Basic Concepts

CPU –I/O Burst Cycle

• Process execution consists of a cycle of CPU execution and I/O wait.

• Process execution begins with a CPU burst, followed by an I/O burst, then another CPU burst ... etc

• The duration of these CPU bursts has been measured.

• An I/O-bound program would typically have many short CPU bursts, A CPU-bound program might have a few very long CPU bursts

• this can help to select an appropriate CPU-scheduling algorithm.

Types of Processes

I/O bound

• Has small bursts of CPU activity and then waits for I/O (eg. Word processor)

• Affects user interaction (we want these processes to have the highest priority)

CPU bound

• Hardly any I/O, mostly CPU activity (eg. gcc, scientific modeling, 3D rendering, etc.)

• Useful to have long CPU bursts

• Could do with lower priorities

CPU Scheduler

The CPU scheduler is the mechanism to select which process has to be executed next and allocates the CPU to that process. Schedulers are responsible for transferring a process from one state to the other.

3 types of schedulers(recall L1):

Long-Term Scheduler

Short-Term Scheduler

Medium-Term Scheduler

When timer interrupt occurs or when running process is blocked on I/O, scheduler run

Scheduler picks another process from the ready queue

Performs a context switch

Preemptive scheduling

抢占式调度

The system may stop the execution of the running process and after that, the context switch may provide the processor to another process. The interrupted process is put back into the ready queue and will be scheduled sometime in future, according to the scheduling policy.

Non-preemptive scheduling

When a process is assigned to the processor, it is allowed to execute to its completion, that is, a system cannot take away the processor from the process until it exits. Any other process which enters the queue has to wait until the current finishes its CPU cycle.

典型process的状态图

Schedulers

-Decide which process should run next

CPU scheduling takes place on 4 circumstances:

1. When the process changes state from Running to Ready ex: when an interrupt occurs .

2. Changes state from Running to Waiting ex: as result of I/O request or wait().

3. Changes state from Waiting to Ready ex: at completion of I/O.

4. Process Terminates.

• Scheduling under 2 and 4 is nonpreemptive - a new process must be selected

• Other scheduling is preemptive - either continue running the current process or select a different one

Dispatcher

Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:

– switching context

– switching to user mode

– jumping to the proper location in the user program to restart that program

Dispatch latency（调度延时） – time it takes for the dispatcher to stop one process and start another running.

Dispatcher is invoked during every process switch; hence it should be as fast as possible

Scheduling Criteria

• Max CPU utilization – keep the CPU as busy as possible

• Max Throughput（吞吐量） – complete as many processes as possible per unit time

• Fairness - give each process a fair share of CPU

• Min Waiting time – process should not wait long in the ready queue

• Min Response time – CPU should respond immediately

Scheduling Algorithms

术语

Arrival Time (AT): Time at which the process arrives in the ready queue.

Completion Time: Time at which process completes its execution.

Burst Time: Time required by a process for CPU execution.

Turnaround Time (TT): the total amount of time spent by the process from coming in the ready state for the first time to its completion.

Turnaround time = Exit time - Arrival time

Waiting Time (WT): The total time spent by the process/thread in the ready state waiting for CPU.

Waiting Time = Turnaround Time – Burst Time

Response time: Time at which the process gets the CPU for the first time.

First-Come, First-Served (FCFS) Scheduling

Processes are executed on first come, first serve basis.

Poor in performance as average wait time is high

Waiting time for P1 = 0; P2 = 24; P3 = 27

Average waiting time: (0 + 24 + 27)/3 = 17

Shortest-Job-First (SJF) Scheduling

Schedule process with the shortest burst time

without preemption

• Advantages

– Minimizes average wait time and average response time

• Disadvantages

– Not practical : difficult to predict burst time

• Learning to predict future

– May starve long jobs

Average waiting time = (0+3+9+16) / 4 = 7

The real difficulty: knowing the length of the next CPU request

- with short-term scheduling, there is no way to know the length of the next CPU burst.

SJF cannot be implemented at the level of the short-term CPU scheduling

Determining Length of Next CPU Burst

Estimate it using lengths of past bursts: next = average of all past bursts.

Exponential averaging: next = average of (past estimate + past actual)

Let tn = actual length of the nth burst;

Tn+1 = predicted value for the next CPU burst;

a = weighing factor, 0 ≤ a ≤ 1

The estimate of the next CPU burst period is:

Commonly, a set to ½

- If a=0, then recent history has no effffect

- If a=1, then only the most recent CPU bursts matter

Shortest Remaining Time First (SRTF) Scheduling

Average waiting time = [(10-1)+(1-1)+(17-2)+(5-3)]/4 = 26/4 = 6.5 msec

Priority Scheduling

Each process is assigned a priority (just a number)

The CPU is allocated to the process with the highest priority (smallest integer)

Priorities may be:

Internal priorities based on criteria within OS. Ex: memory needs.

External priorities based on criteria outside OS. Ex: assigned by

administrators

PROBLEM: Starvation – low priority processes may never execute

SOLUTION: Aging – as time progresses increase the priority of the process

Example: do priority = priority – 1 every 15 minutes

Average waiting time = = (0 + 1 + 6 + 16 + 18) / 5 = 8.2

Round Robin(RR) Scheduling

Each process gets a small unit of CPU time (time quantum or time-slice), usually 10-100 milliseconds.

After this time has elapsed, the process is preempted and added to the end of the ready queue Ready queue is treated as a circular queue

If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.

Performance

q large  RR scheduling = FCFS scheduling

q small  q must be large with respect to context switch, otherwise overhead is too high

e.g.

Multiple-Level Queues Scheduling

• Ready queue is partitioned into separate queues; e.g., two queues containing

o foreground (interactive) processes . May have externally defined priority over background processes

o background (batch) processes

Process permanently associated to a given queue; no move to a different queue

There are two types of scheduling in multi-level queue scheduling:

• Scheduling among the queues.

• Scheduling between the processes of the selected queue.

Must schedule among the queues too (not just processes):

Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.

Time slice – each queue gets a certain amount of CPU time which it can

schedule amongst its processes;

80% to foreground in RR, and 20% to background in FCFS

The various categories of processes can be:

• Interactive processes

• Non-interactive processes

• CPU-bound processes

• I/O-bound processes

• Foreground processes

• Background processes

Multilevel Feedback Queue Scheduling

• Multilevel feedback queues - automatically place processes into priority levels based on their CPU burst behavior.

• I/O-intensive processes will end up on higher priority queues

and CPU-intensive processes will end up on low priority queues.

A process can move between the various queues

- aging can be implemented this way.

A multilevel feedback queue uses two basic rules:

1. A new process gets placed in the highest priority queue.

2. If a process does not finish its quantum, then it will stay at the same priority level otherwise it moves to the next lower priority level

Multilevel-feedback-queue scheduler defined by the following parameters:

– number of queues

– scheduling algorithms for each queue

– method used to determine when to upgrade a process

– method used to determine when to demote a process

– method used to determine which queue a process will enter when that process needs service

Three queues:

Q0 – RR with time quantum 8 milliseconds

▪ Highest priority. Preempts Q1 and Q2 proc’s

Q1 – RR time quantum 16 milliseconds

▪ Medium priority. Preempts processes in Q2

Q2 – FCFS

▪ Lowest priority

A new job enters queue Q0 -> If it does not finish in 8 milliseconds, job is moved to queue Q1 and receives 16 additional milliseconds -> If it still does not complete, it is preempted and moved to queue Q2

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