Differences Between Processes and Threads

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#OS#Thread#Process#Stack#Heap

Overview

A summary of the differences between processes and threads.

What is a Process?

  • A unit of program execution
  • Executed on the CPU
  • Cannot share resources
  • Uses dedicated memory space
    • Maintains a virtual address space
    • Switching processes requires mapping between virtual and physical addresses

Data Structures Maintained by a Process

The data structures maintained in memory by a process are divided into two segments:

  • Text Segment

    • Sequence of program instructions (the program itself to be executed)
      • Read-only

  • Data Segment

    • PDA (Processor Data Area)
      • Contains processor information and data for process management
        • Includes the stack pointer and program counter
          • Stack Pointer
            • When the CPU's internal registers (storage locations within the CPU) are insufficient during program execution, data may be temporarily stored in main memory.
            • This temporary storage location is called the stack, and the stack pointer holds the address of this temporary storage.
          • Program Counter
            • A register that holds the memory address of the next instruction to be executed
    • Data Area
      • Static Area
        • Stores constants and global variables
        • Heap Area - Stores regular variables - The process dynamically increases or decreases this area, so its size is unknown until runtime
    • Stack Area
      • Temporarily stores data such as arguments and local scope data

What is a Thread?

  • An execution unit generated from a single process
  • Can share resources
  • Uses shared memory space
    • Stores data within the memory space of the parent process

Differences Between Processes and Threads During Parallel Execution

Differences between processes and threads when executing programs in parallel:

Process

  • Programs can be executed in parallel by forking the parent process to create multiple child processes
  • When a child process is created, memory space is allocated for it
    • The allocated memory space includes a copy of the parent process's data segment, and a dedicated data segment is reserved for the child process
  • Since the program instructions are the same as the parent process, the text segment refers to the same area as the parent process
  • A child process is still a separate process and cannot directly access the memory of other processes

Thread

  • Programs can be executed in parallel by generating threads from a process
  • When a thread is created, the following values are copied into the parent process's virtual address space:
    • Stack area
    • Stack pointer
    • Program counter
  • Other data is shared with the parent process
  • A thread only holds data about which part of the program it is executing
  • Variables used by a thread may be modified by other threads
    • Programs that can operate without issues in such situations are called thread-safe

Performance

Process

  • Process Switching
    • Requires clearing the cache held by the MMU
      • TLB flush

Thread

  • Thread Switching
    • Only requires switching the stack area, stack pointer, and program counter

Miscellaneous

Stack

  • Automatically allocated and released by the OS or compiler
  • The size is determined during program compilation and linking

Heap

  • Applications need to allocate and release memory as needed
  • The size can be dynamically specified when allocating memory

References