cabletest/docs/measurement.md

# Measurement lessons

Hard-won rules about measuring correctly. Violating these produces numbers that look plausible and are wrong.

## Rate buckets: stamp at read time, never at ticker-fire time

- `direction.capture()` stamps each bucket with `time.Now()` taken immediately after reading the counters — never a timestamp passed in from the sampler ticker.
- Why: a rate is Δcounters/Δt. The sampler runs a variable delay after its tick, so a tick-time label pairs a right numerator with a wrong denominator — and because E[1/x] > 1/E[x], the error biases the rate **upward**; it doesn't cancel.
- Measured: tick-time stamping read 27.55 G / 4.88 Mpps against a true 19.86 G — 35% high. Read-time stamping read within 2%.
- A 1 s console interval hides it (jitter < 0.2%); a 16 ms panel window exposes it.
- Never share one timestamp across directions "so buckets share an instant" — nothing needs it, and it reintroduces the skew.

This rule covers the **NIC-counter** buckets. The per-stream application buckets are a separate system stamped by per-packet MAC RX timestamps (`SO_TIMESTAMPING` cmsg, `rx_filter=ALL` as a hard host check); read-time stamping is not a substitute for those — software stamping was tried and cannot reach the needed precision. NICs without all-packet RX timestamping (X710, 82599) fail the host check; see open-questions.md §2.

## NIC counters are not monotonic

- `/sys/class/net/*/statistics/*` run from boot, not process start, and reset to zero on driver stats resets.
- In unsigned arithmetic that bites twice: a zero baseline charges the machine's whole lifetime to the run; a backward step underflows `now - base` to ~2^64.
- Rule: accumulate only forward motion into a process-local total, then take every baseline from that total — never from a raw reading. Establish all baselines in one place before traffic starts.
- Debugging tell: when a panel's background disagrees with rows drawn from the same value, suspect two vintages of one counter before a color bug.

## Only zero-baseline-loss runs count; small frames measure the host

- A run counts only when `tx frames == rx frames` exactly with a clean cable — any nonzero baseline loss masks real cable faults.
- Small-frame runs are CPU/host-bound, not cable-bound; cable conclusions drawn from them are false.
- Keep to the default config (hardware.md) and judge from steady state — the first ~5 s is settling transient, not residual error.

## Measured performance (AF_PACKET committed path, ice/E810 era)

| Config | Result |
|---|---|
| Full size mix, 7 flow-director streams | Line rate (10.0–10.3 Gb/s/dir), ~1.78 Mpps/dir, zero loss including startup |
| 64 B only | pps-bound at ~5.4 Mpps/dir (frame generation is the limit, not receive drops) → only ~3.8 Gb/s |
| AF_XDP experiment (stashed) | 64 B TX ~14.5 Mpps/dir; RX ~4 Mpps/dir on AF_PACKET vs ~13.5 with AF_XDP RX |

- RX cannot be parallelized by RSS — hardware RSS can't hash raw ethertypes. Flow-steering by ethertype to distinct queues is what gives multiple NAPI contexts.
- The AF_XDP experiment delivered per-packet MAC RX stamps via an XDP-metadata kfunc (E810 datapath only; the committed path gets the same stamps via the `SO_TIMESTAMPING` cmsg). With the test path off the E810 it is parked; its novelty was delivery mechanism and throughput, not the use of hardware stamps.

## Dead ends — measured, do not re-attempt without new hardware

The binding constraint was total CPU across ~28 goroutines on 20 threads.

- **Splitting TX senders from RX streams** — raises TX but collapses RX (RX scales with queue count, capped at 7 on that NIC).
- **CPU pinning** — the Go scheduler beats manual placement; E-cores are poor at TX.
- **Batch sizes above 64** — no gain, worse loss.
- **Chasing the TX `sendmmsg` busy-spin on ENOBUFS** — it only exists when the TX ring is full, which means the wire is the ceiling; recovering that CPU buys no packets. POLLOUT is inert under PACKET_QDISC_BYPASS (skb freed on ENOBUFS, socket always reports writable).

## rxnfc / Flow Director programming (ice) — three traps

1. **Mask polarity is inverted vs `ethtool -n` display.** In raw `m_u` bytes a *set* bit means "must match": a working "match ethertype, ignore MACs" rule has `m_u.ether_spec` = dst 00×6, src 00×6, proto ff ff. `ethtool -n` prints the complement — trusting its display gives an inverted rule that silently matches nothing.
2. **`rule_locs` sits at offset 188, not `sizeof(struct ethtool_rxnfc)` (192), on amd64** — `rule_locs[]` follows `rule_cnt` at 188. Reading from 192 yields garbage locations, so existing rules are never found/deleted.
3. **ice rejects `RX_CLS_LOC_ANY`** with ENOSPC — allocate a free location yourself (capacity from `ETHTOOL_GRXCLSRLCNT`'s `data`; the CLI allocates downward from the top). When a rule inserts but steers nothing, diff the raw bytes of a CLI-made known-good rule against yours.