PMS

• Process


  An active entity that requires a set of resources to perform its function
• Process


  • Has a CPU state (register values)
  • Has an address space (memory contents)
  • Has an environment (connections with other entities)
• Process


  • Requires space in main memory where it resides during its execution
  • Passes through several states from its initial arrival into the computer system to its completion
• Thread


  A lightweight process, a unit smaller than a process, a portion of a process that can run independently
• Processor


  The part of the machine that performs the calculations and executes the programs
• Job scheduling


  The process of allocating system resources to many different tasks by an operating system
• Process scheduling


  The activity of the process manager that handles the removal of the running process from the CPU and the selection of another process on the basis of a particular strategy
• Process Scheduling Queues

  • Job queue
  • Ready queue
  • Device queues

  
• Process Control Block (PCB)


  A data structure used by computer operating systems to store all the information about a process
• Process States

  READY, RUNNING and BLOCKED
• I/O-bound process


  Spends more time doing I/O than computations
• CPU-bound process


  Spends more time doing computations
• Multitasking


  Having multiple processes time slice on the same processor
• Multiprogramming


  Having multiple jobs in the system
• Multiprocessing


  Using multiple processors for the same job or system
• Multiprogramming


  Requires that the processor be allocated to each process for a period of time and de-allocated at an appropriate moment
• Context switch


  The stopping of one process and starting (or restarting) of another process
• Scheduling Algorithms


  The method by which processes are given access to system resources, usually processor time
• Types of Schedulers

  • Long-Term Scheduler
  • Short-Term Scheduler
  • Medium-Term Scheduler
• Preemptive Scheduling


  A process scheduling strategy in which the processing of a job is interrupted and the CPU is transferred to another job
• Non-Preemptive Scheduling


  A job scheduling strategy in which the job captures the processor and begins execution and runs uninterrupted until it issues an IO request or it is finished
• CPU Efficiency


  The average percentage of the hardware (or the CPU) that is actually used
• CPU Utilization


  The main task in which the operating system needs to make sure that the CPU remains as busy as possible
• Throughput


  The number of jobs completed in a unit of time
• Turnaround Time


  The average time from submitting a job until it terminates
• Waiting Time


  The time a job waits (in the ready state) until it runs
• CPU Efficiency


  The main task in which the operating system needs to make sure that the CPU remains as busy as possible
• CPU Efficiency


  • Can range from 0 to 100 percent
  • For the RTOS, it can range from 40 percent for low-level and 90 percent for the high-level system
• CPU Utilization


  The number of jobs completed in a unit of time
• Throughput


  The average time from submitting a job until it terminates
• Turnaround Time


  The time a job waits (in the ready state) until it runs
• Response Time


  Minimize response time by quickly dealing with interactive requests, and letting batch requests wait
• First-Come First-Served (FCFS)


  • A non-preemptive scheduling algorithm that handles jobs according to their arrival time
  • Uses FIFO queue
• Shortest Job Next (SJN)


  • The job which has the shortest burst time gets the CPU
  • Non-preemptive, pre-emptive scheduling algorithm
• Shortest Remaining Time First (SRTF)


  • Preemptive version of a shortest job next scheduling
  • Preemptive in nature (only at arrival time)
  • The processor is allocated to the job closest to completion
  • Can't be implemented on interactive system because it requires advance knowledge of required CPU time
• Priority Scheduling


  • Most common method in batch systems, but may give slow turnaround to some users
  • Gives preferential treatment to important jobs
  • Jobs with the same priority are treated FCFS
  • Priority could be based on: Least amount of memory required, Least number of peripheral devices needed, Shorted estimated CPU time, Time already spent waiting
• Round Robin (RR)


  • Preemptive in nature
  • Preemption based on time slices or time quanta; Time quantum between 10 and 100 milliseconds
  • All user processes are treated to be at the same priority
  • No process is allocated CPU for more than 1 quantum in a row
  • Timer interrupt results in context switch and the process is put at the rear of the ready queue
• Multi-level Feedback Scheduling


  • n priority levels — priority scheduling between levels, round-robin within a level
  • Quantum size decreases with priority level
  • Jobs are demoted to lower priority levels if they don't complete within the current quantum