ef93a4fa14
Source changes (dormant parity infra, retained from iterate 2.AI/2.AO): - xenia-kernel/exports.rs: nt_create_event manual_reset polarity + related event wiring - xenia-gpu/mmio_region.rs: D1MODE_VBLANK_VLINE_STATUS hardcode parity Also lands the audit-runs/ analysis notes (.md/.txt/.json digests) for the iterate 2.x VSync/0x10e8/0x1004 wedge investigation. Raw trace dumps (.jsonl/.gz/.csv/.stdout) and agent worktrees (.claude/) are gitignored as regenerable local artifacts — see memory + HANDOFF for the running findings. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
155 lines
6.3 KiB
Markdown
155 lines
6.3 KiB
Markdown
# Recommendation — Phase C+23
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## Top-line: STAY WITH THE BAND-AID
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After source-reading both engines + characterizing 4 archived canary
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cold runs' jitter shape + reviewing Phase D's H'/H broad outcomes,
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the recommended approach is **(ζ) stay with the band-aid**.
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The 104,607 cap that originally motivated this track is already
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unblocked at the diff-tool layer (Phase D D-extension absorber,
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2026-05-18). The next divergence at idx 105,046 is
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`VdInitializeEngines.return_value` — a VD-subsystem engine bug, NOT
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a scheduling-determinism recurrence. The cost-benefit of pursuing
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γ/β/α is no longer compelling because the immediate symptom is
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resolved and no structural follow-on cap has appeared.
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## Rationale
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### 1. The original target is already unblocked.
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| metric | pre-C+20 (C+19) | post-C+21 | post-Phase-D D-extension | now |
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|---|---|---|---|---|
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| Main matched-prefix | 104,606 | 104,607 | **105,046** | 105,046 |
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| Sister chains | 11/32/3/41/16 | 11/32/3/41/16 | 11/32/4/41/16 | unchanged |
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| Cap class at head | (B) contention | (A) state-mutation | (engine) VD | (engine) VD |
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The matched-prefix advanced **+440** since C+19 through diff-tool work
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that did NOT touch the engines. The cap class at the head is no longer
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scheduling.
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### 2. Phase D Stages 1-4 already built the structural infrastructure.
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Phase D Stage 1 (canary contention emitter), Stage 2 (manifest builder),
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Stage 3 (ours `OrderMode::ContentionReplay` + manifest loader), and
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Stage 4 (diff-tool engine-local kinds) ALL LANDED. The engine code is
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in tree. What's missing is *coverage of the right contention events*:
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the 104,607 divergence was upstream of canary's first
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`contention.observed=true` emit (idx 104,664), so the manifest could
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not target the right call site.
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This means: if we pursue γ (broaden replay to more event classes),
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the entry cost is not "start from scratch" but "extend an existing
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manifest layer." However, the LOC budget for γ is still ~600 across
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both engines, and there is **no proven future cap** that this would
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unblock.
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### 3. The empirical jitter range is small and fully absorbable.
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From `jitter-profile.md`: 4 canary cold samples show 3 distinct
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shapes around the contention window. The C+21 absorber + Phase D
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D-extension already canonicalize ALL 3 shapes to the same matched
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form. Even N=5 or N=10 fresh canary colds would land in one of these
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3 shapes (likely with the same absorber outcome).
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The SID core (`a25a16a4f6f547aa`, `2a70efeeed4f4fb6`,
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`72a4170012353517`) is consistent across cold runs (±20% counts), and
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the shared-global SID recipe (C+18) recomputes them deterministically.
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The transient "top-2" SIDs (which change per-cold) all flow through
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the shared-global absorber.
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### 4. Canary cannot be made deterministic without invalidating it.
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The host-thread-per-XThread model is what makes canary the *oracle*.
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Replacing it (α / β) would require:
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- Reworking ~2000-3000 LOC of canary base+kernel.
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- Re-validating against the broader canary test corpus (other games).
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- Accepting a real risk of breaking Sylpheed-unrelated game-compat.
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Approach γ (record-and-replay) avoids touching canary's scheduling
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philosophy but requires ours to consume a multi-million-entry trace,
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with engineering and runtime cost that should be matched to a *proven*
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future scheduling cap.
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### 5. The Phase B image hash and ours digest are stable.
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`image_loaded_sha256 ea8d160e…` UNCHANGED. Ours default digest
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stable × 3 cold runs. There is no signal of latent divergence in the
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pre-Phase-A surfaces that would benefit from scheduling alignment.
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## What to keep
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1. **Phase D Stages 1-4 infrastructure** stays in tree. Cvar
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`kernel_emit_contention=false` default-off; `XENIA_CONTENTION_MANIFEST_PATH`
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opt-in. Future phases can use them.
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2. **All absorbers** (C+18, C+21, D-extension) stay; they are correct
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and narrow.
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3. **The Stage 0 `OrderMode::ScanQuantum`** stays as a debug knob,
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documented as null-result.
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## What to defer
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1. Approach γ (broader scheduling-trace replay) — defer until a
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future cap demonstrably scheduling-related appears.
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2. Approach β / α (deterministic preemption / cooperative canary) —
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defer indefinitely.
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## What to do next
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The next phase is **C+24** (or whatever the natural next number) on
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the head divergence at idx 105,046: `VdInitializeEngines.return_value`
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(canary=1 ours=0). This is a regular engine bug investigation, ~5-50
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LOC.
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## Fallback: γ trigger criteria
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If a future phase finds a NEW scheduling-determinism cap (defined as:
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two consecutive divergences whose root cause is contention/wakeup-
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ordering across ≥2 guest threads, NOT a guest-code bug or kernel
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emit-completeness gap), then revisit γ. The criteria:
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- The new cap is ≥1,000 events long.
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- The C+21 / D-extension absorbers cannot fold it within their
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current cap (32 pairs).
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- Empirical jitter sampling (≥3 canary colds) confirms structural
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shape divergence, not just SID identity drift.
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If all three hold, γ is justified. Estimated ~600 LOC across 4-5
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sessions.
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## What this recommendation is NOT
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- It is NOT "no scheduling work was useful." Stages 1-4 + D-extension
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produced the matched-prefix advance from 104,606 → 105,046 (+440).
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- It is NOT "the absorbers are perfect forever." They are explicit
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band-aids in spirit of reading-error #23, annotated in schema-v1.md
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v1.5.
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- It is NOT "ours and canary are bit-aligned in contention regions."
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They are *measurably* aligned (matched-prefix) but not *structurally*
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aligned (the underlying guest events still differ; the absorber
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folds the difference).
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## Multi-session budget if we proceed (γ scenario only)
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Sessions estimated 4-5. NOT scheduled now.
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| stage | LOC | est session |
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|---|---|---|
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| γ-Stage 1: extend canary trace to wake/park/yield | ~150 | 1 |
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| γ-Stage 2: extend manifest builder | ~80 | 0.5 |
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| γ-Stage 3: generalized replayer in ours | ~250 | 2 |
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| γ-Stage 4: diff-tool integration | ~50 | 0.5 |
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| γ-Stage 5: validation + sister budgets | n/a | 1 |
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| **total** | **~530** | **~5** |
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## Acceptance for THIS session (planning-only)
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- [x] Planning artifacts in `audit-runs/phase-c23-scheduler-determinism-plan/`.
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- [x] Engine sources UNCHANGED (verified by file listing — only
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documentation + 1 python probe written).
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- [x] Diff tool UNCHANGED.
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- [x] Memory entry to be written next.
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- [x] Recommendation justified against C+21 band-aid + breadth of
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contention regions + multi-session budget.
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