OS Chapter 5
2015-10-29 20:03:28 0 举报AI智能生成
第五章探讨了操作系统的基本概念和功能。首先，它解释了操作系统作为计算机硬件和用户之间的中介的作用。操作系统负责管理和控制计算机的硬件资源，如处理器、内存和存储设备，以确保它们被有效地利用。此外，操作系统还提供了一组服务，使用户能够执行各种任务，如文件管理、内存分配和进程调度。 本章还介绍了操作系统的主要组成部分，包括内核、设备驱动程序、文件系统和用户界面。内核是操作系统的核心部分，负责管理系统资源和提供基本的服务。设备驱动程序允许操作系统与各种硬件设备进行通信。文件系统则负责组织和管理存储在计算机上的数据。最后，用户界面使用户能够与操作系统进行交互，通过命令行或图形界面执行任务。
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basic concept
CPU-I/O BurstCycle
CPU Scheduler
Selects from among the processes in ready queue, and allocates the CPU to one of them
Queue may be ordered in various ways
Preemptive Scheduling
CPU scheduling decisions may take place when a process:
Switches from running to waiting state
Switches from running to ready state
Switches from waiting to ready
Terminates
Scheduling under 1 and 4 is nonpreemptive
All other scheduling is preemptive
Dispatcher:Dispatcher module gives control of the CPU to the process selected by the short-term scheduler
Switching context
Switching to user mode
jumping to the proper location in the user program to restart that program
Dispatch latency – time it takes for the dispatcher to stop one process and start another running
Scheduling Criteria
CPU utilization – keep the CPU as busy as possible
Throughput – # of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process
Waiting time – amount of time a process has been waiting in the ready queue
Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
Optimization Criteria
Max CPU utilization
Max throughput
Min turnaround time
Min waiting time
Min response time
Scheduling Algorithm
First-Come, First-Served (FCFS) Scheduling【nonpreemtion】
Process Burst Time
P1 24
P2 3
P3 3
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: (0 + 24 + 27)/3 = 17
Convoy effect - short process behind long process
Consider one CPU-bound and many I/O-bound processes
Shortest-Job-First (SJF) Scheduling
Associate with each process the length of its next CPU burst
Use these lengths to schedule the process with the shortest time
SJF is optimal – gives minimum average waiting time for a given set of processes
The difficulty is knowing the length of the next CPU request
Could ask the user
Example of Shortest job first
Process Burst Time
P1 6
P2 8
P3 7
P4 3
Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
Determining Length of Next CPU Burst
Can only estimate the length – should be similar to the previous one
Then pick process with shortest predicted next CPU burst
tn+1 = a tn +(1-a)tn
Example of Sortest remaining time first
ProcessA Arrival Time Burst Time
P1 0 8
P2 1 4
P3 2 9
P4 3 5
Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec
Priority Scheduling
A priority number (integer) is associated with each process
The CPU is allocated to the process with the highest priority (smallest integer highest priority)
Preemptive
Nonpreemptive
Problem Starvation – low priority processes may never execute
Solution Aging – as time progresses increase the priority of the process
Example of Priority Scheduling
Process Burst Time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2
Average waiting time = [(6)+(0)+(16)+(18)+(1)]/5 = 8.2 msec
Round Robin (RR)
Each process gets a small unit of CPU time (time quantum q), usually 10-100 milliseconds.
After this time has elapsed, the process is preempted and added to the end of the ready queue.
Timer interrupts every quantum to schedule next process
Performance
q large =》 FIFO
q small =》 q must be large with respect to context switch, otherwise overhead is too high
Example of RR with Time Quantum = 4
Process Burst Time
P1 24
P2 3
P3 3
Typically, higher average turnaround than SJF, but better response
Multilevel Queue
Ready queue is partitioned into separate queues, eg:
foreground (interactive)
background (batch)
Each queue has its own scheduling algorithm:
foreground – RR
background – FCFS
Scheduling must be done between the queues:
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;
i.e., 80% to foreground in RR, 20% to background in FCFS
Multilevel Feedback Queue
A process can move between the various queues; aging can be implemented this way
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
Example of Multilevel Feedback Queue
Three queues:
Q0 – RR with time quantum 8 milliseconds
Q1 – RR time quantum 16 milliseconds
Q2 – FCFS
Scheduling
A new job enters queue Q0 which is served FCFS
When it gains CPU, job receives 8 milliseconds
If it does not finish in 8 milliseconds, job is moved to queue Q1
At Q1 job is again served FCFS and receives 16 additional milliseconds
If it still does not complete, it is preempted and moved to queue Q2
Thread Scheduling
Distinction between user-level and kernel-level threads
contention scope
Kernel thread scheduled onto available CPU is system-contention scope (SCS) – competition among all threads in system
Many-to-one and many-to-many models, thread library schedules user-level threads to run on LWP
Known as process-contention scope (PCS) since scheduling competition is within the process
Typically done via priority set by programmer
Multiple-Processor Scheduling
CPU scheduling more complex when multiple CPUs are available
Asymmetric multiprocessing – only one processor accesses the system data structures, reducing the need for data sharing
Symmetric multiprocessing (SMP) – each processor is self-scheduling, all processes in common ready queue, or each processor has its own private queue of ready processes
Processor affinity – process has affinity for processor on which it is currently running
soft affinity
hard affinity
Variations including processor sets
Operating System Examples
Solaris Scheduling
Scheduler converts class-specific priorities into a per-thread global priority
Thread with highest priority runs next
Runs until (1) blocks, (2) uses time slice, (3) preempted by higher-priority thread
Multiple threads at same priority selected via RR
Windows Scheduling
Windows uses priority-based preemptive scheduling
Highest-priority thread runs next
Dispatcher is scheduler
Thread runs until (1) blocks, (2) uses time slice, (3) preempted by higher-priority thread
Real-time threads can preempt non-real-time
Algorithm Evaluation
How to select CPU-scheduling algorithm for an OS?
Determine criteria, then evaluate algorithms
Queueing Models
Describes the arrival of processes, and CPU and I/O bursts probabilistically
Commonly exponential, and described by mean
Computes average throughput, utilization, waiting time, etc
Computer system described as network of servers, each with queue of waiting processes
Knowing arrival rates and service rates
Computes utilization, average queue length, average wait time, etc
Little’s Formula
n = average queue length
W = average waiting time in queue
λ = average arrival rate into queue
Little’s law – in steady state, processes leaving queue must equal processes arriving, thus n = λ x W
Valid for any scheduling algorithm and arrival distribution
For example, if on average 7 processes arrive per second, and normally 14 processes in queue, then average wait time per process = 2 seconds
Simulations
Simulations more accurate
Programmed model of computer system
Clock is a variable
Gather statistics indicating algorithm performance
Data to drive simulation gathered via
Random number generator according to probabilities
Implementation
Even simulations have limited accuracy
Just implement new scheduler and test in real systems
High cost, high risk
Environments vary
Most flexible schedulers can be modified per-site or per-system
APIs to modify priorities
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