POSIX Thread Libraries
Multi-threading capability is included in the Linux 2.0 kernel. There is an ongoing effort to refine this and make the kernel more reentrant. The multi-thread capability is offered by the clone system call, which creates a new context of execution. This call can be used to create a new process, a new thread or one of a range of possibilities which doesn't fit into either of these categories. The fork system call is actually a call to clone with a specific set of values as parameters, and the pthread_create function call could be a call to clone with a different set of values as parameters.
The clone system call has several flags to indicate how much will be shared between threads. Figure 2 lists each flag.
Threading capabilities are given by different threads packages. Several threading libraries are available for Linux. All libraries referred to in this article make use of POSIX-compliant functionality; however, at the time of this writing, there are no fully POSIX-compliant multi-threading libraries available for Linux.
The libraries we have evaluated are the following:
Provenzano threads (PT): this package offers a user-level POSIX threads library with thread-blocking system calls (read, write, connect, sleep, wait, etc.) and a thread-safe C library (stdio, network utilities, etc.). This implementation currently supports basic functionality, synchronization primitives, thread-specific data and thread attributes.
FSU_Pthreads (FSUT): this is a C library which implements user-level POSIX threads for different operating systems: Solaris 2.x, SCO UNIX, FreeBSD, Linux and DOS. This implementation supports thread management, synchronization, thread-specific data, thread priority scheduling, signals and cancellation.
PC threads (PCT): this is a user-level POSIX threads library that includes non-blocking select, read and write. This library has runtime configurable parameters, such as clock interrupt interval, default thread-scheduling policy, default thread-stack size and the I/O polling interval, among others.
CLthreads (CLT). CLthreads is a kernel-level POSIX-compliant library on top of Linux. CLthreads uses the clone system call to take full advantage of multiprocessor systems.
LinuxThreads (LT). LinuxThreads is an implementation of POSIX threads for Linux. LinuxThreads provides kernel-level threads: they are created using the clone system call, and all scheduling is done in the kernel. This approach can take full advantage of multiprocessor systems. It also results in a simpler, more robust thread library, especially for blocking system calls.
Solaris supports a hybrid model of threads—user-level and kernel-level threads—and a POSIX-compliant library. User-level threads are supported by the library for their creation and scheduling, and the kernel is not aware of these threads. Solaris defines an intermediate level of threads as well, the lightweight process (LWP). LWPs are between user-level threads and kernel-level threads (see Figure 3). LWPs are manipulated by the thread library. The user-level threads are multiplexed onto the LWPs of the process (each process contains at least one LWP), and only threads currently connected to LWPs accomplish work. The rest are either blocked or waiting for an LWP on which they can run.
There is a kernel-level thread for each LWP, and some kernel-level threads run on the kernel's behalf and have no associated LWP. Kernel-level threads are the only objects scheduled within the system.
With this model, any process may have many user-level threads. These user-level threads may be scheduled and switched among kernel-supported lightweight processes without the intervention of the kernel. Each LWP is connected exactly to one kernel-level thread. Many LWPs are in a process, but they are needed only when threads need to communicate with the kernel; if one blocks, the others can continue to execute within the process. In Solaris, users can create new threads permanently bound to an LWP.
Figure 4 summarizes the user-level and kernel-level features of the evaluated libraries.