NTP Sync Quality — Co-located VM Test

Test Matrix

Summary

• All 4 VMs are c8i.16xlarge co-located on the same physical host (droplet 7.233.99.247, DUB68) — same TSC crystal, same NTP network path
• Fedora 44 Cloud, running kernel 7.1.0-rc4+
• NTP via chrony (default Fedora 44 configuration, no tuning) to Amazon Time Sync (stratum 2)
• Fully contended: all 4 VMs share the host's 96 vCPUs (4×64 = 256 vCPUs oversubscribed on 96 physical cores)
• Data collected every 30s via chronyc tracking
• Started: 2026-05-24
• Patched kernel adds: NTP sub-nanosecond precision, NTP precision accounting infrastructure, vmclock_host device
• Baseline kernel: same tree minus the 10 NTP precision/vmclock commits

Why Co-located?

The bare metal test showed clear differences between patched and baseline kernels, but confounders (different crystals, different thermal environments, different network paths) made it hard to attribute differences purely to the kernel patches. By placing all 4 VMs on the same physical host, we eliminate:

• Crystal variation — all VMs read the same TSC
• Thermal drift — same physical environment
• Network path — same NTP route to the time source

Any remaining differences must come from the kernel's timekeeping code or the tick configuration.

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 — 2026-05-30

Data collection restarted with clocksource set to acpi_pm on all four virt hosts. This eliminates the TSC as a variable, testing the NTP precision patches against a slower but independent hardware counter.

Observations

• Collecting data…

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

Same methodology as the bare metal test. 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

• E — periodic + patched
• F — periodic + baseline
• G — nohz + patched
• H — nohz + baseline

CSV Format

Data last fetched: —