#250 - Surface 802.3x pause state and counters

[chloecrozier]

Closes #250.

Found while chasing the ResNet benchmark's 308 Gb/s cap: 802.3x pause was eating 21.7% of
400 GbE line rate with every drop counter at zero, so it read as a slow transmitter. Two
things should have caught it and neither did.

tune_system.py had no pause check. Added --check pause: reports the pause parameters
per NIC, and separately whether the pause counters are non-zero, since pause that never fires
costs nothing. Also Step 10 in the system configuration guide and a warning in the raw
Ethernet benchmarking guide.

Pause counters never reached the xstats dump. They are not in rte_eth_xstats_get on
mlx5 at all, so the dump could not print them. src/net_pause.{h,cpp} reads them from the
kernel netdev instead (ETHTOOL_GPAUSEPARAM / ETHTOOL_GSTATS), and both raw engines call
it: warn at init when pause is enabled, print the counters with the stats dump. Counters are
per-run deltas against an init baseline, because the raw values are cumulative since boot and
would call a run throttled when pause was off for it.

Test plan

• 400 GbE loopback, pause on: warns at init, reports the frames exchanged during the run
• No pause activity: reports 0, no false "throttled" warning despite ~132M lifetime frames
• Port with no netdev (software loopback): silent no-op
• --check pause parsing: both-off, both-on, rx-only, tx-only, RX/TX negotiated lines
• Build clean, clang-format clean, check_doc_refs.py and mkdocs anchors pass

[greptile-apps]

Greptile Summary

The PR surfaces 802.3x pause configuration and activity so throughput throttling is distinguishable from a slow transmitter.

• Adds Linux ethtool-based pause-state and per-run counter collection.
• Integrates pause telemetry into the DPDK and ibverbs raw engines.
• Adds a tune_system.py --check pause readiness check.
• Documents pause diagnosis and configuration guidance.

Confidence Score: 5/5

The PR appears safe to merge because no blocking failure remains in the eligible follow-up review scope.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/net_pause.cpp	Adds best-effort ethtool ioctl inspection, initialization baselines, and pause-counter delta logging.
src/net_pause.h	Defines the internal pause telemetry state and helper interface.
src/engines/dpdk/daqiri_dpdk_engine.cpp	Adds pause-state checks during initialization and pause counters to DPDK statistics output.
src/engines/ibverbs/daqiri_ibverbs_engine.cpp	Adds pause-state checks and counter reporting to the raw ibverbs engine lifecycle.
python/tune_system.py	Adds a focused host check for pause configuration and cumulative pause activity.
src/CMakeLists.txt	Compiles the new Linux-only pause telemetry implementation into the common library.
docs/tutorials/system_configuration.md	Adds operational guidance for checking and disabling Ethernet pause where appropriate.
docs/benchmarks/raw_benchmarking.md	Documents how pause counters explain throughput plateaus without packet drops.

_{Reviews (7): Last reviewed commit: "#252 - Tighten pause documentation" | Re-trigger Greptile}

[cliffburdick]

Couple small comments. Good PR overall

[chloecrozier]

@cliffburdick thanks for reviewing, I just pushed the changes