1. Guy Martin, OSLab, GNU Fall-095
CHAPTER
Process scheduling
Guy Martin, OSLab, GNU Fall-09

2. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Contents
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
Summary
Operating Systems Concepts - Process Scheduling - Guy Martin

3. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Objectives
To introduce CPU scheduling, which is the basis for multi-programmed Operating Systems
To describe various CPU-scheduling algorithms
To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system
Operating Systems Concepts - Process Scheduling - Guy Martin

4. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Contents
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
Summary
Operating Systems Concepts - Process Scheduling - Guy Martin

5. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Basic Concepts
CPU-I/O Burst Cycle
Processes alternate between CPU cycle and I/O wait
Process execution: CPU burst - I/O burst -… -CPU burst – I/O burst…
CPU Scheduler
Or Short-term scheduler selects a process to be run in the CPU from the ready processes queue in memory. The queue is not necessary a FIFO queue.
Preemptive Scheduling
Process switches from the running state to the waiting state
Process switches from the running state to the ready state
Process switches from the waiting state to the ready state
The process terminates
Dispatcher
Gives control of the CPU to the selected process by the short-term scheduler
Dispatch latency: time to stop a process and start another running process
Operating Systems Concepts - Process Scheduling - Guy Martin

6. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Scheduling Criteria
CPU utilization
Keep the CPU as busy as possible
Throughput
Number of completed processes per time unit. Depends on processes’ length
Turnaround time
Interval from the submission time to the finish time
Waiting time
Sum of time spent by a process in the waiting queue
Response time
Time from the submission of a request until the first response is produced
 It is desirable to minimize CPU utilization and throughput and to maximize turnaround time, waiting time and response time
Operating Systems Concepts - Process Scheduling - Guy Martin

7. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Contents
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
Summary
Operating Systems Concepts - Process Scheduling - Guy Martin

8. Scheduling Algorithms (1/6)5
CHAPTER
Scheduling Algorithms (1/6)
Problem: Decide which of the processes in the ready queue is to be allocated the CPU.
First-Come, First-Served Scheduling
The process that requests the CPU first is allocated the CPU first.
Can be implemented with a FIFO queue.
Non preemptive algorithm.
Disadvantage: the average waiting time can be long. Small processes can wait for long time a big process to finish
Operating Systems Concepts - Process Scheduling - Guy Martin

9. Scheduling Algorithms (2/6)5
CHAPTER
Scheduling Algorithms (2/6)
Shortest-Job-First Scheduling
Associates with each process the length of its next CPU burst
CPU is assigned to the process with the smallest next CPU burst. FCFS is used for processes with same CPU bursts
Problem: CPU burst is evaluated as an exponential average of the measured lengths of previous CPU bursts.
Provide the minimum average waiting time for a set of processes
Can be either preemptive or non preemptive
Operating Systems Concepts - Process Scheduling - Guy Martin

10. Scheduling Algorithms (3/6)5
CHAPTER
Scheduling Algorithms (3/6)
Priority Scheduling
Priority is associated with each process
CPU is assigned to the process with the highest priority. Equal priorities are scheduled with FCFS
Can be either preemptive or non preemptive
Problem: indefinite blocking or starvation.
Rumor: when the IBM 7094 has been shut down in 1973 at MIT they found a process that had been submitted in 1967 and that had not yet been run
Solution: Aging
Gradually increase the priority of processes that wait for long time in the system
Operating Systems Concepts - Process Scheduling - Guy Martin

11. Scheduling Algorithms (4/6)5
CHAPTER
Scheduling Algorithms (4/6)
Round-Robin Scheduling
Designed especially for time sharing
Each process uses the CPU for a time quantum or time slice before be preempted
Problem: choose the right time slice (between 10 and 100ms for most OS)
How a smaller time
quantum increases
context switches
Operating Systems Concepts - Process Scheduling - Guy Martin

12. Scheduling Algorithms (5/6)
CHAPTER
Scheduling Algorithms (5/6)
Multilevel Queue Scheduling
Processes are easily classified into groups
Common division for example
Foreground(interactive)
Background(batch)
Multiple queues with different scheduling algorithms
There is a scheduler among the queues
Operating Systems Concepts - Process Scheduling - Guy Martin

13. Scheduling Algorithms (6/6)5
CHAPTER
Scheduling Algorithms (6/6)
Multilevel Feedback Queue Scheduling
Processes can move from one queue to another
The idea: separate processes according to the characteristics of their CPU bursts.
The process which uses too much CPU time will be moved to the lower priority queue.
A process that waits too long in the lower-priority queue may be moved to the higher-priority queue.
Operating Systems Concepts - Process Scheduling - Guy Martin

14. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Contents
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
Summary
Operating Systems Concepts - Process Scheduling - Guy Martin

15. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Thread Scheduling(1/2)
Contention Scope
Two types of threads: user-level and kernel-level
User-level thread are mapped to kernel-level using Lightweight process (LWP)
Process contention scope (PCS): the thread library schedules user thread to run on available LWP
System contention scope (SCS): the kernel decides which kernel thread to schedule onto a CPU.
Operating Systems Concepts - Process Scheduling - Guy Martin

16. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Thread Scheduling(2/2)
Pthread Scheduling
Operating Systems Concepts - Process Scheduling - Guy Martin

17. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Contents
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
Summary
Operating Systems Concepts - Process Scheduling - Guy Martin

18. Multiple-Processor Scheduling(1/5)
CHAPTER
Multiple-Processor Scheduling(1/5)
Approaches to Multiple-Processor Scheduling
Asymmetric multiprocessing: The master server handles all scheduling decisions, I/O processing and other system activities
Symmetric multiprocessing: each processor is self-scheduling.
All processes in a common ready queue
Two processors can choose the same process
Possible lost of processes inside queue
One private ready queue per processor
Operating Systems Concepts - Process Scheduling - Guy Martin

19. Multiple-Processor Scheduling(2/5)
CHAPTER
Multiple-Processor Scheduling(2/5)
Processor Affinity
Keep a process running on the same processor to avoid high cost of invalidating and repopulating caches. We can distinguish between soft and hard affinity
A process that is assigned affinity to a CPU should be allocated memory on the residence board of that CPU by the OS.
Operating Systems Concepts - Process Scheduling - Guy Martin

20. Multiple-Processor Scheduling(3/5)
CHAPTER
Multiple-Processor Scheduling(3/5)
Load Balancing
Keep the workload balanced among all processors
Each processor should have its own private queue
Two approaches:
Push migration: a specific task periodically checks the load on each processor and distributes the load by pushing processes from overloaded to idle or less-busy processors.
Pull migration: an idle processor pulls a waiting task from a busy processor
Operating Systems Concepts - Process Scheduling - Guy Martin

21. Multiple-Processor Scheduling(4/5)
CHAPTER
Multiple-Processor Scheduling(4/5)
Multicore Processors
Memory stall: processors accessing the memory spent significant amount of time waiting for data to become available
Implementation of multithreaded processor cores in which two or more hardware threads are assigned to each core.
2 ways to multithread a processor
Corse-grained multithreading
Fine-grained multithreading C M
Memory stall cycle
Compute cycle
time
Thread
Thread1
Thread0 C M
a. Memory stall C M C M
b. Multithreaded multicore system
Operating Systems Concepts - Process Scheduling - Guy Martin

22. Multiple-Processor Scheduling(5/5)
CHAPTER
Multiple-Processor Scheduling(5/5)
Virtualization and Scheduling
System with virtualization act like multiprocessor
One or more virtual CPU are presented to each of the running virtual machines.
The host OS creates and manages VMs that run guest operating system.
Virtualization will negatively impact any guest OS scheduler that assumes a certain amount of progress in a given amount of time.
Delays are introduced in the scheduling results
Delays can be more catastrophic for real-time guest OS
Operating Systems Concepts - Process Scheduling - Guy Martin

23. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Contents
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
Summary
Operating Systems Concepts - Process Scheduling - Guy Martin

24. Operating System Examples(1/3)5
CHAPTER
Operating System Examples(1/3)
Windows XP Scheduling
Priority-based, preemptive scheduling algorithm
The dispatcher is the portion of the kernel that handles scheduling
32 levels of priorities divided into 2 classes
The variable class: priorities from 1 to 15
The real-time class: priorities from 16 to 31
The memory management thread run at priority 0
The idle thread is executed whenever there is ready thread to execute
Operating Systems Concepts - Process Scheduling - Guy Martin

25. Operating System Examples(2/3)5
CHAPTER
Operating System Examples(2/3)
Linux Scheduling
No distinction between process and thread task
Prior to version 2.5 an old UNIX scheduler is used
Not adequate support for SMP
Not scale well as the number of tasks grows
With version 2.5, an O(1) scheduler is introduced
Support for SMP, processor affinity, load balancing, etc.
Preemptive, priority-based with 2 priority ranges
Real-time: from 0 to 99, static priorities
Nice value: from 100 to 140, dynamic priorities
Operating Systems Concepts - Process Scheduling - Guy Martin

26. Operating System Examples(3/3)5
CHAPTER
Operating System Examples(3/3)
Linux Scheduling
The kernel maintains a list of all runnable tasks in a runqueue data structure.
A runqueue contains two priority arrays
Active: tasks with time remaining in their time slices
Expired: expired tasks
Each processor maintains its own runqueue and schedules it itself
When the active array is empty, the two priority arrays are exchanged; the expired array becomes active, and vice versa
Operating Systems Concepts - Process Scheduling - Guy Martin

27. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Contents
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
Summary
Operating Systems Concepts - Process Scheduling - Guy Martin

28. Algorithm Evaluation(1/4)5
CHAPTER
Algorithm Evaluation(1/4)
Deterministic Modeling
Type of analytic evaluation:
Uses the given algorithm and the system workload to produce a formula or number that evaluates the performance of the algorithm for that workload
Takes a predetermined workload and defines the performances of each algorithm for that workload.
Simple and fast, produces numbers permitting to easily compare algorithms.
Results are applied only to the analyzed cases
Operating Systems Concepts - Process Scheduling - Guy Martin

29. Algorithm Evaluation(2/4)5
CHAPTER
Algorithm Evaluation(2/4)
Queueing Models
Just the distribution of CPU and I/O burst can be determined
Little’s formula: n = λ x W
n is the average queue length, W average waiting time in a queue, and λ average arrival rate for new processes in the queue
Particularly useful because it is valid for any scheduling algorithm and arrival distribution
Difficult to apply on complicated algorithms
Operating Systems Concepts - Process Scheduling - Guy Martin

30. Algorithm Evaluation(3/4)5
CHAPTER
Algorithm Evaluation(3/4)
Simulations
Evaluate the scheduling algorithms using simulators.
Trace tapes: monitor the real system and record the sequence of actual events.
Simulations are expensive and trace tapes require more storage space.
Operating Systems Concepts - Process Scheduling - Guy Martin

31. Algorithm Evaluation(4/4)5
CHAPTER
Algorithm Evaluation(4/4)
Implementation
All the previous method are of limited accuracy
The completely accurate way to evaluate a scheduling algorithm is to code it, put it in the operating system, and see how it works
Difficulties
The real problem here is the high cost
The environment can change
Operating Systems Concepts - Process Scheduling - Guy Martin

32. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Contents
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
Summary
Operating Systems Concepts - Process Scheduling - Guy Martin

33. Operating Systems Concepts - Process Scheduling - Guy Martin5
CHAPTER
Summary
CPU Scheduling is the task of selecting a waiting process from the ready queue and allocating the CPU to it. The CPU is allocated to the selected process by the dispatcher.
Many algorithms are proposed: FCFS, SJF, RR…
Scheduler can be preemptive or non-preemptive
Multiprocessor scheduling must consider processor affinity, load balancing, multicore processing scheduling in virtualization systems.
Some OS provide threads at the kernel level and mechanisms to schedule them.
We need methods to select among the wide variety of the existing algorithm.
Operating Systems Concepts - Process Scheduling - Guy Martin

34. © Modern Operating Systems - Andrew S. Tanenbaum
Operating Systems Concepts - Process Scheduling - Guy Martin