The scheduler is a crucial component of the Linux kernel responsible for managing the allocation of CPU resources among processes and threads. Linux offers various schedulers, each with its own scheduling policies and algorithms tailored to specific use cases and performance requirements. In this article, we'll delve into the Linux kernel supported schedulers, explaining their features, differences, and providing examples to illustrate their usage.
1. Understanding Schedulers:
A scheduler in the Linux kernel is responsible for determining which process or thread should execute on a CPU core at any given time. It aims to optimize system performance by efficiently allocating CPU resources, balancing workload, and maximizing throughput and responsiveness.
2. Linux Kernel Supported Schedulers:
Linux kernel supports multiple schedulers, including:
- Completely Fair Scheduler (CFS)
- Real-Time (RT) Scheduler
- Deadline Scheduler
- Earliest Deadline First (EDF) Scheduler
- Multi-Level Feedback Queue (MLFQ) Scheduler
- Weighted Round-Robin (WRR) Scheduler
3. Completely Fair Scheduler (CFS):
The Completely Fair Scheduler (CFS) is the default Linux scheduler introduced in kernel version 2.6.23. It aims to provide fair CPU resource allocation among processes by implementing a red-black tree-based scheduling algorithm. CFS assigns dynamic time slices (called "vruntime") to each process based on its priority and runtime history.
Example:
# View the current scheduler
cat /sys/kernel/debug/sched_features
# Set the CFS scheduler
echo "schedutil" > /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
4. Real-Time (RT) Scheduler:
The Real-Time (RT) scheduler is designed for deterministic and time-sensitive applications that require predictable response times. It guarantees that real-time tasks (marked with SCHED_FIFO or SCHED_RR policy) have higher priority than non-real-time tasks. The RT scheduler uses priority-based scheduling with fixed priorities.
Example:
#include <sched.h>
int main() {
struct sched_param param;
param.sched_priority = 99; // Real-time priority
sched_setscheduler(0, SCHED_FIFO, ¶m);
return 0;
}
5. Deadline Scheduler:
The Deadline scheduler is suitable for real-time systems with hard deadlines. It guarantees that tasks complete their execution within specified deadlines, optimizing both latency and throughput. The Deadline scheduler assigns tasks to CPUs based on their deadline and priority.
Example:
# Set the Deadline scheduler
echo "deadline" > /sys/block/sda/queue/scheduler
6. Earliest Deadline First (EDF) Scheduler:
The Earliest Deadline First (EDF) scheduler is an alternative to the Deadline scheduler that offers more flexible scheduling policies. It schedules tasks based on their earliest deadline, ensuring timely execution of real-time tasks while maximizing CPU utilization.
Example:
# Set the EDF scheduler
echo "edf" > /sys/block/sda/queue/scheduler
7. Multi-Level Feedback Queue (MLFQ) Scheduler:
The Multi-Level Feedback Queue (MLFQ) scheduler is a dynamic scheduling algorithm that assigns priorities to processes based on their behavior and resource usage. It features multiple priority queues with different time quantum lengths, allowing it to adapt to changing workload conditions.
Example:
# Set the MLFQ scheduler
echo "mq-deadline" > /sys/block/sda/queue/scheduler
8. Weighted Round-Robin (WRR) Scheduler:
The Weighted Round-Robin (WRR) scheduler is a variant of the Round-Robin scheduler that assigns weights to processes based on their priority. It allocates CPU time slices to processes in proportion to their weights, allowing for more efficient CPU resource utilization.
Example:
# Set the WRR scheduler
echo "wrr" > /sys/block/sda/queue/scheduler
Linux kernel provides a range of schedulers catering to diverse workload requirements and real-time constraints. By understanding the features and characteristics of each scheduler and utilizing appropriate scheduler settings and policies, developers can optimize system performance, responsiveness, and determinism for various use cases and workloads.
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