Process Scheduling & Policies

AI Summary

An AI-generated summary of this video lecture.

This lecture introduces Linux process scheduling, beginning with preemptive scheduling and time sharing before comparing the three main policies: SCHED_NORMAL (CFS/EEVDF), SCHED_FIFO, and SCHED_RR. It concludes by transitioning to process creation using the fork() system call.

Chapters

1. 0:00 – 2:00 00:00-02:00

   The lecture opens with a slide titled 'Preemptive Scheduling, Time Sharing & Priority.' The instructor explains that Linux uses preemptive scheduling, meaning a running process can be temporarily interrupted by the scheduler. A numbered diagram illustrates this: step 1 shows 'P1 Running,' followed by red text stating 'Time slice expires / higher priority process arrives.' Step 2 shows the 'Scheduler' is triggered to make a new decision, and step 3 shows 'P2 Running.' The instructor uses a red pen to circle key terms like 'P1 Running' and underline the trigger conditions, emphasizing that this mechanism improves system responsiveness and multitasking.

2. 2:00 – 5:00 02:00-05:00

   The lesson transitions to a new slide titled 'Time Sharing.' The instructor explains that CPU time is divided among multiple processes, with each process getting a small time interval called a 'time slice.' A Gantt chart visualizes this by showing the CPU switching between Process 1 (P1), Process 2 (P2), and Process 3 (P3) over increasing time intervals from T1 to T5. The instructor underlines key phrases such as 'small time interval' and 'After its time slice, the scheduler may switch the CPU to another process.' A color-coded legend distinguishes between the different processes, and a 'Key Point' callout box summarizes how time sharing allows multiple tasks to appear to run simultaneously.

3. 5:00 – 10:00 05:00-10:00

   The lecture moves to priority-based scheduling, defining how the scheduler considers the priority of processes while making decisions. The instructor highlights that in Linux, a lower 'nice value' is associated with higher scheduling priority for normal processes. The text on screen states: 'Higher scheduling priority -> Greater chance/preference for CPU time.' The instructor underlines important phrases like 'priority of processes' and 'nice value.' This section bridges the gap between basic time sharing and the specific Linux scheduling policies, establishing that priority is a core factor in CPU allocation.

4. 10:00 – 15:00 10:00-15:00

   The instructor presents a comprehensive overview of Linux scheduling policies, comparing SCHED_NORMAL (CFS/EEVDF), SCHED_FIFO, and SCHED_RR. Step-by-step flow diagrams illustrate each policy. For SCHED_NORMAL, the diagram shows a 'Fair Scheduler (CFS / EEVDF)' that provides fair CPU sharing. For SCHED_FIFO, the flow shows 'Pick Highest Priority (First In, First Out)' and running until a higher priority process arrives. For SCHED_RR, the diagram shows processes 'Run for Time Quantum (tq)' and asks 'Time Quantum Expired?' to determine if the process is preempted. The instructor uses red annotations to highlight key differences in CPU time sharing and priority handling across the three columns.

5. 15:00 – 19:42 15:00-19:42

   The lecture continues detailing the flows of SCHED_NORMAL, SCHED_FIFO, and SCHED_RR, emphasizing that FIFO uses strict priority-based scheduling without preemption within the same priority, while RR operates on a round-robin basis. The final section transitions to a new topic: 'Process Creation using fork().' A slide explains that fork() is a system call used to create a new process in Linux. A summary table breaks down the return values of fork(), stating that '> 0: Returned to Parent -> value is Child's PID.' This marks the end of the scheduling policies discussion and the beginning of process creation concepts.

The lecture systematically builds understanding of Linux scheduling from fundamental concepts to specific policies. It starts by defining preemptive scheduling and time sharing, using diagrams and Gantt charts to visualize how the CPU is shared among processes. It then introduces priority as a decision factor, specifically mentioning the Linux 'nice value.' The core of the lecture compares the three main scheduling policies (SCHED_NORMAL, SCHED_FIFO, SCHED_RR) using flowcharts to show their distinct logic for CPU allocation. Finally, it transitions to process creation via fork(), linking scheduling decisions back to how processes are initially created. The progression moves from 'what' (preemption/time sharing) to 'how' (policies/flows) to 'where it starts' (fork).