---
id: "bug-1881"
status: "partial"
status_updated: "2026-09-28"
status_note: "mismatch lines now carry a first-difference diff (24b05718); live default-speed replay proof waits on spec scheduler-sim-discrete-clock (task 5.3)"
component: "scheduler"
severity: "wrong-result"
timestamp: "2026-09-28T07:26:09Z"
last_seen: "2026-09-28T07:26:09Z"
occurrences: 1
files: ["src/scheduler/src/sim_verify.c"]
tags: ["scheduler", "sim", "sim-verify", "logging"]
related: []
---

# bug-1881: sim_log_mismatch prints identical expected/actual, hiding the field that differs

## Description
**Observed:**
```
... MISMATCH ... -- expected: DISPATCH jobid=0-12-1790578108-0[49] nhosts=1 -- actual: DISPATCH jobid=0-12-1790578108-0[49] nhosts=1
```
**Expected:** the message names the field(s) that differ, with both values.
**Impact:** a mismatch cannot be diagnosed from the log (bug-1880: 43 such lines, none showing the difference).
**Found by:** user reading the bug-1880 replay log, 2026-09-28

UPDATE (2026-09-28): Fix planned in spec libxce-alloc-preason-equality (Requirement 3, Requirement 5.3, task 5): the libxce equality functions gain an optional why/whylen buffer describing the first difference, and sim_log_mismatch appends it to the MISMATCH line.

## Environment
- OS: NixOS 26.05 (Yarara); NixOS kernel; kernel Linux 6.18.33 x86_64
- libc: glibc 2.42-47 (interpreter /nix/store/wb6rhpznjfczwlwx23zmdrrw74bayxw4-glibc-2.42-47/lib/ld-linux-x86-64.so.2)
- Toolchain: gcc (GCC) 15.2.0, meson 1.9.1
- Build: debug (build.dbg)
- Code: scheduler-framework @ 4c134127 (`xce_scheduler --version`: v0.0.0.1-4c134127-85e8fc00, built 2026-09-28T06:40:21Z)
- Deployment: pc13v: xce_scheduler in simulation mode (pid 93231) replaying a capture of the live pc13v cluster's scheduler (scheduler-replay-20260928-064842.replay, captured from 06:48:42); exact command line and --sim-start-ts not recorded

## Reproduction
1. Any --sim-verify mismatch whose decisions differ only below the jobid/nhosts level (bug-1880's replay)
- Frequency: deterministic (43/43 lines in the pc13v run)
- Evidence: data/xce_scheduler.log

## Root cause
sim_log_mismatch() (src/scheduler/src/sim_verify.c) summarizes each decision as kind + jobid + nhosts, while sim_decision_eq() also compares every host's allocations (resname, value, duration, reserve), so a difference there prints two identical summaries.

## Fix
Spec libxce-alloc-preason-equality (commits 8f25c1c3..dcf3acf3). sim_decision_eq() now compares DISPATCH decisions with libxce's public job_alloc_eq() and returns a caller-owned first-difference description (`char** why`) naming the field path and both values; sim_log_mismatch() appends it as ` -- diff: <why>`, and the replay loop in sim_loop.c passes and frees it (fixed text "no live decision" for leftover recorded decisions) — 24b05718. job_alloc_eq() also compares `from`, so the scheduler now stamps every allocation's `from` with the deciding session's start (scheduler.c sched_make_primary_alloc_, c1960457) and the normal-mode recorder takes beginRound from the same reading (driver.c, 2d7ba660), making `from` reproducible on replay.

Remaining: the simulator's own session-begin clock read (sim_loop.c) still drifts at --sim-speed >= 1, so a live default-settings replay cannot yet be shown to report zero `from` mismatches. That is task 5.3, blocked on spec scheduler-sim-discrete-clock.

## Verification
- Test: TODO: name + file; RED before the fix, GREEN after
- Suites: TODO: suite results and build profile
- Live: TODO: soak/deploy run, or n/a and why

## References
- Related: bug-1880