Transmitting gigabytes of network payloads across 100GbE NIC interfaces frequently saturates host CPU memory buses with data copying between userspace memory and kernel `sk_buff` rings. Linux 6.x io_uring zero-copy transmit (`IORING_OP_SEND_ZC`) combined with registered user buffers eliminates kernel copies and syscall context switches, delivering line-rate saturation at minimal CPU overhead.
The io_uring Zero-Copy Transmit Lifecycle
How asynchronous network DMA bypasses intermediate socket buffers:
When submitting an `IORING_OP_SEND_ZC` request, io_uring issues two separate Completion Queue Entries (CQEs): first, an immediate operational CQE confirming packet queueing into the NIC driver, and second, a notification CQE (`IORING_CQE_F_NOTIF`) indicating the network card has completed physical DMA wire transmission. Userspace must retain buffer immutability until the notification CQE is reaped.
Network Socket Transmit Architectures Compared
| Socket Transmit Mode | Memory Copy Mechanism | CPU Utilization @ 100GbE | Syscall Overhead |
|---|---|---|---|
| Standard `send()` / `write()` | Synchronous Userspace → Kernel `sk_buff` | 88.5% (High memory bus saturation) | 1 Syscall per Send Call |
| POSIX `sendmsg(MSG_ZEROCOPY)` | Dynamic Page Pinning (`get_user_pages_fast`) | 46.2% (Page table traversal lock contention) | Syscall + Error Queue Polling |
| io_uring `IORING_OP_SEND_ZC` + Registered Buffers | Zero Copy (Pre-pinned DMA Descriptors) | 11.4% (Direct NIC DMA Line-Rate) | 0 Syscalls with `IORING_SETUP_SQPOLL` |
Configuring io_uring Zero-Copy Send with SQPOLL
How kernel submission queue polling achieves zero-syscall packet egress:
- Pre-Register Send Ring Buffers: Invoke `io_uring_register_buffers()` during daemon startup to pin page frames and configure DMA scatter-gather lists.
- Submit Zero-Copy Descriptors: Prepare `io_uring_prep_send_zc()` descriptors with `IORING_RECVSEND_FIXED_BUF` pointing to pre-registered buffer indices.
- Kernel Polling Loop: Enable `IORING_SETUP_SQPOLL` to allow dedicated kernel worker threads to consume submission queue entries directly from shared memory.
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