Lever 4c of the single-core throughput campaign (4a send GSO ✓ #55, 4b TUN offload ✓ #56).
The post-4a/4b profiling showed the single-core ceiling is per-packet cost (kernel UDP send sendmsg ~23%, TUN write ~20%, SipHash ~9% — all charged per packet). Raising the tunnel MTU cuts the packet count for the same bytes, so it reduces every per-packet cost proportionally — the cheapest, most broadly-applicable remaining lever.
Scope
- Make the tunnel MTU path-aware rather than a fixed conservative value; probe (PMTUD / PLPMTUD-style) and adapt, falling back cleanly to 1500-safe where the path can't carry more.
- Interacts with FEC symbol sizing and the outer-UDP overhead budget — needs its own design (spec).
Note
- Jumbo frames only help where the whole path supports them; 1500 stays the internet-safe default. The win is largest on controlled/LAN paths.
- Related parked idea: inner-header compression (ROHC-style) — assessed low-priority for yip (loss-fragile + anti-DPI conflict); revisit only after 4c if a header-overhead bottleneck survives.
Lever 4c of the single-core throughput campaign (4a send GSO ✓ #55, 4b TUN offload ✓ #56).
The post-4a/4b profiling showed the single-core ceiling is per-packet cost (kernel UDP send
sendmsg~23%, TUN write ~20%, SipHash ~9% — all charged per packet). Raising the tunnel MTU cuts the packet count for the same bytes, so it reduces every per-packet cost proportionally — the cheapest, most broadly-applicable remaining lever.Scope
Note