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