---
id: "bug-1842"
status: "fixed"
status_updated: "2026-09-24"
status_note: ""
component: "scheduler"
severity: "unknown"
timestamp: "2026-09-22T19:35:00Z"
last_seen: "2026-09-22T19:35:00Z"
occurrences: 1
files: ["src/scheduler/src/sim_verify.c (sim_verify_alloc_eq", "the fix)", "src/lib/xcealloc.c (make_alloc", "the field's origin)", "src/scheduler/testsuite/test_sim_verify.c (new regression test)", "src/scheduler/testsuite/test_sim_live_allocation.c (stale-comment cleanup)"]
tags: ["scheduler", "sim", "sim-verify", "replay", "from-field", "wall-clock", "make_alloc", "xcealloc", "decision-verification", "false-positive", "user-reported", "tdd"]
related: ["bug-1611", "bug-1613", "bug-1779", "bug-1759", "bug-1838"]
legacy: true
gitea_issue: 77
gitea_synced_status: "fixed"
---

# bug-1842: User-reported: replaying a real captured .replay file (scheduler-replay-20260922-095207.replay)…

## Description
User-reported: replaying a real captured .replay file (scheduler-replay-20260922-095207.replay) via --sim --sim-verify logged a MISMATCH for every single DISPATCH decision, even though the printed 'expected:'/'actual:' summaries (jobid, nhosts) were byte-for-byte identical on both sides -- e.g. 'expected: DISPATCH jobid=0-13-1790071174-0[44] nhosts=1 -- actual: DISPATCH jobid=0-13-1790071174-0[44] nhosts=1' still reported as MISMATCH. User asked whether the comparison should stop using a timestamp as part of its check condition.

## Environment
_Not recorded (migrated from buglog.json)._

## Reproduction
_Not recorded (migrated from buglog.json)._

## Root cause
sim_decision_eq()/sim_log_mismatch() (sim_verify.c) never compare the decision's own `ts` field (only log it for diagnostics) -- the user's timestamp hypothesis was right in spirit but aimed at the wrong field. The REAL culprit is buried one level deeper: sim_verify_alloc_eq() compares xce_alloc_t field-by-field including `from`, and make_alloc() (xcealloc.c:30) unconditionally stamps `alloc->from = time(NULL)` -- the real OS wall clock, not core.c's virtualized sim clock -- every time an allocation is built. `expected->from` (decoded from the .replay file) is whatever real second the ORIGINAL recording ran in; `actual->from` (freshly rebuilt by the live seqjob_allocator.c/parjob_allocator.c -> sched_make_allocs() -> make_alloc() chain during THIS replay) is whatever real second THIS replay runs in. Two independent real-world timestamps that essentially never coincide for any replay of a real recording, even though the decision itself (job/host/resource/value/duration) is exactly what was recorded. Confirmed empirically: a clean run of xce_scheduler --sim --sim-verify against the user's own replay file mismatched 120/120 DISPATCH decisions, 100% on this field, zero relation to real-time-vs-batch simulation speed (reproduced identically at --sim-speed=0, 1, and 20). The codebase already had a documented workaround for the same footgun elsewhere: dup_alloc() (xcealloc.c:179) has to re-stamp ->from/->duration from its source AFTER calling make_alloc(), specifically because make_alloc() clobbers them with the current wall time otherwise. A prior session (2026-09-21, see this same discovery independently reached and left unfixed) had already isolated this exact mechanism via test_sim_verify_no_false_mismatch_after_live_dispatch_and_later_confirmation's own header comment, which explicitly (and accurately, until this fix) documented it as accepted 'practically negligible' same-second-window flakiness.

## Fix
FIXED (2026-09-22). Dropped the `a->from == b->from` clause from sim_verify_alloc_eq() (sim_verify.c) with a doc comment explaining why: `from` is reservation-start bookkeeping (real wall-clock, for expiry/accounting purposes), not part of a DISPATCH decision's semantic identity -- unlike `duration`, which is a genuine request-derived value and stays part of the comparison. TDD: new test test_dispatch_decisions_differing_from_timestamp_still_match (test_sim_verify.c) builds two structurally-identical dispatch allocations with deliberately far-apart `from` values (1790000000 vs 1) and asserts sim_decision_eq() still reports a match. Also updated test_sim_live_allocation.c's test_sim_verify_no_false_mismatch_after_live_dispatch_and_later_confirmation header comment, which previously described this as accepted same-second-window flakiness -- now accurately states the flakiness is eliminated by construction. Full scheduler suite: 85/85 Ok, zero regressions (xce-build.sh scheduler asan --incremental). Live-verified against the user's own real replay file: DISPATCH-vs-DISPATCH false mismatches dropped from 120/120 to 0/120 (--sim-speed=0, --sim-verify, --logfile capture, before/after diff). A separate, distinct issue remains in the same file (61 'expected: <none> -- actual: PEND ... main_reason=0' mismatches) -- see bug-1843, not touched by this fix.

## Verification
_Not recorded (migrated from buglog.json)._

## References
- Related: bug-1611, bug-1613, bug-1779, bug-1759, bug-1838
- Gitea: #77