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Vendor AQR datasheet, correct timestamp model (hw rx stamps are a hard requirement), order ConnectX-5, map 82599 bucket-timestamp routes

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flamingcow
2026-08-10 12:50:57 -07:00
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@@ -21,7 +21,7 @@ Any nonzero baseline loss masks real cable faults, so a run counts only when `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; read-time stamping covers bucketing regardless of NIC.
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.