Operating System - CPU Virtualization and Processes

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#operating system #CPU virtualization #process

Related Readings

What is CPU virtualization?

By CPU virtualization, we mean the illusion created by the OS that there are multiple CPUs but in reality there may be only one physical processor on the machine.

Virtualizing CPUs

time shareing
space sharing

Switching Between Processes

Cooperative Approach

wait for system calls
trap instruction
explicit yield system call

Gain control without cooperation

timer interrupt and interrupt handler
A timer can be turned off

context switch

This is about OS switches running processes.

policies -> scheduling policy

historical info
workload knowledge
performance metrics

process -> machine state

memory
registers

Many instructions explicitly read or update registers
special

program counter (instruction pointer)
stack pointer
frame pointer

I/O info

persistent storage info

General APIs

create
destroy
wait
status
miscellanuous control
fork
exec
wait

Process States

Init
Running
Ready
Blocked
Final

Limited Direct Execution

time sharing & control
trap instruction

Three concepts

register
kernel stack
proc_t

Scheduling

Metrics

Definition. Turnaround Time

\( T_{turnaround} = T_{completion} - T_{arrival}\)

Definition. Resposne Time

\( T_{response} = T_{first run} - T_{arrival} \)

There is a fundamental tradeoff: performance vs fairness

If you are willing to be unfair, you can run shorter jobs to completion, but at the cost of response time.

Algorithms

FIFO
Shortest Job First
Shortest-Time-to-Comletion-First (STCF)
Round Robin
Multi-Level Feedback Queue (MLFQ)
Proportional Share

Ticket Assignment
The Linux Comletely Fair Schedule (CFS)

Notion of time slice

time slice: short period of time to run a process
timer-interrupt period

Note that time slice should be a multiple of the timer-interrupt period.

Read a paper of latest scheudling algorithm ?

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Note:

Obtaining high performance while maintaining control is one of the central challenges in building an operating system.

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