Registered Buffer Rings in io_uring for High-Fanout RPC Gateways

Readiness-based epoll servers suffer from an structural memory tax when handling 80,000 mostly idle keep-alive connections: if you pre-allocate a 16 KB read buffer per connection, you burn 1.28 GB of RAM doing nothing; if you wait for EPOLLIN and allocate just before calling recv(), you double your syscall rate per frame.

Kernel-Selected Buffers With PBUF_RING

Linux 5.19+ solved this cleanly via provided buffer rings (io_uring_buf_ring) paired with multishot IORING_OP_RECV. User space registers a pool of 4,096 page-aligned 8 KB buffers per worker thread. Only when packets physically arrive on a socket does the kernel pick a free buffer ID, copy payload bytes directly into it, and post a completion queue entry carrying IORING_CQE_F_BUFFER.

let flags = cqe.flags();
if flags & IORING_CQE_F_BUFFER != 0 {
    let bid = (flags >> IORING_CQE_BUFFER_SHIFT) as u16;
    let slice = buf_ring.borrow_slice(bid, cqe.result() as usize);
    router.dispatch_frame(conn_id, slice);
    // Recycle buffer ID back to the ring tail with Release ordering
    buf_ring.replenish(bid);
}

Guarding Against Ring Exhaustion Under Burst Fanout

If downstream backends slow down and hold references to frame slices across async await points, the provided ring runs dry (-ENOBUFS) and drops multishot registrations. Enforce a strict rule: parse header routing metadata synchronously, forward via scatter-gather SPLICE or registered TX slabs, and return the RX buffer ID before yielding.

End-to-End Verification

Across 100GbE Mellanox ConnectX-6 NICs, multishot receive rings reduced perf kernel syscall time from 34% to 9.8% at 1.1M RPS. You can inspect queue dwell times for this exact architecture using the per-CPU probes in Sub-1% Overhead Tail-Latency Histograms With Per-CPU eBPF Maps and see how batching simplifies quorum heartbeats in Deterministic Raft Lease Quorums Under Asymmetric Cross-AZ Jitter.