Light-weight process
Encyclopedia
In computer operating system
s, a light-weight process (LWP) is a means of achieving multitasking
. In the traditional meaning of the term, as used in Unix System V
and Solaris, an LWP runs in user space
on top of a single kernel thread and shares its address space
and system resources with other LWPs within the same process. Multiple user level threads, managed by a thread library, can be placed on top of one or many LWPs - allowing multitasking to be done at the user level, which can have some performance benefits.
In some operating systems there is no separate LWP layer between kernel threads and user threads. This means that user threads are implemented directly on top of kernel threads. In those contexts, the term "light-weight process" typically refers to kernel threads and the term "threads" can refer to user threads. On Linux
, user threads are implemented by allowing certain processes to share resources, which sometimes leads to these processes to be called "light weight processes". Similarly, in SunOS
version 4 onwards (prior to Solaris) "light weight process" referred to user threads.
For this reason, some user level thread libraries allow multiple user threads to be implemented on top of LWPs. User threads can be created, destroyed, synchronized and switched between entirely in user space without system calls and switches into kernel mode. This provides a significant performance improvement in thread creation time and context switches. However, there are difficulties in implementing a user level thread scheduler that works well together with the kernel.
One solution to this problem is scheduler activation. This is a method for the kernel and the thread library to cooperate. The kernel notifies the thread library's scheduler about certain events (such as when a thread is about to block) and the thread library can make a decision on what action to take. The notification call from the kernel is called an "upcall".
A user level library has no control over the underlying mechanism, it only receives notifications from the kernel and schedules user threads onto available LWPs, not processors. The kernel's scheduler then decides how to schedule the LWPs onto the processors. This means that LWPs can be seen by the thread library as "virtual processors".
UNIX System V
and its modern derivatives IRIX
, SCO OpenServer
, HP-UX
and IBM AIX allow a many-to-many mapping between user threads and LWPs.
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s, a light-weight process (LWP) is a means of achieving multitasking
Computer multitasking
In computing, multitasking is a method where multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for...
. In the traditional meaning of the term, as used in Unix System V
UNIX System V
Unix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
and Solaris, an LWP runs in user space
User space
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
on top of a single kernel thread and shares its address space
Virtual address space
Virtual address space is a memory mapping mechanism available in modern operating systems such as OpenVMS, UNIX, Linux, and Windows NT...
and system resources with other LWPs within the same process. Multiple user level threads, managed by a thread library, can be placed on top of one or many LWPs - allowing multitasking to be done at the user level, which can have some performance benefits.
In some operating systems there is no separate LWP layer between kernel threads and user threads. This means that user threads are implemented directly on top of kernel threads. In those contexts, the term "light-weight process" typically refers to kernel threads and the term "threads" can refer to user threads. On Linux
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....
, user threads are implemented by allowing certain processes to share resources, which sometimes leads to these processes to be called "light weight processes". Similarly, in SunOS
SunOS
SunOS is a version of the Unix operating system developed by Sun Microsystems for their workstation and server computer systems. The SunOS name is usually only used to refer to versions 1.0 to 4.1.4 of SunOS...
version 4 onwards (prior to Solaris) "light weight process" referred to user threads.
Kernel threads
Kernel threads are handled entirely by the kernel. They need not be associated to a process, a kernel can create them whenever it needs to perform a particular task. Kernel threads cannot execute in user mode, LWPs (in systems where they are a separate layer) bind to kernel threads and provide a user-level context. This includes a link to the shared resources of the process to which the LWP belongs to. When an LWP is suspended, it needs to store its user-level registers until it resumes, the underlying kernel thread must also store its own kernel-level registers.Performance
LWPs are slower and more expensive to create than user threads. Whenever an LWP is created a system call must first be made to create a corresponding kernel thread, causing a switch to kernel mode. These mode switches would typically involve copying parameters between kernel and user space, also the kernel may need to have extra steps to verify the parameters to check for invalid behaviour. A context switch between LWPs means that the LWP that is being pre-empted has to save its registers, then go into kernel mode for the kernel thread to save its registers, and the LWP that is being scheduled must restore the kernel and user registers separately also.For this reason, some user level thread libraries allow multiple user threads to be implemented on top of LWPs. User threads can be created, destroyed, synchronized and switched between entirely in user space without system calls and switches into kernel mode. This provides a significant performance improvement in thread creation time and context switches. However, there are difficulties in implementing a user level thread scheduler that works well together with the kernel.
Scheduler activation
While the user threading library will schedule user threads, the kernel will schedule the underlying LWPs. Without coordination between the kernel and the thread library the kernel can make sub-optimal scheduling decisions. Further, it is possible for cases of deadlock to occur when user threads distributed over several LWPs try to acquire the same resources that are used by another user thread that is not currently running.One solution to this problem is scheduler activation. This is a method for the kernel and the thread library to cooperate. The kernel notifies the thread library's scheduler about certain events (such as when a thread is about to block) and the thread library can make a decision on what action to take. The notification call from the kernel is called an "upcall".
A user level library has no control over the underlying mechanism, it only receives notifications from the kernel and schedules user threads onto available LWPs, not processors. The kernel's scheduler then decides how to schedule the LWPs onto the processors. This means that LWPs can be seen by the thread library as "virtual processors".
Supporting operating systems
Solaris has implemented a separate LWP layer since version 2.2. Prior to version 9, Solaris allowed a many-to-many mapping between LWPs and user threads. However, this was retired due to the complexities it introduced and performance improvements to the kernel scheduler.UNIX System V
UNIX System V
Unix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
and its modern derivatives IRIX
IRIX
IRIX is a computer operating system developed by Silicon Graphics, Inc. to run natively on their 32- and 64-bit MIPS architecture workstations and servers. It was based on UNIX System V with BSD extensions. IRIX was the first operating system to include the XFS file system.The last major version...
, SCO OpenServer
SCO OpenServer
SCO OpenServer, previously SCO UNIX and SCO Open Desktop , is, misleadingly, a closed source version of the Unix computer operating system developed by Santa Cruz Operation and now maintained by the SCO Group....
, HP-UX
HP-UX
HP-UX is Hewlett-Packard's proprietary implementation of the Unix operating system, based on UNIX System V and first released in 1984...
and IBM AIX allow a many-to-many mapping between user threads and LWPs.
See also
- Green threadsGreen threadsIn computer programming, green threads are threads that are scheduled by a virtual machine instead of natively by the underlying operating system...
- Computer multitaskingComputer multitaskingIn computing, multitasking is a method where multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for...
- Thread (computer science)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...
- Process (computer science)
- Light Weight Kernel ThreadsLight Weight Kernel ThreadsLight Weight Kernel Threads or LWKT is a term from computer science in general and in DragonFlyBSD in particular. LWKTs differ from normal kernel threads in that they can preempt normal kernel threads. According to Matt Dillon, DragonFlyBSD creator:...
- clone (Linux system call)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...
- POSIX ThreadsPOSIX ThreadsPOSIX 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....
- Fiber (computer science)Fiber (computer science)In computer science, a fiber is a particularly lightweight thread of execution.Like threads, fibers share address space. However, fibers use co-operative multitasking while threads use pre-emptive multitasking. Threads often depend on the kernel's thread scheduler to preempt a busy thread and...