Ryan Malloy
6881489bf6
What the Raspberry Pi time-server guides get wrong on a Pi 4, with the measurements. The headline artifact is a four-line pps-gpio patch: PREEMPT_RT force-threads IRQ handlers, and pps-gpio takes its timestamp inside its handler, so the realtime kernel puts a scheduler between the electrical edge and the clock. IRQF_NO_THREAD takes RMS offset from 2468 ns to 199 ns. - kernel/ the patch - dashboard/ live status page (position hidden by default) - docs-site/ the write-up (Astro/Starlight, brass, no tutorial section)
1.7 KiB
The Cuckoo Escapement
What the Raspberry Pi time-server guides get wrong, and the numbers to prove it.
Docs: cuckoo.warehack.ing
We built a GPS-disciplined Stratum 1 NTP server on a Raspberry Pi 4, followed the published advice, and measured everything. Most of that advice is wrong on this board. One piece of it is wrong on every board.
| Change | RMS offset |
|---|---|
| Baseline | 823 ns |
chrony median filter + prefer on the PPS refclock |
440 ns |
| PREEMPT_RT, as the guides recommend | 2468 ns ← worse |
PREEMPT_RT + our IRQF_NO_THREAD patch |
199 ns |
The one thing worth stealing
kernel/0001-pps-gpio-keep-timestamp-in-hard-irq-under-PREEMPT_RT.patch
PREEMPT_RT force-threads interrupt handlers. pps-gpio takes its timestamp
inside its handler. So the realtime kernel — the marquee upgrade in every guide
— puts a scheduler between the electrical edge and the clock, and triples your
jitter. Four lines fix it.
As far as we can tell this isn't applied anywhere, which means anyone running GPIO-based PPS on a realtime kernel today is silently eating microseconds of jitter with no reason to suspect it. chrony still says Stratum 1. Everything still looks fine.
What's here
kernel/ |
The patch, and how to build it |
dashboard/ |
The live status page (FastAPI + WebSocket, no build step) |
docs-site/ |
The write-up — Astro / Starlight |
n = 1
Every number here comes from one Pi 4 and one GPS module. This is a field report, not a study. We're publishing the method alongside the results precisely so you can check it against your own board rather than take our word for it.
A Supported Systems joint.