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. 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.

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, and a backward step underflows now - base to ~2^64. 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

Any nonzero baseline loss masks real cable faults, so a run counts only when tx frames == rx frames exactly with a clean cable. Small-frame runs are CPU/host-bound, not cable-bound, so cable conclusions drawn from them are false. Keep to the default config (see hardware.md) and judge from steady state — the first ~5 s of any run is a settling transient (flow rules, rings, workers coming up) that can read far below line rate and is not residual error.

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

• Full size mix at 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), so only ~3.8 Gb/s.
• RX cannot be parallelized by RSS (hardware RSS on ice can't hash raw ethertypes) — Flow Director steering by ethertype to distinct queues is what gives multiple NAPI contexts. rxnfc/fdir programming has sharp edges; see the rxnfc notes below.
• Dead ends, measured and not to be re-attempted without new hardware (the binding constraint is total CPU across ~28 goroutines on 20 threads):
  • Splitting tx senders from rx streams — raises tx but collapses rx, since 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.
• The tx sendmmsg busy-spin on ENOBUFS is not worth chasing: it only exists when the tx ring is full, which means the wire is the ceiling, so recovering that CPU buys no packets. POLLOUT is inert under PACKET_QDISC_BYPASS (skb freed on ENOBUFS, socket always reports writable).

The AF_XDP experiment (stashed) removed the tx frame-generation ceiling (64 B tx ~14.5 Mpps/dir) and moved the bottleneck to RX (~4 Mpps/dir on AF_PACKET, ~13.5 with AF_XDP RX). It required per-packet MAC rx timestamps for honest buckets, delivered via an XDP-metadata kfunc — available on the E810 datapath, not the X710. With the test path off the E810 this whole path is parked. Note the committed AF_PACKET path also buckets by per-packet MAC rx stamps — via the SO_TIMESTAMPING cmsg, with rx_filter=ALL enforced as a hard host check — so the stash's novelty was the delivery mechanism and throughput, not the use of hardware stamps. Read-time stamping is how the NIC-counter buckets are labeled (the lesson above), not a substitute for the per-frame stamps; NICs without all-packet rx timestamping (X710, 82599) fail the host check and need a bucketing fallback — see open-questions.md §2.

rxnfc / Flow Director programming (ice), three traps that each cost a debugging round

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, so 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.