NTP Sync Quality — NTP Precision Accounting Test

Test Matrix

Summary

• All hosts are c7i.metal-48xl in eu-west-1a, Fedora 43 Cloud, running kernel 7.1.0-rc2+
• NTP via chrony (default Fedora 43 configuration, no tuning) to Amazon Time Sync (stratum 2)
• Data collected every 30s via chronyc tracking
• Started: 2026-05-23 ~08:19 UTC
• Patched kernel adds the NTP precision series (sub-nanosecond precision plus the per-tick precision-accounting infrastructure) — the 7 commits being posted upstream as v6. Note: the precision accounting is not actively used by chrony in this test — any observable difference comes from the NTP precision patches alone.
• Baseline kernel: same tree minus the 7 NTP precision commits

Test Restart — 2026-05-27

Data collection restarted with Amazon Time Sync Service correctly configured (both 169.254.169.123 and fd00:ec2::123 as preferred servers). Previous Fedora 43 default chrony config did not include these; see RHBZ#2482393. IPv6 also enabled on all instances.

Test Restart — 2026-06-10

Restarted on the reworked v5 patch series — the generic NTP precision commits being prepared for upstream posting as v5. Patched kernel 2fba787174cf (the 5-commit series tip), baseline bc484a509673, both based on 7.1.0-rc2+. All host CSVs were truncated to a common start time for this run.

Test Restart — 2026-06-14

Restarted on the reworked v6 patch series — now seven generic NTP precision commits (the competing time_offset/time_adjust skew settlement added since v5), being prepared for upstream posting as v6. Patched kernel 307b20f7dc0f, baseline bc484a509673, both based on 7.1.0-rc2+. All host CSVs were truncated to a common start time for this run.

Allan Deviation

Allan deviation measures clock stability at different averaging timescales. Lower is better. A clock that's well-disciplined by NTP should show decreasing ADEV at longer timescales (the servo corrects drift). The slope indicates the dominant noise type (white noise = −½, flicker = 0, random walk = +½).

Time Series

The "RMS Offset (chrony)" panel shows chrony's internally-tracked RMS which uses an exponentially-weighted history that predates our test window. The "RMS Offset (cumulative)" panel computes RMS directly from the offset samples collected during this test, giving a fairer comparison unaffected by each host's different convergence history.

Distribution (Box Plot)

CDF of Absolute Offset

Methodology

CSV columns from chronyc -c tracking:

Scripts

• ntp_quality_log.sh — data collector (runs on each host)
• ntp_quality_graph.py — fetch data and generate graphs

Raw Data

• A — periodic + patched
• B — periodic + baseline
• C — nohz + patched
• D — nohz + baseline

CSV Format

Data last fetched: —