bfq

BFQ (Budget Fair Queuing) is a block I/O scheduler for the Linux kernel designed to provide fair and predictable disk I/O performance. It manages disk read and write requests by allocating a budget of service time to each client (process or cgroup) and scheduling requests in a way that respects these budgets while keeping device utilization high.

Operation: The scheduler maintains per-client queues and a credit budget. At each scheduling decision, it selects requests from active clients in a round-robin–like manner, giving priority to those with remaining budget. When a client’s budget is exhausted, its requests are temporarily deprioritized until its budget is replenished, preventing any single process from starving others. BFQ can differentiate I/O requests by priority classes and integrates with Linux control groups to allow policy-based distribution of bandwidth across users or services.

Features: BFQ targets low latency for interactive workloads while preserving high throughput for sequential transfers. It adapts to the device speed and current workload, supports both hard drives and solid-state drives, and can be configured or tuned via kernel parameters. By aiming for predictable latency and fair access, it provides a balance between responsiveness and throughput.

History: BFQ was developed as an alternative to other Linux I/O schedulers by Paolo Valente and collaborators, with the goal of deterministic latency and fair access to storage resources. It has been offered as an option in the Linux kernel and has undergone multiple iterations to refine fairness and latency control. It competes with CFQ and Deadline-style schedulers, offering different trade-offs for desktop and server use.

See also: Linux I/O scheduler, CFQ, MQ-Deadline, Deadline I/O scheduler.