Native POSIX Thread Library
Encyclopedia
The Native POSIX Thread Library (NPTL) is a software feature that enables the Linux kernel
to run programs written to use POSIX Threads
efficiently.
, processes were the 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 provide kernel-level thread support (most of the previous pthread implementations in Linux worked 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 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 in the Linux kernel since version 2.6. It is now a fully integrated part of the GNU C Library
.
There exists a tracing tool for NPTL, called POSIX Thread Trace Tool (PTT). And an Open POSIX Test Suite (OPTS) was written for testing the NPTL library against the POSIX standard.
(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 wake again. 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
An alternative to NPTL's 1×1 model is the m×n model.
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
to run programs written to use POSIX Threads
POSIX Threads
POSIX Threads, usually referred to as Pthreads, is a POSIX standard for threads. The standard, POSIX.1c, Threads extensions , defines an API for creating and manipulating threads....
efficiently.
History
Before the 2.6 version of the Linux kernelLinux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
, processes were the schedulable entities, and there was no real support for threads
Thread (computer science)
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
. However, it did support a system call
System call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
— clone
Clone (Linux system call)
clone is a system call on the Linux kernel related to multithreading. In practice, one should try to avoid calling clone directly, but instead use a threading library which use clone when starting a thread .The syntax for calling clone under a Linux program is: #include int clone clone is a...
— which creates a copy of the calling process where the copy shares the address space of the caller. The LinuxThreads
LinuxThreads
In the Linux operating system, LinuxThreads was a partial implementation of POSIX Threads that has since been superseded by the Native POSIX Thread Library ....
project used this system call to provide kernel-level thread support (most of the previous pthread implementations in Linux worked 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
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
, and NPTL by developers at Red Hat
Red Hat
Red Hat, Inc. is an S&P 500 company in the free and open source software sector, and a major Linux distribution vendor. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina with satellite offices worldwide....
. NGPT was abandoned in mid-2003, at about the same time when NPTL was released.
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
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
website about Java on Red Hat Linux 9.
NPTL has been part of Red Hat Enterprise Linux
Red Hat Enterprise Linux
Red Hat Enterprise Linux is a Linux-based operating system developed by Red Hat and targeted toward the commercial market. Red Hat Enterprise Linux is released in server versions for x86, x86-64, Itanium, PowerPC and IBM System z, and desktop versions for x86 and x86-64...
since version 3, and in the Linux kernel since version 2.6. It is now a fully integrated part of the GNU C Library
GNU C Library
The GNU C Library, commonly known as glibc, is the C standard library released by the GNU Project. Originally written by the Free Software Foundation for the GNU operating system, the library's development has been overseen by a committee since 2001, with Ulrich Drepper from Red Hat as the lead...
.
There exists a tracing tool for NPTL, called POSIX Thread Trace Tool (PTT). And an Open POSIX Test Suite (OPTS) was written for testing the NPTL library against the POSIX standard.
Design
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 callSystem call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
(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 wake again. The primitive used for this is known as a futex
Futex
A futex is a Linux construct that can be used to implement basic locking, or as a building block for higher-level locking abstractions such as semaphores and POSIX mutexes or condition variables....
.
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 (tasks, in the Linux case). This is the simplest possible threading implementation.An alternative to NPTL's 1×1 model is the m×n model.
See also
- LinuxThreadsLinuxThreadsIn the Linux operating system, LinuxThreads was a partial implementation of POSIX Threads that has since been superseded by the Native POSIX Thread Library ....
- Library (computer science)Library (computer science)In computer science, a library is a collection of resources used to develop software. These may include pre-written code and subroutines, classes, values or type specifications....
- Green threadsGreen threadsIn computer programming, green threads are threads that are scheduled by a virtual machine instead of natively by the underlying operating system...
External links
- NPTL Design paper
- NPTL Trace Tool OpenSource tool to trace and debug multithreaded applications using the NPTL.