Process model

process: program in execution (amount of processes depends on the program)

it’s an abstraction that allows OS to simplify resource allocation, accounting, and limiting.

Process table

OS maintains info on resources and internal state of every process

information in Process Table: ID (PID), User (UID), Group (GID), memory address space, hw registers, open files, signals, etc.

process control blocks:

Concurrent processes

in principle, multiple processes are mutually independent (they have nothing at all in common). need explicit means to interact with each other.

the CPU gets allocated to each process in turn

• on OS level: save context of process A (program counter, registers, etc.), switch to B. to go back to process A, simply restore context.

OS (normally) offers no timing or ordering guarantees

Process hierarchies

OS creates only 1 init process (usually) parent process can create a child process results in a tree-like structure and process groups

Process management

fork: create new process

• child is ‘private’ clone of parent
• shares some resources with parent

exec: execute new process image

• used in combination with fork
• replaces the current command

exit: cause voluntary process termination

• exist status returned to parent process

kill: send signal to process (or group)

• can cause involuntary process termination

Process states

OS allocates resources to processes three process states:

• running: process is currently executed by CPU
• blocked: process is waiting for available resources
• ready: process is ready to be selected

scheduler allocates/deallocates the CPU. there is no immediate transition between states because process has to wait for scheduler

Scheduler vs processes

• scheduler periodically switches processes
• sequential processes lie on the layer above
• leads to simple process organisation