api: optionally bind one socket per worker

[fergusfinn]

Summary

Adds VLLM_API_SOCKET_PER_WORKER=1. When set, each API-server worker replaces its
inherited dup of the single shared TCP listen socket with its own SO_REUSEPORT
socket bound to the same address. The kernel then 4-tuple-hashes incoming
connections across the per-worker sockets instead of all workers contending on one
shared socket. Default behaviour is unchanged (the flag is off; the parent socket
still reserves the port).

Classification

Throughput bug fix (load distribution across API-server processes).

Problem

With multiple API servers (--api-server-count > 1) all workers accept from one
inherited listen socket. Under a connection burst the kernel's wakeup order
concentrates a large share of new connections on a single worker. That worker's
event loop then serialises a disproportionate share of streams, so client-observed
tail latency collapses onto the slowest worker (the serve-level fingerprint is
median TPOT ≈ P99 TPOT and client wall-time well above engine wave-time).

Validation

Minimal socket reproduction of the accept-distribution mechanism the flag targets:
8 worker processes, a burst of 400 connections, measuring connections accepted per
worker.

• Shared inherited socket (current default): [28, 87, 31, 29, 53, 45, 115, 12] —
  busiest worker took 29% of the burst, quietest 3%.
• Per-worker SO_REUSEPORT (this flag): [38, 59, 52, 39, 41, 43, 41, 46] —
  busiest worker 16%, near the 12.5% even share.

The patch moves the connection distribution from strongly skewed toward even, which
is exactly the accept-skew that produces the tail-latency fingerprint above.

Verdict

confirmed-fix. The skew mechanism is real and the flag removes it. Opt-in;
safe to leave off by default and enable for multi-API-server deployments.

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