Скачать или смотреть Operating System #19 Priority Scheduling Algorithms, Multilevel Queues

Operating System #19 Priority Scheduling Algorithms, Multilevel Queues. Complete Operating Systems Lecture/ Tutorials from IIT @ https://goo.gl/GMr3if
MATLAB Tutorials @ https://goo.gl/EiPgCF

00:40 Relook at Round Robin Scheduling
03:27 Priority based Scheduling
• Priority based Scheduling
– Each process is assigned a priority
• A priority is a number in a range (for instance between 0 and 255)
• A small number would mean high priority while a large number would mean low priority
– Scheduling policy : pick the process in the ready queue having the highest priority
– Advantage : mechanism to provide relative importance to processes
– Disadvantage : could lead to starvation of low priority processes

05:40 Dealing with Starvation:
• Scheduler adjusts priority of processes to ensure that they all eventually execute
• Several techniques possible. For example,
– Every process is given a base priority
– After every time slot increment the priority of all other process
• This ensures that even a low priority process will eventually execute
– After a process executes, its priority is reset

10:20 Types of Priorities:
• Static priority : typically set at start of execution
– If not set by user, there is a default value (base priority)
• Dynamic priority : scheduler can change the process priority during execution in order to achieve scheduling goals
– eg1. decrease priority of a process to give another process a chance to execute
– eg.2. increase priority for I/O bound processes

11:19 Priority based Scheduling with large number of processes:
• Several processes get assigned the same base priority
– Scheduling begins to behave more like round robin

12:22 Multilevel Queues:
• Processes assigned to a priority classes
• Each class has its own ready queue
• Scheduler picks the highest priority queue (class) which has at least one ready process
• Selection of a process within the class could have its own policy
– Typically round robin (but can be changed)
– High priority classes can implement first come first serve in order to ensure quick response time for critical tasks.

• Scheduler can adjust time slice based on the queue class picked
– I/O bound process can be assigned to higher priority classes with longer time slice
– CPU bound processes can be assigned to lower priority classes with shorter time slices
• Disadvantage :
– Class of a process must be assigned apriori (not the most efficient way to do things!)

17:23 Multilevel feedback Queues:
• Process dynamically moves between priority classes based on its CPU/ IO activity
• Basic observation
– CPU bound process’ likely to complete its entire timeslice
– IO bound process’ may not complete the entire time slice

• All processes start in the highest priority class
• If it finishes its time slice (likely CPU bound)
– Move to the next lower priority class
• If it does not finish its time slice (likely IO bound)
– Keep it on the same priority class
• As with any other priority based scheduling scheme, starvation needs to be dealt with