Mastering Linux I/O Schedulers: When to Use CFQ, Deadline, or Noop

Linux I/O Scheduler Overview

The I/O scheduler resides in the Linux kernel’s I/O scheduling layer, which is one of seven layers in the overall I/O stack:

VFS layer – virtual file system abstraction.

File system layer – specific file‑system implementations.

Page cache layer – caching of pages.

Block layer – generic block‑device abstraction.

I/O scheduler layer – decides the order of block‑device requests.

Block device driver layer – hardware‑specific driver interface.

Block device layer – the physical device itself.

The scheduler’s goal is to improve overall block‑device performance, especially for mechanical disks where seek time dominates.

1. CFQ – Completely Fair Queueing

CFQ is the default scheduler for most general‑purpose workloads. It creates a separate queue for each process and allocates I/O time slices to ensure fairness.

CFQ also supports priority classes (RT, BE, IDLE) and per‑process I/O priority settings.

Key data structures:

cfq_data – global scheduler state.

cfq_group – represents a cgroup, stored in a red‑black tree keyed by vdisktime (total I/O time used).

service_tree – seven trees per group handling RT, BE, and IDLE requests for read/write and async operations.

CFQ parameters (found under /sys/class/block/<em>dev</em>/queue/iosched/) include: back_seek_max and back_seek_penalty – control backward seeks. fifo_expire_async / fifo_expire_sync – timeout for async/sync requests. slice_idle and group_idle – idle wait times to improve sequential I/O. low_latency and target_latency – enable low‑latency mode. quantum, slice_sync, slice_async, slice_async_rq – control request batch sizes and time slices.

CFQ also integrates with cgroup blkio control to enforce I/O quotas per group.

CFQ Design Highlights

Each process’s queue is placed in a red‑black tree ordered by its accumulated service time; the scheduler always selects the queue with the smallest vdisktime, ensuring fairness across processes and cgroups.

2. Deadline Scheduler

Deadline is simpler and focuses on maximizing throughput while preventing request starvation.

It maintains four queues: read‑sort, write‑sort (ordered by sector), and read‑fifo, write‑fifo (ordered by request age).

When dequeuing, the scheduler prefers reads, checks for any request that has exceeded its deadline ( read_expire or write_expire), and processes it to avoid starvation; otherwise it serves the next request in sector order.

Adjustable parameters include: read_expire / write_expire – request timeout in milliseconds. fifo_batch – number of requests processed per batch. writes_starved – threshold for when writes become eligible. front_merges – enable/disable forward merges.

3. Noop Scheduler

Noop implements a simple FIFO queue with minimal merging; it is ideal for SSDs where complex scheduling provides no benefit.

Choosing the Right Scheduler

• CFQ : General‑purpose, desktop, and server workloads where fairness among processes is important.

• Deadline : Workloads with heavy I/O pressure on a few processes (e.g., databases) where throughput and bounded latency are critical.

• Noop : SSD or other devices with negligible seek time; the simplest scheduler yields the best performance.

Images illustrating the I/O stack and CFQ data structures: