thread (computer science)
{{About||the form of code consisting entirely of
subroutine calls|Threaded code|the collection of posts|Internet forum#Thread}}
thumb|A process with two threads of execution.In
computer science, a
thread of execution is a
fork of a
computer program into two or more
concurrently running
tasks. The implementation of threads and
processes differs from one
operating system to another, but in general, a thread is contained inside a process and different threads in the same process share some resources (most commonly
memory), while different
processes do not.On a single processor,
multithreading generally occurs by
time-division multiplexing (as in
multitasking): the
processor switches between different threads. This
context switching generally happens frequently enough that the user perceives the threads or tasks to be running at the same time. On a
multiprocessor or
multi-core system, the threads or tasks actually do run at the same time, with each processor or core running a particular thread or task. However, if you are using a scripting language such as
Python, the script interpreter may implement a software lock that does not permit an interpreter instance to run different threads at the same time on multiple cores; the cPython interpreter implements a
Global Interpreter Lock (GIL) that disables the processing of multiple threads on multiple cores at one time. cPython`s threads are user threads; on the other hand, kernel threads will indeed enable true concurrency.Many modern
operating systems directly support both time-sliced and multiprocessor threading with a process
scheduler. The operating system
kernel allows programmers to manipulate threads via the
system call interface. Some implementations are called a
kernel thread, whereas a
lightweight process (LWP) is a specific type of
kernel thread that shares the same state and information.Programs can
user-space thread when threading with timers, signals, or other methods to interrupt their own execution, performing a sort of
ad-hoc time-slicing.
Threads compared with processes
Threads are distinguished from traditional
multitasking operating system processes in that processes:
Multiple threads, on the other hand, typically share the state information of a process, and share
memory and other
resources directly.
Context switching between threads in the same process is typically faster than context switching between processes. Systems like
Windows NT and
OS/2 are said to have "cheap" threads and "expensive" processes; in other operating systems there is not so great a difference except the cost of
address space change which implies a
TLB flush. Multithreading is a popular programming and execution model that allows multiple threads to exist within the context of a single process, sharing the process' resources but able to execute independently. The threaded programming model provides developers with a useful abstraction of concurrent execution. However, perhaps the most interesting application of the technology is when it is applied to a
single process to enable
parallel execution on a
multiprocessor system.This advantage of a multithreaded program allows it to operate faster on
computer systems that have multiple
CPUs, CPUs with multiple cores, or across a
cluster of machines. This is because the threads of the program naturally lend themselves to truly
concurrent execution. In such a case, the
programmer needs to be careful to avoid
race conditions, and other non-intuitive behaviors. In order for data to be correctly manipulated, threads will often need to
rendezvous in time in order to process the data in the correct order. Threads may also require
atomic operations (often implemented using
semaphores) in order to prevent common data from being simultaneously modified, or read while in the process of being modified. Careless use of such primitives can lead to
deadlocks.Operating systems schedule threads in one of two ways.
Preemptive multithreading is generally considered the superior approach, as it allows the operating system to determine when a
context switch should occur.
Cooperative multithreading, on the other hand, relies on the threads themselves to relinquish control once they are at a stopping point. This can create problems if a thread is waiting for a resource to become available. The disadvantage to preemptive multithreading is that the system may make a context switch at an inappropriate time, causing
priority inversion or other bad effects which may be avoided by cooperative multithreading.Traditional mainstream computing hardware did not have much support for multithreading as switching between threads was generally already quicker than full process
context switches. Processors in
embedded systems, which have higher requirements for real-time behaviors, might support multithreading by decreasing the thread switch time, perhaps by allocating a dedicated register file for each thread instead of saving/restoring a common register file. In the late 1990s, the idea of executing instructions from multiple threads simultaneously has become known as
simultaneous multithreading. This feature was introduced in
Intel's
Pentium 4 processor, with the name ''
hyper threadingProcesses, kernel threads, user threads, and fibers
A
process is the "heaviest" unit of kernel scheduling. Processes own
resources allocated by the operating system. Resources include memory,
file handles, sockets, device handles, and windows. Processes do not share address spaces or file resources except through explicit methods such as inheriting file handles or shared memory segments, or mapping the same file in a shared way. Processes are typically preemptively multitasked. However, Windows 3.1 and older versions of Mac OS used cooperative or non-preemptive multitasking.A
kernel thread is the "lightest" unit of kernel scheduling. At least one kernel thread exists within each process. If multiple kernel threads can exist within a process, then they share the same memory and file resources. Kernel threads are preemptively multitasked if the operating system's process
scheduler is preemptive. Kernel threads do not own resources except for a
stack, a copy of the
registers including the
program counter, and
thread-local storage (if any).Threads are sometimes implemented in
userspace libraries, thus called
user threads. The kernel is not aware of them, they are managed and scheduled in
userspace. Some implementations base their
user threads on top of several
kernel threads to benefit from
multi-processor machines (N:M model). In this article the term "thread" (without kernel or user qualifier) defaults to referring to kernel threads. User threads as implemented by
virtual machines are also called
green threads.
Fibers are an even lighter unit of scheduling which are
cooperatively scheduled: a running fiber must explicitly "yield" to allow another fiber to run, which makes their implementation much easier than kernel or user threads. A fiber can be scheduled to run in any thread in the same process. This permits applications to gain performance improvements by managing scheduling themselves, instead of relying on the kernel scheduler (which may not be tuned for the application). Microsoft SQL Server 2000's user mode scheduler, running in fiber mode, is an example of doing this. Parallel programming environments such as
OpenMP typically implements their tasks through fibers.
Thread and fiber issues
Concurrency and data structures
Threads in the same process share the same address space. This allows concurrently-running code to
couple tightly and conveniently exchange data without the overhead or complexity of an
IPC. When shared between threads, however, even simple data structures become prone to
race hazards if they require more than one CPU instruction to update: two threads may end up attempting to update the data structure at the same time and find it unexpectedly changing underfoot. Bugs caused by race hazards can be very difficult to reproduce and isolate.To prevent this, threading APIs offer synchronization primitives such as
mutexes to
lock data structures against concurrent access. On uniprocessor systems, a thread running into a locked mutex must sleep and hence trigger a context switch. On multi-processor systems, the thread may instead poll the mutex in a
spinlock. Both of these may sap performance and force processors in
SMP systems to contend for the memory bus, especially if the
granularity of the locking is fine.
I/O and scheduling
User thread or fiber implementations are typically entirely in
userspace. As a result, context switching between user threads or fibers within the same process is extremely efficient because it does not require any interaction with the kernel at all: a context switch can be performed by locally saving the CPU registers used by the currently executing user thread or fiber and then loading the registers required by the user thread or fiber to be executed. Since scheduling occurs in userspace, the scheduling policy can be more easily tailored to the requirements of the program's workload.However, the use of blocking system calls in user threads or fibers can be problematic. If a user thread or a fiber performs a system call that blocks, the other user threads and fibers in the process are unable to run until the system call returns. A typical example of this problem is when performing I/O: most programs are written to perform I/O synchronously. When an I/O operation is initiated, a system call is made, and does not return until the I/O operation has been completed. In the intervening period, the entire process is "blocked" by the kernel and cannot run, which starves other user threads and fibers in the same process from executing.A common solution to this problem is providing an I/O API that implements a synchronous interface by using non-blocking I/O internally, and scheduling another user thread or fiber while the I/O operation is in progress. Similar solutions can be provided for other blocking system calls. Alternatively, the program can be written to avoid the use of synchronous I/O or other blocking system calls.SunOS 4.x implemented "
light-weight processes" or LWPs.
NetBSD 2.x+, and
DragonFly BSD implement LWPs as kernel threads (1:1 model).
SunOS 5.2 through
SunOS 5.8 as well as NetBSD 2 to NetBSD 4 implemented a two level model, multiplexing one or more user level threads on each kernel thread (M:N model).
SunOS 5.9 and later, as well as NetBSD 5 eliminated user threads support, returning to a 1:1 model.
weblinkThe use of kernel threads simplifies user code by moving some of the most complex aspects of threading into the kernel. The program doesn't need to schedule threads or explicitly yield the processor. User code can be written in a familiar procedural style, including calls to blocking APIs, without starving other threads. However, kernel threading on uniprocessor systems may force a context switch between threads at any time, and thus expose race hazards and concurrency bugs that would otherwise lie latent. On SMP systems, this is further exacerbated because kernel threads may actually execute concurrently on separate processors.
Models
1:1
1:1 threads created by the user are in 1-1 correspondence with schedulable entities in the kernel. This is the simplest possible threading implementation. On
Linux, the
usual C library implements this approach (via the
NPTL).
N:M
N:M maps some N number of application threads onto some M number of kernel entities, or "virtual processors". This is a compromise between kernel-level ("1:1") and user-level ("N:1") threading. In general, "N:M" threading systems are more complex to implement than either kernel or user threads, because both changes to kernel and user-space code are required. In the N:M 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.
N:1
An N:1 model implies that all application-level threads map on to a single kernel-level scheduled entity; the kernel has no knowledge of the application threads. With this approach, context switching can be done very fast and, in addition, it can be implemented even on simple kernels which do not support threading. One of the major drawbacks however is that it cannot benefit from the hardware acceleration on
multi-threaded processors or
multi-processor computers: there is never more than one thread being scheduled at the same time.
Implementations
There are many different and incompatible implementations of threading. These include both kernel-level and user-level implementations. They however often follow more or less closely the
POSIX Threads interface.
Kernel-level implementation examples
User-level implementation examples
Hybrid implementation examples
- Scheduler activations used by the NetBSD native POSIX threads library implementation (an N:M model as opposed to a 1:1 kernel or userspace implementation model)
- Marcel from the PM2 project.
Fiber implementation examples
Fibers can be implemented without operating system support, although some operating systems or libraries provide explicit support for them.
- Win32 supplies a fiber API(1) (Windows NT 3.51 SP3 and later)
See also
References
-
[CreateFiber, MSDN]
- David R. Butenhof: Programming with POSIX Threads, Addison-Wesley, ISBN 0-201-63392-2
- Bradford Nichols, Dick Buttlar, Jacqueline Proulx Farell: Pthreads Programming, O'Reilly & Associates, ISBN 1-56592-115-1
- Charles J. Northrup: Programming with UNIX Threads, John Wiley & Sons, ISBN 0-471-13751-0
- Mark Walmsley: Multi-Threaded Programming in C++, Springer, ISBN 1-85233-146-1
- Paul Hyde: Java Thread Programming, Sams, ISBN 0-672-31585-8
- Bill Lewis: Threads Primer: A Guide to Multithreaded Programming, Prentice Hall, ISBN 0-13-443698-9
- Steve Kleiman, Devang Shah, Bart Smaalders: Programming With Threads, SunSoft Press, ISBN 0-13-172389-8
- Pat Villani: Advanced WIN32 Programming: Files, Threads, and Process Synchronization, Harpercollins Publishers, ISBN 0-87930-563-0
- Jim Beveridge, Robert Wiener: Multithreading Applications in Win32, Addison-Wesley, ISBN 0-201-44234-5
- Thuan Q. Pham, Pankaj K. Garg: Multithreaded Programming with Windows NT, Prentice Hall, ISBN 0-13-120643-5
- Len Dorfman, Marc J. Neuberger: Effective Multithreading in OS/2, McGraw-Hill Osborne Media, ISBN 0-07-017841-0
- Uresh Vahalia: Unix Internals: the New Frontiers, Prentice Hall, ISBN 0-13-101908-2
- Alan L. Dennis: .Net Multithreading , Manning Publications Company, ISBN 1-930110-54-5
- Tobin Titus, Fabio Claudio Ferracchiati, Srinivasa Sivakumar, Tejaswi Redkar, Sandra Gopikrishna: C Threading Handbook, Peer Information Inc, ISBN 1-86100-829-5
- Tobin Titus, Fabio Claudio Ferracchiati, Srinivasa Sivakumar, Tejaswi Redkar, Sandra Gopikrishna: Visual Basic .Net Threading Handbook, Wrox Press Inc, ISBN 1-86100-713-2
External links
{{Parallel Computing}}
خيط (حاسوب)Fil d'execucióVlákno (program)Thread (Informatik)LõimHilo de ejecuciónProcessus léger스레드ThreadÞráðurThread (informatica)תהליכוןSzál (számítástechnika)Thread (informatica)スレッド (コンピュータ)Tråd (informatikk)Wątek (informatyka)Thread (ciência da computação)МногопоточностьThread (multithreading)Нит (рачунарство)Tråd (datavetenskap)ThreadНить多线程
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