Native POSIX Thread Library

From Wikipedia, the free encyclopedia

In the GNU/Linux operating system, the Native POSIX Thread Library (NPTL) is a software feature that enables the Linux kernel to run programs written to use POSIX Threads very efficiently.

In tests, NPTL succeeded in starting 100,000 threads on a IA-32 in two seconds. In comparison, this test under a kernel without NPTL would have taken around 15 minutes [1][2].

1 History
2 Determining version
3 Applications using NPTL
4 See also
5 External links

[edit] History

Before the 2.6 version of the Linux kernel, processes were schedulable entities, and there was no real support for threads. However, it did support a system call - clone() - which creates a copy of the calling process, where the copy shares the address space of the caller. The LinuxThreads project used this system call to simulate thread support entirely in userland. Unfortunately, it had a number of issues with true POSIX compliance, particularly in the areas of signal handling, scheduling, and inter-process synchronization primitives.

To improve upon LinuxThreads, it was clear that some kernel support and a re-written threads library would be required. Two competing projects were started to address the requirement: NGPT (Next Generation POSIX Threads) worked on by a team which included developers from IBM, and NPTL by developers at Red Hat. NGPT was abandoned in mid-2003, at about the same time when NPTL was released.

NPTL uses a similar approach to LinuxThreads, in that the primary abstraction known by the kernel is still a process, and new threads are created with the clone() system call (called from the NPTL library). However, NPTL requires specialized kernel support to implement (for example) the contended case of synchronisation primitives which might require threads to sleep and be re-awoken. The primitive used for this is known as a futex.

NPTL is a so-called 1×1 threads library, in that threads created by the user (via the pthread_create() library function) are in 1-1 correspondence with schedulable entities in the kernel (processes, in the Linux case). This is the simplest possible threading implementation.

An alternative to NPTL's 1×1 model is the m×n model where there are typically more userland threads than schedulable entities. In this implementation, the threading library is responsible for scheduling user threads on the available schedulable entities; this makes context switching of threads very fast, as it avoids system calls. However, this increases complexity and the likelihood of priority inversion, as well as suboptimal scheduling without extensive (and expensive) coordination between the userland scheduler and the kernel scheduler.

NPTL was first released in Red Hat Linux 9. Old-style Linux POSIX threading is known for having trouble with threads that refuse to yield to the system occasionally because it does not take the opportunity to preempt them when it arises, something that Windows was known to do better at the time. Red Hat claimed that NPTL fixed this problem in an article on the Java website about Java on Red Hat Linux 9.

NPTL has been part of Red Hat Enterprise Linux since version 3, and is now a fully integrated part of glibc.

[edit] Determining version

Because NPTL is being introduced gradually to different distributions of Linux, and in particular patched versions of Red Hat Linux, one command line test to determine the presence in the kernel is:

getconf GNU_LIBPTHREAD_VERSION

This will report the default threading library on any distribution of Linux.

[edit] Applications using NPTL

Linux version of Google Earth (client) lists NPTL as a requisite. Linux version of Firstclass Server lists NTPL as a prerequisite. It is assumed that that's a typo and that it is in fact NPTL that is required on 2.4 kernels.