io_uring is a Linux asynchronous I/O interface built around two shared ring buffers: the application places requests in the submission queue, and the kernel places results in the completion queue. The queues carry information in opposite directions; understanding that distinction explains how an operation travels from submission to completion.
What the two queues do
An application sets up the rings and uses shared memory to communicate with the kernel. The Linux Programmer’s Manual describes the programming model in io_uring(7); ring setup and layout are described in io_uring_setup(2).
| Queue | Direction | What it contains | Who consumes it |
|---|---|---|---|
| Submission queue (SQ) | Application to kernel | Submission queue entries (SQEs) describing operations, such as reads, writes, or socket accepts | The kernel consumes entries from the queue |
| Completion queue (CQ) | Kernel to application | Completion queue events (CQEs) reporting finished operations | The application reads events and checks their results |
The application adds SQEs at the SQ tail; the kernel consumes them from the head. When an operation finishes, the kernel posts a CQE at the CQ tail, and the application reads it from the head. A CQE’s res field carries the operation’s result. Its user_data field can carry an identifier the application supplied with the request, helping it match a completion to the right operation.
How a request moves through io_uring
- Create an SQE. Describe the operation the application wants performed.
- Publish it to the SQ. Add the entry to the submission queue using the required ring-index and memory-ordering rules.
- Notify or wait with
io_uring_enter(2). The call can notify the kernel about queued work and can also wait for a requested number of completions. - Read the CQE. When the operation completes, inspect its result and use its
user_datavalue, if set, to identify the corresponding request.
The shared queues can support batching multiple requests, but that does not mean every operation in every configuration avoids system calls. The exact submission and waiting behavior depends on how the interface is used.
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What queue order does—and does not—guarantee
Submission order is not completion order. The kernel attempts requests in submission order, but that does not guarantee their execution or completion order. If several operations are in flight, the application should use a request identifier such as user_data to associate each CQE with its request rather than relying on where the completion appears in the queue.
When one operation depends on another, use the documented ordering mechanisms and respect the constraints for those specific operations. A shared queue alone does not create a dependency between requests.
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Keep I/O buffers valid until completion
Buffers used by IORING_OP_READ or IORING_OP_WRITE must remain valid until the corresponding operation completes. Do not assume that because an SQE has been submitted, the kernel has finished using its I/O buffer. Other pointer-based metadata may have different lifetime rules, so check the documentation for the operation rather than generalizing from the buffer rule.
Shared rings still require synchronization
Because user space and the kernel access shared ring memory, ring indices must be published and consumed with the required ordering. The io_uring(7) manual discusses these rules and points to Linux memory-barrier and C11/kernel memory-model documentation. Code that manipulates the rings directly must follow those requirements; mapping shared memory does not remove the need for synchronization.
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Setup and kernel-version differences
Applications typically call io_uring_setup(2) and map the ring regions into user space with mmap(2). Setup returns parameters, offsets, entry counts, and feature flags that describe the available rings and supported features. Use those returned values rather than assuming a fixed mapping layout.
| Setup feature | Kernel availability stated by the manual | Practical meaning |
|---|---|---|
IORING_FEAT_SINGLE_MMAP |
Linux 5.4 | Allows the SQ and CQ rings to be mapped together; SQEs remain separately allocated. |
IORING_SETUP_NO_MMAP |
Linux 6.5 | A version-specific setup option; check whether the running kernel supports it. |
IORING_SETUP_NO_SQARRAY |
Linux 6.6 | A version-specific setup option; check whether the running kernel supports it. |
These are availability facts from the io_uring_setup(2) manual, not assumptions that every deployed kernel supports the same options. Check setup results and handle unsupported features or setup errors.
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What this model does not tell you
The two-queue model explains how requests and results move between an application and the kernel. It does not, by itself, establish that io_uring is faster than another I/O approach. Any performance comparison needs evidence for the workload and configuration being compared.
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