Operating Systems- process

Seehwan YooMse.cis.dku

Review

• Computer systems orga-nization (hardware)• Main components

• CPU: instruction execu-tion

• Memory: store data/in-struction (program)

• I/O devices: interact with outer world

• Bus: an inter-connect across hw• Address• Data• Control

• Input/output device operation• Accessing registers

• Mem-mapped I/O• I/O-mapped I/O

• Event checking• Interrupt: notify of

event to cpu• Polling: cpu manually

checks status• DMA

• Bulk data transfer• Avoid cpu interven-

tion

Review

• Operating systems structure• Kernel• User programs

• Dual-mode OS: protection support by hw• User mode• Kernel mode

• System call• OS service call• Across protection boundary (from user to kernel)• No API, but ABI

• Multi-user systems• Multi-programming: running multiple user applications

Process & execution con-text• Program in execution• Abstraction for processor• Every process has its own execution ‘context’

• CPU register values• Incl. PC, SP, A0, V0, …

• Address space (memory)• State• Open files

Program vs. process

Program • Stored in disk• No active execution

context• No PC, no SP, no run-

time data

Process• Loaded in memory• Has execution context• PC, SP, live registers,

open files, memory con-tents

Process Memory or address space• Address • of your home?• Address is given to memory location• LW/SW (load/store) data to some memory location

• Address space• Set of address {from begin to end}

• {0, …, 4G} (for 32bit machine)• Process has its own address space• Every process runs only within his own address space• Will be addressed, later (in virtual memory chapter)

Memory layout

• Process should have memory region for• Text: program code• Data• Stack• Heap• And kernel

• All above within 0~4G address space• Written in your program

Time-sharing system

• Multiple processes run concurrently• assumption: 1 CPU system

• OS makes illusion• Feels like each process has its own CPU• And all processes run at the same time

• Conceptually,• Make process A run, then stop• Make process B run, then stop• …

Time-sharing system’s im-plementation• Timer runs • Generates timer interrupts periodically

• 1s/ 10ms/ 1ms• Like heartbeat

• When CPU gets timer interrupt, • Account CPU usage of the process

• Time quantum (or time slice): max. time given to a process without interruption

• When a process consumes all its quantum, change the running process

• Scheduler determines which process to run (at when)

Process states

• New• Running• Waiting• Ready• Terminated

Process state transition di-agram

Process state transition when?• New ready• Ready running• Running ready• Running waiting• Waiting ready• Running terminated

Process state transition when?• New ready : process creation completion• Ready running : scheduler dispatch• Running ready : time quantum expired, pre-

empted by another process• Running waiting : I/O req.• Waiting ready : I/O completion• Running terminated : exit(), unexpected excep-

tions

PCB – process control block

• Data structure of process inside kernel• task_struct in Linux

• pid t_pid; /* process identifier */ long state; /* state of the process */ unsigned int time_slice /* scheduling information */ struct task_struct *parent; /* this process’s parent */ struct list_head children; /* this process’s children */ struct files_struct *files; /* list of open files */ struct mm_struct *mm; /* address space of this process */

Context switching

• Triggered by various events• Time quantum expired, I/O completion, etc.

• Exchange running process with one of ready pro-cesses

Scheduler - Switching the execution context

Analogy in context switching, interrupt handling

• When interrupt/context switching occurs,• Stop the current execution

• Save regs on the PCB• Handle interrupt / exchange PCB (by scheduler)• Return to the previous execution

• Load regs (values) back from PCB

• PC / SP ?• Help of hw (arch.)

• Return address reg.• Separate pc/sp for user applications

Scheduling – Choose a process to run• Scheduling queue: data structure for storing

process in various states• Ready queue• Device queue

Scheduling queues

Overall picture of schedul-ing

Let’s look into Scheduling in the next class!

Summary page

• Process concept• Cf. program

• Memory layout• Address space

• Execution context• PCB

• Context switching• Scheduler operation• Process state diagram