Programs, Processes and Threads

Course Code: U10814

Course Name: Networking and Operating Systems

Credits: 20

Module Leaders: Dr Alexios Louridas & Mr Danny Werb

Material created by Seb Blair, modified by Danny Werb

Programs

  • Code that is stored on a computer that can complete a certain task
  • Can be built into the operating system
  • Can be specific task
    • AKA applications
  • Typically stored on a disk or in nonvolatile memory in executable format. Let’s break that down to understand why.

Programs - Memory

  • Volatile

    • Is temporary and processes in real time.
    • It’s faster, easily accessible, and increases the efficiency of the computer.
    • Not permanent. When your computer turns off, this type of memory resets.

  • Non-Volatile

    • Is permanent unless deleted.
    • It’s slower to access, it can store more information. So, that makes it a better place to store programs.

  • A file in an executable format is simply one that runs a program. It can be run directly by your CPU (that’s your processor).

    • Examples of these file types are .EXE in Windows and .APP in Mac.

Programs - Compiled

  • Most programs are written in a compiled language (processed by the compiler)
    • C, C++, C#, Java, Python, Rust, ... etc
  • The end result is a text file of code that is compiled into binary form
  • The text file speaks directly to your computer.
  • They’re typically fast
  • More control over things like memory management
  • They are also fixed compared to interpreted programs.
  • Platform dependent
  • If you have to change something in your code, it typically takes longer to build and test.

Programs - Interpreted

  • Some of the most common interpreted programming languages (executed from source code form, by an interpreter)
    • Basic, JavaScript, PHP, Ruby... etc.
    • BASH, Powershell (including scripting languages)
  • Require an additional program to take the program instructions and translate that to code for the computer.
  • Compared with compiled languages, these types of programs are platform-independent (just have to find a different interpreter, instead of writing a whole new program)
  • Typically take up less space.

Programs - Execution

  • Programs have to run in binary because your computer’s central processing unit (CPU) understands only binary instructions.
  • Needs to be loaded into memory:
    • The executing program needs resources from the operating system and memory to run.
    • When a program is loaded into memory along with all the resources it needs to operate, it is called a process

Process - Essential Resources

  • Register contains data that may be needed by a process, like instructions, storage addresses, or other data.

  • Program counter Also known as an instruction pointer. It keeps track of where a computer is in its program sequence.

  • Stack. A stack is a data structure that stores information about the active subroutines of a computer program. It is used as scratch space for the process. It is distinguished from dynamically allocated memory for the process that is known as the heap.

Process - State

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Process Control Block (PCB)

  • Process state
  • Program counter
  • CPU registers
  • CPU-scheduling information
  • Memory-management information
  • Accounting information
  • I/O status information

Process - Instances

  • Multiple instances of a single program

    • Multiple processes, one for each program
    • Separate memory address space
      • Independent
      • Isolated

  • Kill a malfunctioning program and keep working with minimal disruptions

Threads - ?

  • Unit of execution within a process.
  • Process can have 1:n threads

Thread - Single

  • Process starts
    • Receives an assignment of memory and other computing resources.
    • Each thread in the process shares that memory and resources.
    • With single-threaded processes, the process contains one thread.

Thread - Multi

Process contains more than one thread

  • Process is accomplishing a number of things at virtually the same time (concurrency).
  • Each thread will have its own stack.
  • All the threads in a process will share the heap.
  • It is easy to communicate between the threads.
  • One malfunctioning thread in a process can impact the viability of the process itself.

Process and Threads Flow

Here’s what happens when you open an application on your computer...

Interprocess Communication (IPC)

  • (a) Shared Memory

    • Processes exchange information by writing and reading in the shared memory

  • (b) Message passing

    • Processes exchange information via a messaging system

Program, Process, Thread

Using htop to see systemd (System Daemon)
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Concurrency and Parallelism

Can processes or threads can run at the same time?

It depends.

  • Multiple processors or CPU cores multiple processes or threads can be executed in parallel.
  • On a single processor, though, it is not possible to have processes or threads truly executing at the same time.
    • CPU is shared among running processes or threads using a process scheduling algorithm that divides the CPU’s time and yields the illusion of parallel execution.
    • The time given to each task is called a “time slice.”
    • Context switching back and forth between tasks happens so fast it is usually not perceptible.

Concurrency and Parallelism

  • Parallelism

    • genuine simultaneous execution

  • Concurrency

    • interleaving of processes in time to give the appearance of simultaneous execution

- Process state. Storing the current state of the process. - Program counter. Address of next instruction. - CPU registers. Index registers, stack pointers, and general-purpose registers, state information for correct process state change. - CPU-scheduling information. Process priority and scheduling information. - Memory-management information. Page tables, or the segment tables etc. - Accounting information. This information includes the amount of CPU and real time used, time limits, account numbers, job or process numbers, and so on. - - I/O status information. List of I/O devices allocated to the process, a list of open files, and so on.