Internships/ProjectIdeas/IOUring

io_uring AIO engine

Summary: Add io_uring support to QEMU for high-performance disk I/O on Linux

The io_uring interface supersedes the Linux AIO API for asynchronous I/O. The core functionality of asynchronous I/O APIs is I/O request submission (reads/writes/flushes) and completion processing at a later point in time. Unlike a blocking read(2)/write(2) syscall, this allows the application threads to perform other activity while one or more I/O requests are in flight.

QEMU currently supports two asynchronous I/O engines: aio=threads (a thread-pool that invokes preadv(2)/pwritev(2)/fdatasync(2)) and aio=native (Linux AIO). This project will add io_uring support, which should achieve better performance than Linux AIO. This is because io_uring offers several optimizations:

• Memory buffers can be registered (pinned) ahead of time to avoid pinning on each request
• File descriptors can be held long-term to avoid the need to acquire them on each request
• A single system call can both submit and complete I/O requests or polling mode can be used to avoid system calls altogether

This project consists of the following tasks:

• Understanding the io_uring userspace ABI
• Extending block/file-posix.c to use io_uring
• Benchmarking io_uring against Linux AIO and aio=threads
• Adding polling mode support for completions (similar to existing Linux AIO polling code in QEMU)
• Stretch goal: Adding polling mode support for submissions
• Stretch goal: Adding a fast path when QEMU block layer features are not in use

Links:

• io_uring article on LWN.net
• QEMU linux-aio.c
• io_uring example program

Details:

• Skill level: intermediate
• Language: C
• Mentor: Stefan Hajnoczi <stefanha@redhat.com> ("stefanha" on IRC)