Threads Model

• In shared memory multiprocessor architectures, such as SMPs, threads can be used to implement parallelism.
• In the threads model of parallel programming, a single process can have
  • multiple concurrent,
  • execution paths.
• Most simple analogy for threads is the concept of a single program that includes a number of subroutines:

Figure 2: Threads model.

• a.out (main program) loads and acquires all of the necessary system and user resources to run.
• Main program performs some serial work,
• and then creates a number of tasks (threads) that can be scheduled and run by the OS concurrently.

• Each thread has local data, but also, shares the entire resources of main program.

  Figure 3: Thread shared memory model.

  
• This saves the overhead associated with replicating a program's resources for each thread.
• Each thread also benefits from a global memory view because it shares the memory space of program.
• Any thread can execute any subroutine at the same time as other threads.

• Threads communicate with each other through global memory (updating address locations).
• Changes made by one thread to shared system resources (such as closing a file) will be seen by all other threads.
• This requires synchronization constructs to insure that more than one thread is not updating the same global address at any time.

  Figure 4: Threads Unsafe! Pointers having the same value point to the same data.

  

• Threads can come and go, but main program remains present to provide the necessary shared resources until the application has completed.
• From a programming perspective, threads implementations commonly comprise:
  1. A library of subroutines that are called from within parallel source code
  2. A set of compiler directives embedded in either serial or parallel source code
• In both cases, the programmer is responsible for determining all parallelism.