Content unchanged by
|
||
> the move; its dependencies are restated in that track index.
|
||
|
||
> Format: `product/story-iteration-template`. Part of
|
||
> [Story — one language, one runtime, one database, one binary](../language-runtime-database/00-story.md)
|
||
> — the track this iteration was authored in before the 2026-08-26 move.
|
||
>
|
||
> **Inserted 2026-08-15.** The write-path optimization, and deliberately the
|
||
> LAST database performance iteration: it only earns its complexity once
|
||
> there is a measured fsync-per-commit baseline to beat (iteration 22) and a
|
||
> multithreaded runtime to overlap against (iteration 8). Doing it earlier
|
||
> would optimize a number nobody had measured, against a runtime that
|
||
> couldn't use it.
|
||
>
|
||
> **Supporting evidence for staying last (iteration 1, 2026-08-27):** the write
|
||
> path is *not* where memory pressure bites. Inserting 900 000 rows inside a
|
||
> 64 MiB cap with swap cost **~1%** (148 s vs 150 s uncapped), because appending
|
||
> never re-touches its cold pages. Random *reads* over the same oversized table
|
||
> cost **273×**. So the pressure is on the read path, and the io_uring question
|
||
> that may actually matter is the one iteration 2 deferred here — io_uring for
|
||
> `resident: keys` row reads — not group-commit for writes.
|
||
>
|
||
> **No spec exists yet.** ~~The forks in *Info* are genuine decisions.~~
|
||
>
|
||
> **REFINED 2026-08-20: the four forks are SETTLED as their recorded
|
||
> leanings** (developer confirmation, no code): (1) drop-in behind
|
||
> `wo_wal_commit` first, an async variant only if the arc's scheduler
|
||
> proves the blocking boundary is the bottleneck; (2) raw
|
||
> `io_uring_setup`/`io_uring_enter` syscalls — libc-only doctrine holds,
|
||
> ring layout documented normatively; (3) the batch boundary is the shard
|
||
> tick (the 8+11 arc's quantum), single-writer fallback batches whatever
|
||
> accumulated; (4) startup auto-probe + an env override so CI proves both
|
||
> paths on one kernel — AMENDED: the override is the arc-wide
|
||
> `WO_IO=uring|epoll` (the arc's T4 owns the probe and the per-shard
|
||
> ring; `WO_WAL_MODE` is subsumed). Position — RE-SEQUENCED 2026-08-21:
|
||
> FIFTH in the concurrency chain (32, WAL checkpoint, follows it —
|
||
> added 2026-08-21), **stage 3 → 22 → 31 → 24 → 23 → 32**
|
||
> (supersedes the 2026-08-20 old-id ordering "9e → 8+11 → 9f"); the
|
||
> per-shard ring already exists (arc T4 landed 2026-08-20,
|
||
> `WO_IO=uring|epoll`) — this iteration adds the WAL's WRITE+FSYNC
|
||
> chains to it. AMENDED 2026-08-20 (io_uring-first
|
||
> directive): the WAL's WRITE+FSYNC chains ride the SAME per-shard ring
|
||
> T4 creates for fiber parking — one event loop per shard, readiness ops
|
||
> and durability ops together, exactly the linux reference project's
|
||
> "single event loop" card. One composition
|
||
> note added since iteration 18: a `transaction { }` already IS a staged
|
||
> batch — under io_uring it becomes exactly one submission, so the two
|
||
> features compose without either knowing the other.
|
||
|
||
## Goals
|
||
|
||
- **Replace fsync-per-commit with io_uring group-commit** on the WAL write
|
||
path: batch a tick's committed records into one submission, let the kernel
|
||
overlap the write and the durability barrier, and acknowledge each writer
|
||
only after the barrier its record rode has completed — the same
|
||
ack-after-durable contract, at a fraction of the syscall cost.
|
||
- **Overlap durability with work.** With the shard-actor runtime
|
||
(iteration 8) the shard thread submits its batch and keeps executing ready
|
||
statements while the ring drains, instead of blocking one thread on one
|
||
fdatasync — the multithreading the throughput number has been waiting for.
|
||
- **Keep the durability promise byte-for-byte.** Every guarantee iterations 9
|
||
and 22 proved — replay-whole-or-not-at-all, torn-tail drop, no
|
||
acknowledged write ever lost — holds identically; io_uring changes HOW the
|
||
bytes reach the platter, never WHETHER an ack means durable.
|
||
|
||
## 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.
|
||
|
||
## Out Of Scope
|
||
|
||
- **io_uring for the network/accept path.** This iteration is the WAL write
|
||
path only; the socket side is the shard-actor runtime's and the network
|
||
layer's concern.
|
||
- **io_uring for reads.** For a fully-resident table reads never touch a
|
||
descriptor, so there is nothing to accelerate on the read path. This is a
|
||
write-durability optimization, full stop. **Note (2026-08-26):** principle 7's
|
||
residency half was amended, so a table declaring `resident: keys`
|
||
([databasev2 2](02-table-storage-modes.md)) *does* `pread` rows from the log —
|
||
and accelerating that read path with io_uring becomes a real, separate
|
||
question. It is not this iteration's, and it should not be folded in: this
|
||
one is about the commit path and its acceptance is a durability number.
|
||
- **Registered buffers / fixed files / SQPOLL tuning** beyond what the
|
||
benchmark shows is worth it. Start with the plain submit/complete model;
|
||
add ring features only when 22's number says a specific one pays.
|
||
- **Replacing the WAL format or the commit contract.** The bytes on disk and
|
||
the meaning of an ack are iteration 9's; this changes the syscall, not the
|
||
format.
|
||
|
||
## Info
|
||
|
||
Forks the spec must settle:
|
||
|
||
**1. How much of the ring model, and behind what abstraction?** The write
|
||
path today is `pwrite` + `fdatasync` in `database/src/wal.c`; io_uring adds a
|
||
submission/completion queue and a durability barrier op
|
||
(`IORING_OP_FSYNC`/`IORING_FSYNC_DATASYNC` or `O_DSYNC` writes). The fork:
|
||
wrap it behind the existing `wo_wal_commit` boundary (drop-in, the engine
|
||
never learns) or expose an async-commit primitive the shard scheduler drives
|
||
(faster overlap, but couples the WAL to iteration 8's loop). Leaning:
|
||
drop-in behind `wo_wal_commit` first — it is the correctness-preserving
|
||
step and 22 can measure it standalone — then an async variant only if 8's
|
||
scheduler shows the blocking boundary is the remaining bottleneck.
|
||
|
||
**2. liburing or raw syscalls?** liburing is the ergonomic wrapper but is a
|
||
new external dependency, against the libc-only doctrine; the raw
|
||
`io_uring_setup`/`io_uring_enter` syscalls are a few hundred lines and keep
|
||
the doctrine. Leaning: raw syscalls (the doctrine is load-bearing and this is
|
||
a bounded surface), with the mmap'd ring setup written down in the binding
|
||
doc the way the WAL format is — normative, versioned.
|
||
|
||
**3. What is the batch boundary?** Per-statement commit (today) is the
|
||
simplest correct thing and the slowest; a group commit needs a boundary — a
|
||
tick (iteration 8's scheduler quantum), a count, or a short time window.
|
||
Leaning: the shard tick once iteration 8 lands (a batch is "everything
|
||
committed this tick"), with a single-writer fallback that batches whatever
|
||
accumulated between one `wo_wal_commit` call and the ring draining.
|
||
|
||
**4. How is the fallback chosen and tested?** A kernel probe at startup
|
||
(attempt `io_uring_setup`, fall back on ENOSYS/EPERM) is the mechanism; the
|
||
question is how CI proves BOTH paths without two kernels. Leaning: an
|
||
environment override (`WO_WAL_MODE=fsync|uring`) so the test matrix runs the
|
||
crash battery and the benchmark on both on any capable machine, and the
|
||
auto-probe is what production uses.
|
||
|
||
## Proposed Solution
|
||
|
||
- **Brainstorm the spec** after iterations 8 and 22 exist — this iteration is
|
||
meaningless without a multithreaded runtime to overlap against and a
|
||
measured baseline to beat, and its plan's acceptance is literally "22's
|
||
durable number improved, 22's crash battery still green, fsync fallback
|
||
still correct".
|
||
- Expected shape: a `wo_wal` write-mode switch (fsync vs uring), the raw ring
|
||
setup + submit/complete in `database/src/wal.c` (or a `wal_uring.c`
|
||
beside it), the startup probe + `WO_WAL_MODE` override, the binding doc's
|
||
WAL section extended with the ring layout, and iteration 22 re-run on both
|
||
paths with the delta committed.
|