Process Control

Process control allows us to talk about execution, threads or threads, which are a sequence of tasks chained so small that they can be executed by an operating system

Process model with two states

A process can be in one of two basic states:

• Execution
• No execution

Processes not running

With a single queue of non-running processes, the scheduler would have to traverse it to find a process that is not blocked. FIFO (First-in, First-out) strategy is not used

Two types of non-execution processes:

• Ready to run
• Locked: Expect to complete an I/O operation

Model of five states

• Execution
• Ready
• Locked
• New
• Finished



Processes suspended

The operating system may decide to “evict” a process from main memory and transfer it to disk

Two new states:

• Locked/Suspended: The process is in secondary memory waiting for an event
• Ready / Suspended: The process is in secondary memory available for execution



Signals and exceptions

Signals and exceptions are the mechanisms that uses an operating system to notify a process of the occurrence of a particular event

The signals are used in POSIX and exceptions are used in Windows

Signals

A signal is the interruption of a process

Receiving the signal:

• The process stops its execution
• Branches to execute a signal processing routine (code must be part of the process itself)
• Once the processing routine is executed, follow the execution of the process

Sending the signal:

• The source can be the operating system or a process
• At POSIX, it is done using the kill service

Exceptions

Event that occurs during the execution of a program and that require the execution of a piece of code outside the normal flow of execution

Can be generated by the hardware or the software

Exception handling requires programming language support. For example, in Java:

try {
	Block where exception can occur
}
catch {
	Block to be executed if an exception occurs in the previous block
}

Threads

A modern process manager separates dynamic execution from static aspects of the computational environment from the process:

• Modern process: Unit of resource ownership
• Thread (or light process): Unit or engine running

A process can contain a single ground thread or several threads of execution

States of a thread

States:

• Execution
• Ready
• Locked

The state suspension belongs to the concept of process

Basic operations related to thread state change:

• Creating/destroying a thread
  • When you create a process, you also create a thread
  • A thread of a process can create other threads within the same process
• Lock/Unlock
  • Reservation-specific resource from the thread

Processes in a multi-threaded environment

Own information for each thread:

• Processor Logs: counter, battery, status, etc.
• Thread Status: Execution, Ready or Locked

Information shared by all threads in the same process:

• Address Space: code and data
• Global Variables
• Open files
• Other shared resources

Benefits of threads

Facilitates modularity by allowing you to encapsulate each task in a thread independent

Increases job execution speed:

• It takes less time to create/finish a new thread than a process
• It takes less time to switch context between two threads of the same process
• Because threads in the same process share memory and files, they can communicate with each other without invoking the kernel

Allows concurrent programming

Process server

A server is a process that is pending receiving work orders that come from other processes (clients)

Once the order is received, it executes it and responds to the customer with the result

Communication between the client process and the server is done through the ports

Daemon

They are booted when you start the system. They are always active and do not die

Running in the background and are not associated with a terminal or login process

They are typically waiting for an event or perform a task periodically

They don't do the work directly: they throw other processes (or threads) to do their homework

Examples: FTP server, web server, ...