1 processes

Instance of a program that is being executed.

1.1 concepts

A process can be described as either:

• IO-bound process - spends more time doing IO than computations; many short CPU bursts
• CPU-bound process – spends more time doing computations; few very long CPU bursts

1.1.1 structure

• text: code
• data: global/static variables
• stack: temporary quick access data
• heap: dynamically allocated memory during runtime

Stack and heap is growable/shrinkable.

1.1.2 states

• new: The process is being created
• ready: The process is waiting to be assigned to a processor
• running: Instructions are being executed
• waiting: The process is waiting for some event to occur
• terminated: The process has finished execution

1.1.3 process control block (PCB)

Stores information about running processes. pid, process state, CPU registers, CPU scheduling info (priorities, queue), memory limits, stats etc.

1.1.4 context switch

Switching from one process to another. Requires saving current PCB and loading the one that is needed. Context switches are expensive to perform.

1.2 scheduling

Maximize/optimize CPU use. Decides which process is executed next.

Processes move around these queues:

• job queue
• ready queue
• device queue

1.2.1 types

• short-term scheduler: selects the next process and allocates CPU (fast, often runs)
• long-term scheduler: selects processes into the read queue (slower, rarely runs)
• medium-term scheduler: swaps out/in processes from/to disk to/from memory

1.3 operations

Processes are a tree: parents can create its own processes called children.

• parent and children execute concurrently
• parent has to wait until children terminate

1.3.1 fork

Creates a new child process with a copy of the current process’ memory, registers, etc.

1.3.2 exec family

Executing other programs with arguments.

1.3.3 process termination

exit() exits current process with a given status code.

kill(pid, SIGNAL) kills a process.

• If no parent was waiting for its child, the child process becomes a zombie
• If parent terminates before the child, the child process becomes an orphan (some operating systems don’t allow such behavior and all children are killed if the parent terminated)

All resources are deallocated by the operating system.

1.3.3.1 wait

Waits for child to exit. Returns the exit code and pid of the child process.

1.3.4 inter-process communication (IPC)

Either by shared memory or message passing.