CFQ

CFQ, short for Completely Fair Queuing, is an input/output (I/O) scheduler for block devices in the Linux kernel. Its primary goal is to allocate disk I/O bandwidth fairly among all processes and threads that issue requests, preventing any single task from starving others while preserving reasonable latency for interactive applications.

Operation: CFQ sits in the block layer and maintains per-process and per-device request queues. Each task can be assigned a weight, and the scheduler distributes service time in a time-slice fashion to achieve proportional fairness. CFQ tracks in-flight requests, prioritizes older or latency-sensitive traffic, and attempts to reduce head-of-line blocking by balancing throughput with responsiveness. It also supports request merging and limits on per-queue depth to manage overall I/O pressure.

History and status: CFQ was a long-standing default I/O scheduler in many Linux distributions for desktop and server workloads. In recent years, other schedulers such as BFQ and Deadline have gained attention for specific scenarios, and some distributions have moved away from CFQ as the default option. Nevertheless, CFQ remains available in modern kernels as an alternative or legacy choice, depending on the distribution and kernel configuration.

Performance considerations: For mixed workloads that include interactive tasks and background processes, CFQ often improved fairness and responsiveness. Critics have noted that the per-task accounting and scheduling overhead can lead to higher latency in certain workloads, particularly under heavy parallel I/O. On solid-state drives, alternative schedulers may offer lower latency or different performance characteristics, leading some users to prefer BFQ or Deadline over CFQ.

See also: Linux I/O scheduling, BFQ, Deadline, NOOP.