6 Sep 2013 ... Process Control Block (PCB). OS bookkeeping information associated with each
process: • Process state. • Program counter. • CPU registers.
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What Is A Process? A process is a program in execution. #include int main(int argc, char*argv[]) { int v; printf(“hello world\n”); scanf(“%d”, &v); return 0; }
Notice: This set of slides is based on the notes by Professor Perrone of Bucknell and the textbook authors Silberschatz, Galvin, and Gagne
CSCI 315 Operating Systems Design
Program test‐process is running
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Process States
Process Concept • A process includes:
As a process executes, it changes state:
– program counter – stack – data section
heap
• Process execution must progress in sequential progress in sequential fashion. • Program Counter (PC) designates the next instruction to be program counter executed
new: The process is being created. running: Instructions are being executed. waiting: The process is waiting for some event to occur. ready: The process is waiting to be assigned to a processor. – terminated: The process has finished execution.
– – – –
stack
data
code
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CSCI 315 Operating Systems Design
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OS bookkeeping information associated with each process:
created exit
admitted
interrupt
terminated
• • • • • • •
running
ready scheduler dispatch I/O or event completion
I/O or event wait
waiting
Process state Program counter CPU registers CPU registers CPU scheduling information Memory‐management information Accounting information I/O status information
CSCI 315 Operating Systems Design
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process id process state program counter registers memory limits list of open files
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Process Control Block (PCB)
Process State Transition Diagram new
CSCI 315 Operating Systems Design
CSCI 315 Operating Systems Design
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A Sample Linux PCB
CPU Switching
• http://www.eecs.harvard.edu/~margo/cs161/ videos/sched.h.html • http://www.tldp.org/LDP/tlk/ds/ds.html • Look for struct kf task_struct k i h in the source code d distribution of Linux.
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Process Scheduling Queues
CSCI 315 Operating Systems Design
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Processes and OS Queues
• Job queue – set of all processes in the system. • Ready queue – set of all processes residing in main memory, ready and waiting to execute. • Device queues – set of processes waiting for an I/O device. Processes migrate between the various queues.
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CSCI 315 Operating Systems Design
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Process Scheduling
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Schedulers CPU
ready queue
I/O
CSCI 315 Operating Systems Design
I/O queue
• Long‐term scheduler (or job scheduler) – selects which processes should be brought into the ready queue • Short‐term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU p
I/O request time slice expired
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child executes
fork a child
interrupt occurs
wait for interrupt
CSCI 315 Operating Systems Design
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CSCI 315 Operating Systems Design
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Schedulers
Context Switch
• Short‐term scheduler is invoked very frequently (milliseconds) (must be fast) • Long‐term scheduler is invoked very infrequently (seconds, minutes) (may be slow; controls the degree of multiprogramming) • Processes can be described as either:
• When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process. – What is in context? (Information in PCBs)
– I/O‐bound process – spends more time doing I/O than computations, many short CPU bursts
• Context‐switch time is overhead; the system does no useful work while switching.
– CPU‐bound process – spends more time doing computations; few very long CPU bursts
• Time dependent on hardware support.
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CSCI 315 Operating Systems Design
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CSCI 315 Operating Systems Design
Operations on Processes
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Process Creation • Parent process create children processes, which, in turn can create other processes, forming a tree of processes.
• Creation • Termination
• Resource sharing: – Parent and children share all resources, – Children share subset of parent’s resources, – Parent and child share no resources.
• Execution: – Parent and children execute concurrently, – Parent may wait until children terminate.
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Process Creation (Cont.)
CSCI 315 Operating Systems Design
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Process Tree (Linux Example)
• Address space: – Child has duplicate of parent’s address space, or – Child can have a program loaded onto it.
• UNIX examples: – fork system call creates new process and returns with a pid (0 in child, > 0 in the parent), – exec system call can be used after a fork to replace the process’ memory space with a new program. See example by running “ps –aef | less” 9/6/2013
CSCI 315 Operating Systems Design
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Process Termination
Create a Process in Linux #include #include int main(int argc, char *argv[]) { pid_t pid; if ((pid = fork())
