shoney.arickathil
a798cf6698
- developer directive: pending iteration IDs now ARE the priority order; LANDED iterations keep historical numbers (code comments and commit history cite them — records, not a queue); 8/11 (the half-landed arc), 17 (parked, artifacts on a branch), 18 (next, artifacts named) also frozen - mapping (recorded in 00-story): 19<-20 Float+Bytes, 20<-9c attach, 21<-9d keypair, 22<-9e benchmarks, 23<-9f io_uring WAL, 24<-19 chat, 25<-10 services, 26<-12 blue-green, 27<-9g query corpus, 28<-14 skillhost, 29<-13 metaprogramming - 11 story files renamed; every doc reference re-numbered (word-boundary sweep for the lettered 9x ids, phrase-level for numeric ones); the iterations table rewritten with Seq == priority and "(was N)" notes; story-scoped link check: zero broken - merge-recovery folded in: the partial master merge had dropped the chat story, the fibers exploration note, the arc spec+plan, the framework-v2 plan, and the iteration-17 spec+plan — all restored from their branches and renumbered consistently Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
116 lines
6.1 KiB
Markdown
116 lines
6.1 KiB
Markdown
# Iteration 23 — io_uring group-commit write path
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> Format: fiberloom `product/story-iteration-template`. Part of
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> [Story — one language, one runtime, one database, one binary](../00-story.md).
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>
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> **Inserted 2026-08-15.** The write-path optimization, and deliberately the
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> LAST database performance iteration: it only earns its complexity once
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> there is a measured fsync-per-commit baseline to beat (iteration 22) and a
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> multithreaded runtime to overlap against (iteration 8). Doing it earlier
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> would optimize a number nobody had measured, against a runtime that
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> couldn't use it.
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>
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> **No spec exists yet.** The forks in *Info* are genuine decisions.
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## Goals
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- **Replace fsync-per-commit with io_uring group-commit** on the WAL write
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path: batch a tick's committed records into one submission, let the kernel
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overlap the write and the durability barrier, and acknowledge each writer
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only after the barrier its record rode has completed — the same
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ack-after-durable contract, at a fraction of the syscall cost.
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- **Overlap durability with work.** With the shard-actor runtime
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(iteration 8) the shard thread submits its batch and keeps executing ready
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statements while the ring drains, instead of blocking one thread on one
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fdatasync — the multithreading the throughput number has been waiting for.
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- **Keep the durability promise byte-for-byte.** Every guarantee iterations 9
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and 22 proved — replay-whole-or-not-at-all, torn-tail drop, no
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acknowledged write ever lost — holds identically; io_uring changes HOW the
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bytes reach the platter, never WHETHER an ack means durable.
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## Acceptance Criteria
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- What to achieve?
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- **Given** the io_uring write path under the iteration-22 crash battery
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(concurrent writers, kill -9 mid-stream, reboot, replay),
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- **when** it runs,
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- **then** every acknowledged write is present after replay and no
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unacknowledged partial write is ever visible — the exact result the
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fsync path gives, so durability is provably unchanged.
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- What to achieve?
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- **Given** the iteration-22 durable write benchmark,
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- **when** it is run on the fsync-per-commit path and then the io_uring
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group-commit path on the same machine,
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- **then** the io_uring path's write throughput is materially higher and
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its p99 commit latency lower, with the before/after numbers recorded —
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the payoff, measured, not asserted.
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- What to achieve?
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- **Given** a kernel without io_uring (old, or restricted by seccomp),
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- **when** the runtime starts,
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- **then** it falls back to the pwrite + fdatasync path automatically and
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correctly — io_uring is an accelerator, never a hard dependency, and a
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binary that runs everywhere is the whole project's premise.
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## Out Of Scope
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- **io_uring for the network/accept path.** This iteration is the WAL write
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path only; the socket side is the shard-actor runtime's and the network
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layer's concern.
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- **io_uring for reads.** RAM is authoritative — reads never touch a
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descriptor (phase-B doctrine), so there is nothing to accelerate on the
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read path. This is a write-durability optimization, full stop.
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- **Registered buffers / fixed files / SQPOLL tuning** beyond what the
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benchmark shows is worth it. Start with the plain submit/complete model;
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add ring features only when 22's number says a specific one pays.
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- **Replacing the WAL format or the commit contract.** The bytes on disk and
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the meaning of an ack are iteration 9's; this changes the syscall, not the
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format.
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## Info
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Forks the spec must settle:
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**1. How much of the ring model, and behind what abstraction?** The write
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path today is `pwrite` + `fdatasync` in `database/src/wal.c`; io_uring adds a
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submission/completion queue and a durability barrier op
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(`IORING_OP_FSYNC`/`IORING_FSYNC_DATASYNC` or `O_DSYNC` writes). The fork:
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wrap it behind the existing `wo_wal_commit` boundary (drop-in, the engine
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never learns) or expose an async-commit primitive the shard scheduler drives
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(faster overlap, but couples the WAL to iteration 8's loop). Leaning:
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drop-in behind `wo_wal_commit` first — it is the correctness-preserving
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step and 22 can measure it standalone — then an async variant only if 8's
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scheduler shows the blocking boundary is the remaining bottleneck.
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**2. liburing or raw syscalls?** liburing is the ergonomic wrapper but is a
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new external dependency, against the libc-only doctrine; the raw
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`io_uring_setup`/`io_uring_enter` syscalls are a few hundred lines and keep
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the doctrine. Leaning: raw syscalls (the doctrine is load-bearing and this is
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a bounded surface), with the mmap'd ring setup written down in the binding
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doc the way the WAL format is — normative, versioned.
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**3. What is the batch boundary?** Per-statement commit (today) is the
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simplest correct thing and the slowest; a group commit needs a boundary — a
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tick (iteration 8's scheduler quantum), a count, or a short time window.
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Leaning: the shard tick once iteration 8 lands (a batch is "everything
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committed this tick"), with a single-writer fallback that batches whatever
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accumulated between one `wo_wal_commit` call and the ring draining.
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**4. How is the fallback chosen and tested?** A kernel probe at startup
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(attempt `io_uring_setup`, fall back on ENOSYS/EPERM) is the mechanism; the
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question is how CI proves BOTH paths without two kernels. Leaning: an
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environment override (`WO_WAL_MODE=fsync|uring`) so the test matrix runs the
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crash battery and the benchmark on both on any capable machine, and the
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auto-probe is what production uses.
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## Proposed Solution
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- **Brainstorm the spec** after iterations 8 and 22 exist — this iteration is
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meaningless without a multithreaded runtime to overlap against and a
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measured baseline to beat, and its plan's acceptance is literally "22's
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durable number improved, 22's crash battery still green, fsync fallback
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still correct".
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- Expected shape: a `wo_wal` write-mode switch (fsync vs uring), the raw ring
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setup + submit/complete in `database/src/wal.c` (or a `wal_uring.c`
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beside it), the startup probe + `WO_WAL_MODE` override, the binding doc's
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WAL section extended with the ring layout, and iteration 22 re-run on both
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paths with the delta committed.
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