Operating Systems - Process Priority | CMSC 412, Study notes of Operating Systems

Download Operating Systems - Process Priority | CMSC 412 and more Study notes Operating Systems in PDF only on Docsity!

Announcements

z

Program #1– Is on the web– Additional info on elf file format is on the web

z

Reading– Chapter 6

Selecting a process to run

z

called scheduling

z

can simply pick the first item in the queue– called round-robin scheduling– is round-robin scheduling fair?

z

can use more complex schemes– we will study these in the future

z

use alarm interrupts to switch between processes– when time is up, a process is put back on the end of the

ready queue

• frequency of these interrupts is an important parameter
  • typically 3-10ms on modern systems• need to balance overhead of switching vs.

responsiveness

Process Creation

z

Who creates processes?– answer: other processes– operations is called fork (or spawn)– what about the first process?

z

Have a tree of processes

• parent-child relationship between processes

z

what resources does the child get?

• new resources from the OS• a copy of the parent resources• a subset of the parent resources

z

What program does the child run?

• a copy of the parent (UNIX fork)
  • a process may change its program (execve call in

UNIX)

• a new program specified at creation (VMS spawn)

CPU Scheduling

z

Manage CPU to achieve several objectives:– maximize CPU utilization– minimize response time– maximize throughput– minimize turnaround time

z

Multiprogrammed OS– multiple processes in executable state at same time– scheduling picks the one that will run at any give time (on a

uniprocessor)

z

Processes use the CPU in bursts– may be short or long depending on the job

Process State Transitions

New

Exit

Ready,suspend

Ready

Running

Blocked

Blocked,suspend

Long-term scheduling

Medium-termscheduling

Short-termscheduling

Eventwait

Long-term scheduling

z

Determine which programs admitted to system forprocessing - controls degree of multiprogramming

z

Once admitted, program becomes a process, either:– added to queue for short-term scheduler– swapped out (to disk), so added to queue for medium-term

scheduler

z

Batch Jobs– Can system take a new process?

•^

more processes implies less time for each existing one

• add job(s) when a process terminates, or if percentage of

processor idle time is greater than some threshold

• Which job to turn into a process
  • first-come, first-serve (FCFS), or to manage overall

system performance (e.g. based on priority, expectedexecution time, I/O requirements, etc.)

Scheduling criteria

z

Per processor, or system oriented– CPU utilization

• maximize, to keep as busy as possible
  • throughput
    • maximize, number of processes completed per time unit

z

Per process, or user oriented– turnaround time

• minimize, time of submission to time of completion.
  • waiting time
    • minimize, time spent in ready queue - affected solely by

scheduling policy

• response time
  • minimize, time to produce first output• most important for interactive OS

Scheduling criteria

non-performance related

z

Per process– predictability

• job should run in about the same amount of time,

regardless of total system load

z

Per processor– fairness

• don’t starve any processes, treat them all the same
  • enforce priorities
    • favor higher priority processes
      • balance resources
        • keep all resources busy