Threads

• Thread


  A lightweight process created within a normal (heavyweight) process
• Multithreading


  The ability of an operating system to support multiple threads of execution within a single process
• Threads


  • Processes have at least one thread of control
  • Multiple threads run in the same address space, share the same memory areas
  • The creation of a thread only creates a new thread control structure, not a separate process image
• It depends on the actual kernel implementation how threads are managed
• Thread Control Block


  A saved thread context when not running, containing: thread identifier, stack pointer, program counter, state, register values, pointer to process control block
• Thread states


  Running, Ready, Blocked
• There are four basic operations for managing threads: Spawn/create, Block, Unblock, Finish
• User-level threads


  Operating system unaware of threads, completely controlled by user program
• User-level threads


  • Thread library functions manage threads
  • Light thread switching in user mode
  • No mode switch necessary
  • Can implement own thread scheduling
• Kernel-level threads


  Operating system manages threads, better mapping to multiple CPU cores
• Kernel-level threads


  • Kernel manages each thread separately
  • Fine-grained scheduling by kernel
  • If one thread blocks, another can be scheduled without blocking the whole process
  • Threads can be distributed to multiple processors and run in parallel
• Kernel-level threads have significant overhead and increase in kernel complexity
• A mode switch to kernel mode is required to transfer control from one kernel-level thread to another in a process