Threads

Single threading

In a traditional operating system, a process includes:

• an address space (code and data of the associated program).

• a set of communication channels with I/O devices.

• a single thread of control, which incorporates the processor registers (including the program counter) and a stack.

However, these components can be managed separately.

In this model, thread appears as an execution component within a process.

Multithreading

Several independent threads can coexist in the same process, thus sharing the same address space and the same I/O context.

• This is referred to as multithreading.

Threads can be seen as light weight processes.

Structure of a multithreaded program

Each thread is typically associated to the execution of a function that implements some specific activity.

Communication between threads can be done through the process data structure, which is global from the threads point of view.

• It includes static and dynamic variables (heap memory).

The main program, also represented by a function that implements a specific activity, is the first thread to be created and, in general, the last to be destroyed.

Implementations of multithreading

User level threads

Threads are implemented by a library, at user level, which provides creation and management of threads without kernel intervention.

• versatile and portable.

• when a thread calls a blocking system call, the whole process blocks.

  • because the kernel only sees the process.

Kernel level threads

Threads are implemented directly at kernel level.

• less versatile and less portable.

• when a thread calls a blocking system call, another thread can be schedule to execution.

Advantages of multithreading

Easier implementation of applications - in many applications, decomposing the solution into a number of parallel activities makes the programming model simpler.

• since the address space and the I/O context is shared among all threads, multithreading favors this decomposition.

Better management of computer resources – creating, destroying and switching threads is easier then doing the same with processes.

Better performance – when an application involves substantial I/O, multithreading allows activities to overlap, thus speeding up its execution.

Multiprocessing – real parallelism is possible if multiples CPUs exist.