Processes - Chapter 3

Process - program in execution

Job - process

Batch system - completes jobs sequentially

Time shared system - completes jobs concurrently, based on allocated time slots

Memory Structure

Text - contains program code

Data section - stores global variables

Stack - stores local data (i say data because it includes functions)

Heap - stores dynamically allocated memory

Process State

admitted - new process is made and is put in ready queue

scheduler dispatch - process is now at the front of the queue and is set to running

interrupt - an interrupt occurs and the process is sent back to the ready queue

IO wait - the process requires IO the process is set to waiting

IO completion - the IO has completed and the process is then sent to the ready queue

exit - the process is terminated, along with its memory space

Process Control Block (PCB)

Process State - i.e. running, waiting, ready

Process Number - the ID of that process

Program Counter - points to the next instruction, multi-THREADS need multiple program counters

Registers - the last value of the registers in the cpu

Memory limits - memory allocated to that process

List of open files - lists out all currently open files

Accounting information - time limits, time since process start...

Process Scheduling

Ready queue - processes ready in main memory

Job queue - all processes in the system

Device queue - process waiting for IO

Process scheduler (general term) - selects next process to execute out of the ready/ job/ device queues

CPU scheduler (short-term) - selects the next process to be executed by the CPU

Job scheduler (long-term) - selects processes to be added to the ready queue (process might be in virtual memory)

Medium-term scheduler - will remove a process by putting it in main memory and bring another into the ready queue and primary memory

Context switches

When switching processes, the processes state is saved via the PCB

Child Processes and Memory sharing

Children can share all, apart of, or none of the parents resources

Processes refure to one another via their PID

UNIX commands for process management

fork() - create a new process

exec() - replaces parents memory space with childs

wait() - moves parent to waiting queue, until child termination

exit() - terminates the process

abort() - kills child

Killing children

Children should not exist without a parent

Zombie - parent did not wait for child to terminate (baso child is still running with parent that left it)

Orphan - parent died, with out waiting

Interprocess communication (IPC)

Coperation processes - processes that communicate with others

Done by:

• Message passing - done by using a queue, bound or un-bound (unlimited in size)

• Shared memory

Shared Memory communication

Symmetry - both processes know each others id

Asymmetric - only one knows the id (the sender)

Messages are passed by mailboxes (port)

Blocking (synchronous messaging, i.e. cannot read/write till message is ready/read)

Non-blocking (well guess retard ^^^)