What is IORING_OP_SEND_ZC and how does it eliminate memory copies in Linux socket transmission?

In standard Linux socket transmission (send() / IORING_OP_SEND), userspace data is copied by the CPU into kernel socket buffers (sk_buff) before the network card can DMA it onto the wire. For high-bandwidth servers (10GbE to 100GbE), this memory copy saturates CPU memory bus bandwidth and pollutes CPU caches. Introduced in Linux 6.0, IORING_OP_SEND_ZC pins user pages directly into the kernel page pool, instructing the NIC hardware DMA engine to pull data straight out of userspace memory without an intermediate kernel buffer copy.

Why does IORING_OP_SEND_ZC emit two distinct Completion Queue Entries (CQEs)?

Zero-copy networking requires asynchronous two-stage lifecycle synchronization: 1) The first CQE indicates that the data was accepted by the networking stack and queued for physical transmission. This CQE has the IORING_CQE_F_MORE flag set, signaling that further notifications are pending. 2) The second CQE (the notification CQE, marked with IORING_CQE_F_NOTIF) is posted only after the NIC hardware confirms over PCIe DMA that the physical transmission is finished. Only after receiving the second CQE is it safe for userspace to overwrite, reuse, or free the memory buffer.

How does splice() enable zero-copy proxying between two network sockets?

The splice() system call (and its io_uring equivalent IORING_OP_SPLICE) moves data between two file descriptors via an in-kernel circular pipe buffer without copying data to userspace memory. For reverse proxies (like HAProxy, NGINX, or Envoy), data received on an incoming client socket is spliced into a kernel pipe and spliced directly out to an upstream backend socket. The payload bytes stay in kernel memory pages throughout the entire transit.

What performance benefits do pre-registered buffers (IORING_REGISTER_BUFFERS) provide to SEND_ZC?

When sending arbitrary user buffers via SEND_ZC, the kernel must pin and unpin physical pages (get_user_pages / put_page) on every single transmission, which adds CPU overhead. By pre-registering a memory pool using io_uring_register_buffers(), pages are pinned once at application startup. Subsequent IORING_OP_SEND_ZC operations use pre-pinned virtual-to-physical translations with zero per-request page-pinning overhead.

Linux io_uring Zero-Copy Network Transmit Studio | IORING_OP_SEND_ZC & splice()

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Linux Kernel 6.0+ io_uring IORING_OP_SEND_ZC Zero-Copy Splicing

Linux io_uring Zero-Copy Network Transmit Studio

Simulate Linux 6.0+ zero-copy socket transmission and kernel pipe splicing with io_uring. Inspect two-stage CQE lifecycle tracking, page pinning overhead, and export production liburing C code.

1. Network Transmit & Buffer Parameters

Transmission Mechanism: Network Interface Bandwidth:

Payload Chunk Size:

SQPOLL Thread:

2. Two-Stage CQE Notification Stepper

Zero-Copy Socket Telemetry & CPU Efficiency

Memory Bandwidth Saved

23.4 GB/s

Zero memcpy CPU bypass

System Call Overhead

0 Syscalls/sec

SQPOLL kernel polling

Buffer Safety State

Safe to Reuse

Second CQE (NOTIF) received

CPU Utilization (Core %)

8.4%

-68% vs standard send()

io_uring SEND_ZC Two-Stage Completion Architecture

Userspace • Submission Queue (SQE) • Kernel / NIC DMA • Completion Queue (CQE)

3. Production liburing C Implementation

Linux Network Transmission Mechanisms Comparison

Transmission API	io_uring SEND_ZC	io_uring SPLICE	Standard send()
CPU Memory Copy	Zero (Direct DMA)	Zero (In-kernel pipe pages)	Full CPU memcpy to sk_buff
Completion Sync	Two-Stage CQE (F_MORE + NOTIF)	Single linked CQE	Immediate return upon buffer copy
Optimal Workload	High-throughput web servers, databases	Reverse proxies (Envoy / NGINX / HAProxy)	Small messages (< 4 KB)
Linux Kernel Version	Linux 6.0+	Linux 5.7+	All Linux kernels

Linux io_uring Zero-Copy Network Transmit Studio

1. Network Transmit & Buffer Parameters

2. Two-Stage CQE Notification Stepper

Zero-Copy Socket Telemetry & CPU Efficiency

io_uring SEND_ZC Two-Stage Completion Architecture

3. Production liburing C Implementation

Linux Network Transmission Mechanisms Comparison

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