A thread is an independent flow of control that operates within the same address space as other independent flows of controls within a process. In previous versions of AIX, and in most of UNIX systems, thread and process characteristics are grouped into a single entity called a process. In other operating systems, threads are sometimes called "lightweight processes," or the meaning of the word "thread" is sometimes slightly different.
In the following pages, we will learn the differences between a thread and a process, and see what "thread" really means in AIX.
Read the following to learn more about threads in AIX:
In traditional single-threaded process systems, a process has a set of properties. In multi-threaded systems, these properties are divided between processes and threads. For more information, see Thread Properties.
A process in a multi-threaded system is the changeable entity. It must be considered as an execution frame. It has all traditional process attributes, such as:
A process also provides a common address space and common system resources:
A thread is the schedulable entity. It has only those properties that are required to ensure its independent flow of control. These include the following properties:
An example of thread-specific data is the error indicator, errno. In multi-threaded systems, errno is no longer a global variable, but usually a subroutine returning a thread-specific errno value. Some other systems may provide other implementations of errno.
Threads within a process must not be considered as a group of processes. All threads share the same address space. This means that two pointers having the same value in two threads refer to the same data. Also, if any thread changes one of the shared system resources, all threads within the process are affected. For example, if a thread closes a file, the file is closed for all threads.
When a process is created, one thread is automatically created. This thread is called the initial thread. It ensures the compatibility between the old processes with a unique implicit thread and the new multi-threaded processes. The initial thread has some special properties, not visible to the programmer, that ensure binary compatibility between the old single-threaded programs and the multi-threaded operating system. It is also the initial thread that executes the main routine in multi-threaded programs.
A thread is the schedulable entity, which means that the system scheduler handles threads. These threads, known by the system scheduler, are strongly implementation-dependent. To facilitate the writing of portable programs, libraries provide another kind of thread.
A kernel thread is a kernel entity, like processes and interrupt handlers; it is the entity handled by the system scheduler. A kernel thread runs within a process, but can be referenced by any other thread in the system. The programmer has no direct control over these threads, unless writing kernel extensions or device drivers. See AIX 5L Version 5.1 Kernel Extensions and Device Support Programming Concepts for more information about kernel programming.
A user thread is an entity used by programmers to handle multiple flows of controls within a program. The API for handling user threads is provided by a library, the threads library. A user thread only exists within a process; a user thread in process A cannot reference a user thread in process B. The library uses a proprietary interface to handle kernel threads for executing user threads. The user threads API, unlike the kernel threads interface, is part of a portable programming model. Thus, a multi-threaded program developed on an AIX system can easily be ported to other systems.
On other systems, user threads are simply called threads, and lightweight process refers to kernel threads.
User threads are mapped to kernel threads by the threads library. The way this mapping is done is called the thread model. There are three possible thread models, corresponding to three different ways to map user threads to kernel threads.
The mapping of user threads to kernel threads is done using virtual processors. A virtual processor (VP) is a library entity that is usually implicit. For a user thread, the virtual processor behaves as a CPU for a kernel thread. In the library, the virtual processor is a kernel thread or a structure bound to a kernel thread.
In the M:1 model all user threads are mapped to one kernel thread; all user threads run on one VP. The mapping is handled by a library scheduler. All user threads programming facilities are completely handled by the library. This model can be used on any system, especially on traditional single-threaded systems.
In the 1:1 model, each user thread is mapped to one kernel thread; each user thread runs on one VP. Most of the user threads programming facilities are directly handled by the kernel threads.
In the M:N model, all user threads are mapped to a pool of kernel threads; all user threads run on a pool of virtual processors. A user thread may be bound to a specific VP, as in the 1:1 model. All unbound user threads share the remaining VPs. This is the most efficient and most complex thread model; the user threads programming facilities are shared between the threads library and the kernel threads.
The contention scope of a user thread defines how it is mapped to a kernel thread. There are two possible contention scopes:
A system contention scope user thread is a user thread that is directly mapped to one kernel thread. All user threads in a 1:1 thread model have system contention scope.
A process contention scope user thread is a user thread that shares a kernel thread with other (process contention scope) user threads in the process. All user threads in a M:1 thread model have process contention scope.
In an M:N thread model, user threads can have either system or process contention scope. Therefore, an M:N thread model is often referred as a mixed-scope model.
The concurrency level is a property of M:N threads libraries. It defines the number of VPs used to run the process contention scope user threads. This number cannot exceed the number of process contention scope user threads, and is usually dynamically set by the threads library. The system also sets a limit to the number of available kernel threads.
AIX provides a threads library, called libpthreads.a, based on the POSIX 1003.1c industry standard for a portable user threads API. Any program written for use with a POSIX thread library can easily be ported for use with another POSIX threads library; only the performance and very few subroutines of the threads library are implementation-dependent. For this reason, multi-threaded programs written for this version of AIX will work on any future version of AIX.
To enhance the portability of the threads library, the POSIX standard made the implementation of several programming facilities optional. See Threads Library Options for more information about checking the POSIX options.
AIX provides binary compatibility for existing multi-threads applications that were coded to Draft 7 of the POSIX thread standard. These applications will run without re-linking.
The libpthreads_compat.a library is actually provided for program development. AIX 4.3 provides program support for both Draft 7 of the POSIX Thread Standard and Xopen Version 5 Standard, which includes the final POSIX 1003.1c Pthread Standard.
See Developing Multi-Threaded Programs for more information.
Chapter 9, Parallel Programming