76 lines
2.4 KiB
Markdown
76 lines
2.4 KiB
Markdown
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---
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title: The measurements
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description: Every number on this site, with the methodology that produced it. Check our work.
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sidebar:
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order: 2
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---
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All numbers from **one** Raspberry Pi 4 + BerryGPS-IMU v4 (u-blox CAM-M8C), PPS on
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GPIO18. n = 1. Check them against your own board.
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## Headline progression
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| Change | RMS offset | Root dispersion |
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|---|---|---|
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| Baseline (stock kernel, stock chrony) | 823 ns | 16.8 µs |
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| chrony `filter 10` + `prefer` on PPS refclock | 440 ns | 5 µs |
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| PREEMPT_RT, unpatched | **2468 ns** ← *worse* | 11.6 µs |
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| **PREEMPT_RT + [`IRQF_NO_THREAD`](/reference/the-patch/)** | **199 ns** | 6.3 µs |
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## Raw PPS jitter (kernel-timestamped)
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| Kernel | jitter (σ) | peak-to-peak |
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|---|---|---|
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| Stock | 2134 ns | 11 µs |
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| PREEMPT_RT (threaded handler) | 6947 ns | 38 µs |
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| PREEMPT_RT + patch (hard-irq handler) | 2568 ns | 18 µs |
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## The dashboard's tax
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A/B/A, 62 s per round. [Why this matters](/explanation/the-observer-effect/).
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| | Dashboard off | Dashboard on |
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|---|---|---|
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| Before fix | 1304 ns | 1912 / 2179 ns (**+36%**) |
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| After fix | 1437 ns | 1169 / 1450 ns (**no penalty**) |
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## Things that did nothing
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| Change | Result |
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|---|---|
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| Baud 9600 → 115200 | PPS offset **−1 ns** either way. Identical. |
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| SBAS disabled | No measurable change to PPS. |
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| `isolcpus` alone | Inconclusive-to-harmful (concentrates load onto the PPS core). |
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## Methodology
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**Don't trust chrony's own stats for this.** `chronyc sourcestats` reports a
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windowed, median-filtered figure that lags reality and hides what you're trying to
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see. Measure the kernel's PPS timestamps directly:
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```bash
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sudo timeout 62 ppstest /dev/pps0 | awk '
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/assert/ {
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split($0, a, "assert "); split(a[2], b, ","); t = b[1] + 0;
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if (prev > 0) {
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d = (t - prev - 1.0) * 1e9; # deviation from exactly 1.000000000 s, in ns
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n++; sum += d; sumsq += d*d;
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if (d > max || n == 1) max = d;
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if (d < min || n == 1) min = d;
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}
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prev = t
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}
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END {
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mean = sum/n; sd = sqrt(sumsq/n - mean*mean);
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printf "n=%d jitter_sd=%.0f ns p2p=%.0f ns\n", n, sd, max-min
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}'
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```
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Each pulse should be exactly 1.000000000 s after the last. The deviation *is* the
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jitter.
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**Always run A/B/A**, never A/B. Clock behaviour drifts on the scale of minutes;
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if you measure on-then-off you cannot tell a real effect from thermal drift or a
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satellite geometry change. Go on → off → on, and require the two "on" rounds to
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agree before you believe the middle one.
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