Process Scheduling Chapter 4

A schematic of scheduling

Scheduling-related Concepts and Terms

Kinds of Scheduling Non-preemptive scheduling: A process runs to completion when scheduled Preemptive scheduling: A process may be preempted by the kernel, and another process may be scheduled. Hence a set of processes are processed in an overlapped manner.

Processes for scheduling

Non-preemptive scheduling Performance criteria: Throughput Turnaround times of processes Weighted turnaround Change the order in which processes are serviced Non-preemptive scheduling policies: - First come first served (FCFS) - Shortest request first Q: Which policy will provide better throughput under what conditions?

Scheduling using FCFS and SRN policies

Performance of FCFS and SRN scheduling

Preemptive scheduling policies Performance criteria: Throughput Response time to a subrequest Scheduling policies Round-robin with time slicing (RR) Least completed next (LCN) Shortest time to go (STG): shortest remaining time Highest response ratio (HRN): defined later

Round-robin with time slicing (RR)

Least completed next (LCN)

Shortest time to go (STG)

Scheduling using preemptive scheduling policies

Operation of preemptive scheduling policies

Performance of preemptive scheduling policies

Variation of average response time with time slice

Highest response ratio next (HRN) Two ways of defining response ratio of a process Response ratio = Time in the system / service time recd Response ratio = (Time in the system + execution time)/ service time received We use the first definition in the next slide HRN eliminates the drawbacks of LCN and STG policies. Q: How?

Performance of HRN scheduling policy

Long, medium and short-term scheduling

Event handling and scheduling

Long, medium and short-term scheduling in a time sharing system

Mechanism and policy modules of process scheduler

Simple priority-based scheduling

Priority-based scheduling

Priority-based scheduling What should the kernel do if there aren’t any ready processes? Schedule a dummy process that does nothing Put the processor to sleep (conserves power!)

Round-robin scheduling

Ready lists in a time sharing system

Practical scheduling policies Provide a good balance of response time and overhead Vary the time slice! Vary the priority Provide fair service to processes (Q: How defined?) In round-robin, all process receive approx similar service. What if one application has 5 processes, while another application has only 1 process?

Multilevel scheduling

Ready lists in a multilevel scheduler

Multi-level adaptive scheduling Vary the priority of a process depending on its recent behaviour If the process uses up its time slice, it must be (more) CPU bound than assumed, so reduce its priority If a process is starved of CPU attention for some time, increase its priority

Example of multi-level adaptive scheduling

Fair share scheduling

Scheduling in Unix Processes are assigned priorities that are integer values A larger value implies a lower priority Priority of a process is varied based on its CPU usage So as not to have an LCN-like effect (starvation!), the effect of CPU time used is decays with time, hence only recent CPU usage affects priority of a process Priority of a process = Base priority + nice value + f(CPU time used by the process

Operation of a Unix-like scheduling policy when process perform I/O

Fair-share scheduling using a Unix-like policy Groups of processes are defined for ensuring fair share of CPU time Priority of a process depends on the CPU time used by all processes its group Priority of a process = Base priority + nice value + f(CPU time used by the process) + f(CPU time used by all processes in the same group)

Operation of a fair share scheduling

Real time scheduling Perform scheduling such that each process can meet its deadline Procedure: Consider precedences between processes, i.e. which process should complete before which other process Find the deadline of each application Use a scheduling policy which ensures that deadlines are met (in a soft real time system, it is adequate to try to meet the deadlines, and succeed most of the time)

Real time scheduling

Process precedence graph (PPG) for a simple real time system

Deadlines for processes of previous slide Total execution time = 25 seconds If the deadline for the application as a whole is 35 seconds process P6 has a deadline of 35 seconds processes P4 and P5 have deadlines of 30 seconds Q: What are the deadlines of other processes?

Feasible schedule An application has a feasible schedule if there exists at least one sequence in which its processes can be scheduled so that deadlines of all processes are met Justify the following statement: “If a feasible schedule exists for an application, then earliest deadline first (EDF) scheduling can meet all deadlines.”

Performance of Earliest Deadline First Scheduling

Rate Monotonic Scheduling (RMS) Priority of a process is inversely proportional to its periodicity Priority-based scheduling is now used Q: Does RMS guarantee that deadlines will be met if an application has a feasible schedule?

Rate Monotonic Scheduling (RMS) Consider three processes: P1 P2 P3 Time period 10 15 30 Execution time 3 5 9 What are their priorities? Does a feasible schedule exist? Consider summation of (execution time/time period) Feasible schedule exists if summation is < 1. Will RMS find a feasible schedule?

Summary of performance analysis