BUG: macOS CPU monitoring calculates incorrect percentages using delta on instantaneous values

[noahgift]

Summary

The macOS CPU monitoring implementation incorrectly applies delta-based percentage calculation to instantaneous CPU percentages from top, resulting in incorrect CPU usage readings.

Bug Location

src/monitor/collectors/cpu.rs:166-221 (macOS parse_proc_stat implementation)

Root Cause

On macOS, the code:

1. Parses instantaneous CPU percentages from top -l 1 (e.g., "45.67% user, 3.21% sys, 51.12% idle")
2. Truncates float to u64, losing decimal precision (45.67 -> 45)
3. Multiplies by 100 to "scale like Linux jiffies" (45 -> 4500)
4. Stores these as CpuStats which are meant for cumulative tick counts
5. calculate_percentage() then computes deltas between two readings

The problem: Linux's /proc/stat gives cumulative CPU tick counts, so delta calculation works. But macOS top gives instantaneous percentages already - computing deltas between two instantaneous percentage readings is mathematically incorrect.

Example of Bug

First reading:  user=45%, idle=50%  -> stored as CpuStats{user: 4500, idle: 5000}
Second reading: user=30%, idle=65%  -> stored as CpuStats{user: 3000, idle: 6500}

Delta calculation:
  total_delta = (3000+6500) - (4500+5000) = 0  (or negative!)
  Result: 0% or NaN instead of ~30%

Expected Behavior

On macOS, the CPU percentage should directly use the instantaneous percentages from top without delta calculation.

Falsification Test (ttop-demo.md Claim 41)

Per specification Section 10.3, Claim 41: "CPU % within ±2% of top"

This claim is currently falsified - the bug causes readings to deviate significantly more than ±2%.

Fix Approach

For macOS:

1. Store raw percentage directly from top as the CPU usage value
2. Skip delta calculation since we already have instantaneous percentages
3. Maintain precision by using f64 throughout

References

• ttop-demo.md Section 10.3, Claim 41
• tests/playbooks/macos_collectors.yaml (accuracy_tests)
• Popperian falsification checklist item 41

Labels

• bug
• macos
• cpu-monitoring
• falsification-failed

[noahgift]

Triage (2026-03-31): Bug report. macOS CPU monitoring uses delta calculation on instantaneous top values — wrong math. No local repo to verify fix. Keeping open.

[noahgift]

Triage: P2 — macOS-specific bug. Incorrect CPU percentage calculations using delta on instantaneous values. Platform-specific monitoring logic issue.