jimmy
50 lines
3.4 KiB
Markdown
50 lines
3.4 KiB
Markdown
# Windows clock sync (w32time) for latency-gated work
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> Reusable failure mode. Diagnosed 2026-10-11 on Tony's desktop (physical ASUS box) while building a latency-gated paper harness whose spec voids any candidate whose NTP skew exceeds 250 ms.
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## Symptom
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Anything that gates on the local clock fails: an NTP gate rejects every candidate, timestamps look wrong, and the clock **drifts** instead of holding. On this box the offset went from ~−1.8 s, to ~+10 ms right after a sync, then slid to **−284 ms and kept going** (~4–5 ms/min).
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## Diagnose
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```bash
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sc.exe query w32time # STATE: 1 STOPPED == no time sync at all
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w32tm /query /status # Leap Indicator, Stratum, Last Successful Sync, Poll Interval
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```
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Then measure the offset **independently** of w32time (a stdlib NTP query against `pool.ntp.org`, `time.google.com`, `time.cloudflare.com`). Multiple servers agreeing confirms it is a real local-clock error, not one bad source. HTTP `Date` headers are useless for sub-second work — they are **whole-second** granularity.
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## Root causes seen (in order encountered)
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1. **The service was stopped.** `w32time` not running ⇒ no sync at all.
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2. **Registry writes went to the wrong key path.** `MinPollInterval`, `MaxPollInterval`, `MaxAllowedPhaseOffset`, `LocalClockDispersion` live under
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`HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Config` — **not** the service root (`…\Services\W32Time`). `reg add` reported success but nothing applied. `SpecialPollInterval`/`NtpServer` live under `…\W32Time\TimeProviders\NtpClient`.
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3. **Default poll is 1024 s**, so a bad RTC drifts far between syncs.
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4. **`MaxAllowedPhaseOffset = 1` (s) makes w32time *slew*, not *step*** — it never nulls a sub-second residual. The clock sat stable but stuck at ~−180 ms.
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## Fix (all elevated)
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```bat
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set CFG=HKLM\SYSTEM\CurrentControlSet\Services\W32Time\Config
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set NTP=HKLM\SYSTEM\CurrentControlSet\Services\W32Time\TimeProviders\NtpClient
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sc config w32time start= auto
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w32tm /config /manualpeerlist:"time.google.com,0x1 time.cloudflare.com,0x1 pool.ntp.org,0x1" /syncfromflags:manual /reliable:no /update
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reg add "%CFG%" /v MinPollInterval /t REG_DWORD /d 6 /f REM 64 s
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reg add "%CFG%" /v MaxPollInterval /t REG_DWORD /d 10 /f
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reg add "%CFG%" /v LocalClockDispersion /t REG_DWORD /d 3 /f
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reg add "%CFG%" /v MaxAllowedPhaseOffset /t REG_DWORD /d 0 /f REM force a STEP to null the residual
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reg add "%NTP%" /v SpecialPollInterval /t REG_DWORD /d 64 /f REM honoured only with the 0x1 flag
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net stop w32time & net start w32time
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w32tm /resync /force
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```
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Result: **−284 ms drifting → ±2 ms stable** (held over 150 s across four NTP servers).
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## Lessons
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- A `reg add` that "completed successfully" can be a **no-op** if the key path is wrong. Verify with `w32tm /query /configuration`, never with the command's exit code.
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- `MaxAllowedPhaseOffset: 0` forces a step on every sync — right for nulling a residual; if constant stepping is unwanted, raise it to a small tolerance afterwards.
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- Post-step, `w32tm /query /status` can read `Leap Indicator: 3 (not synchronized)` / `Stratum 0` while the time is actually correct — trust an independent NTP measurement, not the status flag.
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- `Date` headers (1 s granularity) can never satisfy a sub-second freshness/skew gate; measure freshness from **local timestamps** instead.
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- Requires elevation (UAC). A physical box with a bad RTC has no host time-integration to fall back on, unlike a VM. |