How does Linux io_uring Zero-Copy Send (IORING_OP_SEND_ZC) eliminate memory copies?

In standard socket transmissions (send/sendto/write), the Linux kernel copies payload bytes from userspace memory into kernel-allocated socket buffers (sk_buff / skb) before initiating network interface DMA. Under high packet rates (100GbE+), this memory copy saturates CPU memory bandwidth and evicts CPU caches. With IORING_OP_SEND_ZC, the kernel pins userspace memory pages (via pin_user_pages) and instructs the network interface card (NIC) to DMA bytes directly from userspace virtual addresses, eliminating memory copying entirely.

What is the two-phase Completion Queue Event (CQE) lifecycle in io_uring zero-copy?

Because the NIC DMAs directly from userspace memory, the application must not mutate or free the buffer while transmission is underway. io_uring handles this via two distinct CQEs: First, the Send CQE indicates the request was queued to the socket and the buffer length accepted. Second, once the NIC has physically completed the DMA transfer and the skb is freed by the network driver, io_uring posts a Notification CQE (tagged with IORING_CQE_F_NOTIF). Only after receiving the notification CQE can the application safely reuse the memory buffer.

Why should applications use IORING_REGISTER_BUFFERS (Fixed Buffers) with zero-copy?

In un-registered zero-copy, the kernel must execute pin_user_pages() and page table walks for every single send request, adding measurable syscall overhead. By pre-registering a pool of buffers via io_uring_register_buffers(), the kernel locks and pins the virtual-to-physical address mappings once during initialization. Subsequent IORING_OP_SEND_ZC operations reference buffer indices directly, bypassing all runtime page translation and locking overhead.

At what payload size does zero-copy outperform standard kernel copy?

Zero-copy incurs overhead: pinning pages and handling two-stage notification CQEs costs CPU cycles. For tiny payloads (< 1-2 KB), copying bytes into pre-allocated kernel sk_buff structures is faster because CPU L1/L2 caches copy small chunks almost instantly. Zero-copy becomes decisively superior once payload sizes exceed 3 KB to 4 KB (such as video streaming, database backups, or large RPC payloads), where memory bus saturation dominates.

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Linux Kernel 6.x io_uring Zero-Copy

io_uring Fixed Buffers & Zero-Copy Network Studio

Simulate high-throughput Linux network transport via IORING_OP_SEND_ZC. Model page pinning, fixed buffer registration, two-phase Notification CQEs, and compare zero-copy vs copy memory bandwidth.

1. Buffer & Transmission Parameters

Payload Block Size: Memory Buffer Mode: Notification Flushing Policy:

2. Transmission Dispatch & Hardware Rings

SQ Ring Entries: 0 in flight

CQ Ring Status: 0 posted (0 notifs)

NIC DMA State: Idle (Descriptor ready)

Zero-Copy Hardware Transfer Pipeline

Kernel Ring & Hardware DMA Telemetry

Memory Bandwidth & Cache Saturation Metrics

Memory Copy Cost

0 B (Zero-Copy)

vs 16 KB copied in send()

Throughput Speedup

+142%

Over standard send() at 100GbE

L1/L2 Cache Thrashing

Zero Eviction

Payload bypasses CPU caches

CQE Completion Events

2-Phase

Send CQE + Notification CQE

io_uring Fixed Buffers & Zero-Copy Network Studio

1. Buffer & Transmission Parameters

2. Transmission Dispatch & Hardware Rings

Zero-Copy Hardware Transfer Pipeline

Kernel Ring & Hardware DMA Telemetry

Memory Bandwidth & Cache Saturation Metrics

Production C Code (liburing Zero-Copy Pipeline)

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