The Mac/iOS client's wall around ~380 Mbps on a 2.5 G path is the receive
drain, not the transport: a loopback speed-test pushes 380/600/1000 Mbps at
0.0% loss, but Darwin has no recvmmsg(2), so the macOS client was doing one
recv() syscall per packet — ~40-90k syscalls/s on one core. When the recv loop
can't drain fast enough the kernel socket buffer backs up and drops, which the
client sees as a sustained stream stalling/freezing in the 300-400 Mbps range
(and an immediate "session ended" when a 500 Mbps+ first keyframe bursts in).
- core/transport: flip recvmsg_x (the batched Darwin recv, ~30x fewer syscalls)
from opt-in to default ON, opt-out via PUNKTFUNK_RECVMSG_X=0. Keeps the
auto-fallback to the scalar loop on any unexpected syscall error. The Apple CI
swift-test loopback now exercises this path by default.
- packaging/kde host.env: enable PUNKTFUNK_GSO=1 — UDP segmentation offload on
the host send path (one sendmsg per ~64 packets), the dominant lever above
~1 Gbps. Already wired (send_sealed -> send_gso) with sendmmsg auto-fallback.
- apple SpeedTestSheet: lengthen the bandwidth probe 2 s -> 5 s so the measured
number stops swinging wildly (50 vs 900 Mbps on the same link) — long enough
for steady-state send + recv drain to settle. Matches host MAX_PROBE_MS.
- host capture: PUNKTFUNK_SYNTH_NOISE synthetic high-entropy source for
reproducible throughput testing of the encode->FEC->send->recv path.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
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