COMP 3000 Essay 1 2010 Question 5

Question

Compare and contrast the evolution of the default BSD/FreeBSD and Linux schedulers.

Answer

One of the most difficult problems that operating systems must handle is process management. In order to ensure that a system will run efficiently, processes must be maintained, prioritized, categorized and communicated with all without experiencing critical errors such as race conditions or process starvation. A critical component in the management of such issues is the operating system’s scheduler. The goal of a scheduler is to ensure that all processes of a computer system get access to the system resources they require as efficiently as possible while maintaining fairness for each process, limiting CPU wait times, and maximizing the throughput of the system. As computer hardware has increased in complexity, for example multiple core CPUs, schedulers of operating systems have similarly evolved to handle these additional challenges. In this article we will compare and contrast the evolution of two such schedulers; the default BSD/FreeBSD and Linux schedulers.

Linux Schedulers

(Note to the other group members: Feel free to modify or remove anything I post here. I'm just trying to piece together what you've all posted in the discussion section and turn it into a single paragraph. You know. Just to see how it looks.)

Older Versions

(This work belongs to Sschnei1)

In Linux 1.2 a scheduler operated with a round robin policy using a circular queue, allowing the scheduler to be efficient in adding and removing processes. When Linux 2.2 was introduced, the scheduler was changed. It now used the idea of scheduling classes, thus allowing it to schedule real-time tasks, non real-time tasks, and non-preemptible tasks. It was the first scheduler which supported SMP.

With the introduction of Linux 2.4, the scheduler was changed again. The scheduler started to be more complex than its predecessors, but it also has more features. The running time was O(n) because it iterated over each task during a scheduling event. The scheduler divided tasks into epochs, allowing each tasks to execute up to its time slice. If a task did not use up all of its time slice, the remaining time was added to the next time slice to allow the task to execute longer in its next epoch. The scheduler simply iterated over all tasks, which made it inefficient, low in scalability and did not have a useful support for real-time systems. On top of that, it did not have features to exploit new hardware architectures, such as multi-core processors.