Date: September 1, 2024

Topic: Process Lifecycle States

Recall

Lifecycle States include:

• New
• Ready
• Running
• Waiting
• Terminated

In the Running state, it is possible to go back to:

• Ready (interrupt)
• Waiting (I/O or event wait)
• Terminated (process conclusion)

Notes

Process Lifecycle States

Process lifecycle states. Possible states are shown in the nodes

Starting a Process

• When a process is created, it enters a new state
• OS performs admission control and if determined to be OK, the OS will allocate and initiate a PCB and initial resources
• This is provided that there are some minimum available resources

Ready and Running

• The process is then in a ready state, where the scheduler dispatch schedules it on the CPU
• When a process is running, it can be interrupted and context switched to idle (ready state)
• A scheduler dispatch can execute the process again on the CPU, moving to a running state

Running State Branches

Interrupt:

• A running process can be interrupted such that a context switch is performed, going back to the ready state

I/O or Event Wait:

• A running process may need to initiate a longer operation like reading from a disk / waiting for a timer / input from a keyboard etc.
• The process then enters a waiting state until the timer or event completes, moving back to the ready state

Termination:

• When the process finishes or encounters some error, it returns the appropriate exit code (either SUCCESS or ERROR)

<aside> 💡

CPU is able to execute processes in either a running or ready state

</aside>

Processes can spawn child processes

Child processes can be created either by:

• fork: child continues from same point
• exec: child starts from first instruction

Process Creation

All processes come from a single root

In OS, a process can create child processes

• Some processes are privileged (root) processes
• When the initial boot process is done and the OS is loaded on the machine , it creates some initial processes
• When a user logs in, a user shell process is created
• When a user types in commands like ls or emcas, new processes are spawned from shell parent process

Mechanisms for Process Creation

Fork - child continues from same point

• Copies the parent PCB into the new child PCB
• When the fork is complete, both processes (parent and child) continue at the exact same point

Exec - child starts from first instruction

• Replace child image, loading a new program and starts from the first instruction
• A fork is initially called, then the child image is replaced

Parent Processes

For UNIX-based OS, the parent of all processes is init

For Android OS, the parent of all processes is zygote

<aside> 📌 SUMMARY: Processes can have the following lifecycle states: new; ready, running, waiting; terminated. Processes are created from another processes either through fork or exec

</aside>

Date: September 1, 2024

Topic: Process Scheduling

<aside> 📌 SUMMARY: The CPU scheduler determines which process from a ready queue should run next. This operation should be minimized so that the actual process running time $T_p$ is maximized. Process scheduling is done in a few ways; I/O request, expiring time slice, forking a child and waiting for an interrupt.

</aside>