writeonce/docs/stories/databasev2/01-ram-ceiling-measurement.md

track	iteration	status	readiness
databasev2	1	in-progress	ready

databasev2 1 — the RAM ceiling: measure the breaking point

Part of Story — databasev2: the database beyond RAM.

Refined 2026-08-27; the three forks are settled below and the decisions are locked. No spec document: the deliverable is numbers plus a harness leg, and the design fits in this file — the same call 7 makes.

First because the repo's own doctrine says so. "Always inspect crashsites. Always measure. Never assume." Two other iterations already cite numbers this one was supposed to produce: 2's resident-footprint budget defaults to a fraction of host memory whose value comes from here, and 3's before/after replay criterion has no "before" because bench/baseline.json carries 75 metrics and zero for replay, restart, boot or recovery. Iteration 22 proved restart correctness; it never timed it.

The design, as settled

Measure the curve, not the cliff. Everything dies at the ceiling; what matters is the shape on the way down — where p99 leaves its 1µs baseline, what insert throughput does as slabs stop coming from a warm allocator, and how much warning there is between "fine" and "unusable".

Fork 1 — the limit mechanism: rootless cgroup v2 via systemd-run --user

systemd-run --user --scope -p MemoryMax=N -p MemorySwapMax=M. Verified on the dev box: the memory controller is delegated to user.slice/user-<uid>.slice, a scope's memory.max reads back exactly as set, and no passwordless sudo is needed. Being cgroup-scoped also isolates the measurement from whatever else the box is doing, which matters — the dev box was at 22.9 of 31.7 GiB with 4.6 GiB of swap already in use when this was refined.

ulimit -v is rejected: it bounds address space, not resident set, which is the wrong quantity for an engine that mallocs slabs — and it is actively broken under ASan, whose huge virtual reservations trip it long before any real memory pressure.

If the mechanism is unavailable (no systemd, no delegation), the harness skips the growth legs loudly and names why. It must never silently fall back to measuring an uncapped box, because "it survived on a 32 GiB workstation" measures the workstation.

Fork 2 — the reference shapes: both, reported separately

Per-row footprint differs substantially between an Int-only row and a text-heavy one, because a Text column is a separate db_text allocation per row on top of the slab slot. Measured 2026-08-27: 96.5 B/row Int-only vs 320.6 B/row with three Text columns — 3.3×. An earlier draft of this section said "an order of magnitude"; that was an unmeasured guess and this iteration exists to replace exactly that kind of claim. 3.3× is still more than enough to make a single "bytes per row" number useless, which is the decision it was supporting.

db-bench already supplies half of this: items (k: Int, v: Int, plus a bucket ref) is the Int-only reference as it stands. The work is one text-heavy shape beside it, with footprint reported per shape.

Fork 3 — swap: in scope, as a controlled dimension

Not a confound to wish away — MemorySwapMax is the knob that separates the two exits this iteration exists to characterise. Both were measured, and both turned out differently than this iteration predicted.

Leg	Predicted	Measured
swap-off	catchable WO_T_OOM from checked malloc	SIGKILL, signal 9 (shell rc 137). No trap, no message
swap-on	latency collapse	no degradation at all: 148 s vs 150 s uncapped

Prediction 1 was wrong because of overcommit. With vm.overcommit_memory = 0 malloc succeeds and the process dies when it touches the pages, so table storage never gets the chance to report failure. The trap path is real but belongs to a different allocator:

Allocator	Ceiling	Failure mode
VM object arena	WO_HEAP_MB, checked	trap 4 / WO_T_OOM, exit 1, reportable
table storage (slabs + heap values)	none	SIGKILL under overcommit

This is the strongest argument available for iteration 2's byte budget: a declared budget is the only way table storage can acquire a checked ceiling, because malloc under default overcommit will never tell it there is a problem.

Prediction 2 was wrong because of access pattern. 900 000 Int rows under a 64 MiB cap with 256 MiB of swap finished in 148 s with RSS pinned at 62 MiB; the same workload uncapped took 150 s at 165 MiB RSS. Swap cost approximately nothing. The reason is that inserting is append-mostly: cold pages are written out once and never read again, so paging is sequential and off the critical path. The swap is a real disk file (/swap.img, no zram, zswap disabled), so this is genuine disk paging, not compressed RAM.

The correct generalisation is narrower than "swap is fine". This measures an append-mostly workload. A workload that reads randomly across a table larger than the cap is the one that collapses, and this iteration did not measure that — see Outstanding.

Progress

Piece	State
Wide text-heavy reference shape (db-bench/types.wo)	✅
growth N int|text — insert, per-decile RSS and read latency	✅
the sample reads its OWN RSS via /proc/self/status	✅ — the driver polls every 250 ms and would miss the value at a decile boundary
growth-verify — the survivor is a contiguous intact prefix	✅
rootless cap wrapper, swap on/off legs	✅ 4 footprint legs
footprint metric = median of marginals, doublings counted separately	✅
ceiling leg: dies at the cap, then replay must be intact	✅ gated
baseline + tolerance policy	✅ 121 checks; footprint at ±10%, kill-timing metrics at ±100%
perf-targets.md §5	✅
resident-footprint fraction for iteration 2	⬜ not delivered — the premise it rested on is false, see Outstanding
replay/restart baseline for iteration 3	⬜ not delivered
the random-read-over-cap collapse	⬜ not measured

Measured

Footprint, reproducible inside 2% across runs:

What	Int-only (Item)	Text-heavy (Wide)
steady-state footprint	96.5–100 B/row	320.6–324 B/row
doubling steps	3 (at ~24k and ~48k rows)	2
base process RSS	≈ 3.9 MiB, excluded from the per-row figure	same

Ratio 3.3× — not the "order of magnitude" an earlier draft asserted. Enough on its own to make a single "bytes per row" number useless, which is the decision it was supporting (2, fork 5).

The ceiling, 60 000 Int rows under an 8 MiB cap with swap off and WO_DATA set:

Question	Answer
how does it die?	SIGKILL, signal 9. No refusal, no diagnostic
what survives?	a contiguous intact prefix — ~40 000 rows, every v correct, no holes, not reported as corruption

Ack-after-fsync holds through an OOM kill. That is the one shutdown path which skips every cleanup handler, and the durable prefix came back whole.

The finding that matters most is the swap leg succeeding. It did not fail, did not warn, and returned 0. A deployment in that state looks healthy while serving from disk. That is the exit with no error signal, and it is why iteration 5's back-pressure must act at a declared threshold rather than at exhaustion — exhaustion either kills without warning or silently does not arrive.

Acceptance Criteria

Met:

• Given the growth workload under a fixed cap, when it runs twice, then RSS-per-row agrees inside tolerance and the slope is recorded per shape. ✅ inside 2%; perf-targets.md §5.
• Given the swap-off leg, when the cap is exceeded, then the exit is identified and recorded. ✅ SIGKILL, signal 9 — not the catchable trap this criterion originally expected, which is the whole point of measuring. The "process keeps serving" half of the original wording is void: nothing survives a SIGKILL.
• Given a cap exceeded with WO_DATA set, when the process is killed at exhaustion, then replay shows the acked writes present. ✅ ~40 000 rows, contiguous, no holes, no corruption report. Gated as the ceiling leg.
• Given the swap-on leg, when the same point is reached, then the degradation is quantified and the absence of any error signal recorded. ✅ degradation is nil for this workload (148 s vs 150 s uncapped) and the silence is total. Both halves are findings; the first inverted the prediction.
• Given the extended baseline, when a growth metric is doctored, then the gate fails on exactly that metric. ✅ text footprint +20% → FAIL gate.growth.text.noswap.bytes_per_row -- 388 vs baseline 324, 1 of 104.
• Given a host without the cap mechanism, when the harness runs, then the legs are skipped with a named reason and the rest still passes. ✅ cap_wrapper returns None unless the memory controller is delegated; there is no uncapped fallback.

Outstanding:

• The resident-footprint fraction for iteration 2's budget default. NOT delivered, and the premise is false. It was to be derived from the swap-onset point — but there is no onset: swap-off jumps straight from working to SIGKILL, and swap-on shows no degradation to detect an onset in. Iteration 2 must pick its budget on other grounds (host RAM fraction, or an explicit developer-declared figure) rather than waiting on a number this iteration cannot produce. This is the most important thing this slice learned and it removes a dependency rather than satisfying it.
• The random-read-over-cap collapse. Not measured. This is where the "latency collapse" prediction may still be true, and it is the workload that matters for 2's resident: keys, whose whole premise is reading rows back from a log larger than RAM. Needs a read-heavy leg over a table exceeding the cap. The single most valuable follow-up.
• Given rising fractions of the cap, when latency is sampled, then the p99 departure point is recorded. Partially: the sampler and metric exist and are gated, but the footprint legs never approach their 512 MiB cap, so p99_departure_decile is legitimately 0 and proves nothing. It becomes meaningful only with the read-heavy leg above.
• A replay/restart baseline for iteration 3. Not delivered; growth exercises WO_DATA but nothing times replay. Cheap to add, still absent from bench/baseline.json.

Out Of Scope

• Any fix. This measures. Declared budgets are 2, eviction is 5, tiering is 2's resident: keys.
• Changing the OOM behaviour. The checked-malloc code is untouched. The measurement showed it is largely unreachable for table storage under default overcommit — a finding to hand to 2, not a bug to fix here, and emphatically not a licence to start setting vm.overcommit_memory.
• A memory profiler or allocator instrumentation — observability is language iteration 30. RSS from /proc plus time.ticks is enough for a curve.
• Comparing against SQLite at the ceiling. bench/compare/go-sqlite exists, but SQLite's paged architecture is the design this project rejected, so the numbers would inform no decision here.
• Multi-host scaling — one binary owns its data.

Info — the forks, settled

1. The limit mechanism is rootless cgroup v2 via systemd-run --user --scope -p MemoryMax -p MemorySwapMax. ulimit -v was rejected: it bounds address space, not resident set, and ASan's virtual reservations trip it long before real pressure. No sudo needed; it also isolates the run from the rest of the box, which mattered — the dev box sat at 22.9 of 31.7 GiB throughout.
2. Both reference shapes, reported separately. 3.3× apart; one number would be a fiction.
3. Swap is a dimension, not a footnote — settled by getting it wrong first. An early run looked like the cap was unenforced because the process held 400 MiB inside a 64 MiB limit; it was swapping, which is the phenomenon under study.
4. Footprint is read as the median of per-decile marginals, not a two-point slope, so a slab doubling does not smear into the per-row figure. Doublings are counted as their own metric.
5. Kill-timing metrics carry ±100% tolerance. rows_recovered depends on where the SIGKILL landed; gating it tightly would be gating the scheduler. The invariant asserted instead is the shape of the survivor.
6. The ceiling leg asserts rc, never records it. When iteration 2's byte budget lands, death should become a checked refusal — the gate must not fail on that improvement.

History — four corrections worth keeping

"An order of magnitude" was a guess. The per-shape difference is 3.3×. An iteration whose purpose is replacing unmeasured claims had one in its own premise.

The clean-exit premise was wrong. This file and the residency spec both asserted the ceiling surfaces as a catchable WO_T_OOM. It is a SIGKILL. Overcommit means the allocator never learns there is a problem.

The latency-collapse premise was wrong too. Swap cost ~1% on an append-mostly workload (148 s vs 150 s). The prediction was not merely imprecise, it had the wrong sign. The narrower claim that survives is that a random-read workload over an oversized table is the one at risk, and that remains unmeasured.

A SIGKILL was once labelled a "checked refusal" by the harness, because subprocess reports signal death as a negative returncode (-9) while the shell spells the same event 137. The leg existed specifically to tell those two apart. Fixed, and the distinction is now spelled out at the comparison.