We Replaced MMAP with Io_uring in Our Rust Query Engine. It Got Slower

The engineering team at Conviva recently documented an unexpected performance regression while optimizing their Rust-based query engine. Seeking to improve I/O efficiency, the team replaced the traditional memory-mapped file (mmap) approach with Linux's modern io_uring interface. Contrary to expectations, the transition resulted in slower query execution times. The technical deep dive explores the nuances of asynchronous I/O, the overhead associated with system calls, and the specific characteristics of their query engine's workload that made mmap more effective. The article serves as a cautionary tale for developers assuming that newer kernel interfaces automatically guarantee performance gains. It provides a detailed analysis of why the overhead of managing io_uring submission and completion queues outweighed the benefits in their specific database architecture, highlighting the importance of benchmarking against real-world application patterns rather than relying on theoretical performance improvements.