Threads. They are peers in the same process. Motivation for threads. What is a thread? Fork is expensive.

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1 1 2 Threads They are peers i the same process. What is a thread? A thread is a executio stream withi a process with its ow stack, local variables, ad program couter. There may be more tha oe executio stream i a process. A multi-threaded process ca perform several tasks cocurretly. A thread shares resources with other threads executig i the same address space. All threads executig i the same process address space are called siblig threads. No paret/child relatioship betwee the creator thread ad the createe thread. A thread ca create as may threads as it pleases. But there is o relatioship amog them after creatio. Motivatio for threads Fork is expesive. IPC is required for data exchage betwee paret ad child. A thread is lightweight. Thread creatio is about times faster tha process creatio. Thread resources All threads withi a process share: Text segmet Data segmet Heap Ope files Sigal hadlers Curret workig directory UID ad GID 3 4 However, each thread has its ow (kow as thread-private) TID Set of registers, icludig program couter ad stack poiter Stack (for local variables ad retur addresses) erro variable Sigal mask Priority A thread also has its thread-specific data (TSD). Data structures depedig o applicatios. Thread implemetatios Threads may be implemeted as a kerel-level abstractio (kerel-supported threads), user-level abstractio, or a combiatio of the two. Kerel-supported threads require kerel data structures. The OS is aware of each thread. Kerel threads are required to support user-level threads. The kerel must cotai system-level code for each specified thread fuctio. This approach is good for supportig parallelism with multiple threads ruig o multiple processors. A user-level abstractio is represeted by data structures withi a process ow address space. Does ot require direct support from the OS. It rus o top of the OS ad is trasparet to it. The OS maitais a rutime system to maage thread activities. Has the potetial to execute oly whe associated with a kerel process. User-level threads are multiplexed oto a kerel process for executio.

2 I geeral, user-levels threads are desiged to share resources with other threads withi their ow process space ruig o a sigle processor. The combied model offer both multiplexed ad boud user-level threads. A user-level thread is boud (oe-to-oe mappig) to a kerel thread. A kerel thread is also kow as a lightweight process (LWP). 5 However, it does provide for both multiplexed ad boud threads. The relatios betwee user-level threads ad kerel LWPs may be: 1-to-1 M-to-1 (May-to-Oe) M-to-N (May-to-May) Both implemetatios ca support exactly the same APIs. 6 LWPs are also called virtual processors by some authors. Differet UNIX systems may have their ow implemetatio of threads. Or, multiple user-level threads are multiplexed oto a kerel LWP(s). The umber of kerel LWPs available for the multiplexed user threads may be either implemetatio-depedet, or tuable by the applicatio. The UNIX Threads Iterface does ot defie the implemetatio. POSIX threads are portable amog UNIX systems that are POSIX-compliat. It is kow as IEEE c AKA Pthreads Other major threads implemetatios (ot a complete list): Wi32 Threads 7 8 OS/2 Threads DCE Threads Solaris Threads Thread operatios Thread creatio A thread is created by aother thread. Oce a thread is created, it has its ow set of attributes. Either give by the iitiatig thread or system default. has its ow executio stack. iherits its sigal mask ad schedulig priority from the callig thread. does ot iherit ay pedig sigals. does ot iherit ay TSD data. it pthread_create() Thread executio Threads of a process execute i a sigle UNIX process eviromet. All resources available i this eviromet are shared by the siblig threads ad oe or more thread executio eviromets. A thread executio eviromet cotais the schedulig policy, priority, ad the dispositio of sigals for a thread. I terms of executio, a process has oe or more kerel LWPs that provide the executio vehicle for the threads. The threads of a process are either multiplexed oto a available kerel LWP, or are boud (mapped oe-to-oe) to a specific LWP for executio. I case a kerel call blocks, the correspodig user level thread(s) also blocks.

3 9 10 Thread termiatio A thread termiates whe either its executio reaches the last statemet i the thread, is sigaled to quit or it exits volutarily (a call to pthread_exit()). Whe a thread exits, ormally a siblig ca request the exit status of the termiated thread. However, all threads termiate if oe thread calls exit(), or executio falls off the bottom of mai(). Use pthread_exit() i mai() to avoid premature termiatio of the program. A UNIX process will termiate whe its last thread exits. void pthread_exit() it pthread_cacel() Thread maagemet A thread ca be either detached or odetached. A detached thread will clea up after itself upo termiatio. Resources to retur for reuse iclude its thread structure, TSD array, stack, ad heap. A odetached thread will clea up after itself oly after it has bee joied. Nodetached threads are the default. it pthread_joi() it pthread_detach() Thread sychroizatio Mutual exclusio locks A mutual exclusio (mutex) lock idicates that the use of a shared resource is mutually exclusive betwee competig threads. To use a resource, a thread must first lock the mutex guardig the resource. Whe the use is complete, the thread must ulock the mutex, thereby permittig other threads to use the resource. The sectio of code maipulatig the shared resource is ofte referred to as a critical sectio. The itegrity of the shared resource is esured oly if all threads usig the resource follow the lock-ulock covetio. Coditio variables A coveiet mechaism to otify iterested threads of a evet. How it works: 11 A thread obtais a mutex (a coditio variable always has a associated mutex) ad evaluates the coditio uder the mutex s protectio. If the coditio is true, the thread completes its task, releasig the mutex whe appropriate. If the coditio is false, the mutex is released by the system ad the thread goes to sleep o the coditio variable. Whe the value of the coditio variable is chaged by aother thread, it ca wake up the thread(s) sleepig o the variable. The awakeed thread will reevaluate the coditio variable agai. A typical example of usig a coditio variable would be for a thread to susped its executio util a message is received. it pthread_mutexattr_iit() it pthread_mutexattr_setpshared() 12

4 13 14 it pthread_mutex_iit() it pthread_mutex_lock() it pthread_mutex_ulock() it pthread_mutex_trylock() it pthread_mutex_destroy() Barriers A mechaism for a set of threads to syc up. A barrier is iitialized to the umber of threads to be usig it. Whe a thread reaches it, its executio is suspeded util all of the participatig threads arrive at the barrier. At this poit, all threads are permitted to resume executio. A barrier provides a redezvous poit for threads cooperatig i the barrier. Thread schedulig Commo schedulig policies: First-come-first-serve Shortest-job first Priority-based Roud-robi Global ad local schedulig If a thread is boud (oe-to-oe with a LWP), its schedulig is determied by the kerel schedulig algorithms. It is kow as global schedulig. Its schedulig class is said to have a System Cotetio Scope. If a thread is uboud, the thread library has full cotrol which thread will be scheduled o a LWP. It is kow as local schedulig. It is said to have a Process Cotetio Scope. Schedulig of threads ivolves three factors: Cotetio scope Schedulig policy Thread priority *Note: Most thread implemetatios today use a priority-based, preemptive (a thread ca be removed by a thread of higher priority), otime slicig algorithm to schedule thread activities. It is also recommeded that you, as a programmer, to sped little time thikig about issues of thread schedulig. void pthread_setschedparam() void pthread_getschedparam() TSD maipulatio TSD provides a mechaism of hadlig global data i a thread. TSD is globally accessible to all fuctios i a thread but still uique to the thread. A TSD value is refereced usig a thread specific poiter ad a associated key. 15 To make use of TSD, a thread must create ad bid (associate) the key with the TSD data. The TSD keys i a thread are global to all fuctios i the thread. Also, data storage for TSD eeds to be explicitly allocated i the thread. A destructor fuctio for cleaup ca be specified at the time of creatig a TSD key. However, the curret implemetatio still requires the destructor be ivoked explicitly. To esure data itegrity, mutual exclusio is desired for accessig TSD. Primitives: pthread_key_create() pthread_key_delete() pthread_getspecific() pthread_setspecific() 16

5 17 18 Thread erro hadlig I geeral, pthread fuctios do ot set the stadard UNIX erro variable. Whe a error occurs, the erro value is the retur value of the fuctio. O may eed to use a variable to save the retur value. Therefore, each thread has, i effect, its ow erro variable. Thread attributes Attributes defied ad their values: cotetioscope PTHREAD_SCOPE_PROCESS PTHREAD_SCOPE_SYSTEM detachstate PTHREAD_CREATE_JOINABLE PTHREAD_CREATE_DETACHED stackaddr NULL (valid address) stacksize NULL (withi system limits) priority *Note: The data type for thread priorities is it sched_priority. It is defied i the sched_param structure foud i the header file <sched.h>. However, POSIX gives o advice o how to use the priority levels provided. policy SCHED_OTHER SCHED_FIFO SCHED_RR iheritsched PTHREAD_EXPLICIT_SCHED Gettig/Settig attributes it pthread_attr_iit() it pthread_attr_getscope() it pthread_attr_setscope() it pthread_attr_getdetachstate() it pthread_attr_setdetachstate() it pthread_attr_getstackaddr() it pthread_attr_setstackaddr() it pthread_attr_getstacksize() it pthread_attr_setstacksize() it pthread_attr_getschedparam() it pthread_attr_setschedparam() it pthread_attr_getschedpolicy() it pthread_attr_setschedpolicy() it pthread_attr_getiheritsched() it pthread_attr_setiheritsched() Sigal hadlig i threads Two types of sigals i threads Sychroous: Sigals delivered to the thread that geerated the exceptio. Ex: SIGFPE (divide by zero) Asychroous: Sigals delivered to a ospecific or o-offedig thread. Ex: SIGHUP (hag up) Differet uses of sigals Three applicatios Error reportig Situatio reportig Iterruptio Methods of hadlig sigals These 3 differet situatios are mixed together i sigle threaded processes, ad hadled idifferetly. I multithreaded programmig, the distictios become importat. They are hadled differetly. Oly oe set of sigal hadlers per process No thread-specific sigal hadlers. However, each thread ca have its ow sigal mask. I terms of sigal delivery

6 For error reportig, the thread library guaratees that a sigal will be delivered to the offedig thread. For situatio reportig, the thread library decides which thread should receive a specific sigal ad arrages for the executio of the associated sigal hadler. 21 For iterruptio, there is o geeral method of esurig that a sigal gets delivered to the iteded thread. A dirty fix, mask out the sigal o all but oe thread. Ratioale of havig oe set of sigal hadlers for all threads i a process Sigals are used for asychroous evets. However, multithreadig is itself asychroous eough. A multithreaded program ca simply spaw a ew thread to wait for a evet of iterest. Therefore, to hadle sigals effectively i threads, a programmer, Needs to be cocered of the most is probably the thread sigal mask. Employs a simple solutio by desigatig oe thread to take care of sigals i a process. Maskig out all asychroous sigals o all threads but oe, ad let this oe hadles the asychroous sigals of the process. Be aware that sigal hadlers are processwide. No thread-specific sigal hadlers. it pthread_kill() it pthread_sigmask() it sigwait() it sigtimedwait() it sigwaitifo() 22 Thigs you should kow about threads MT safe fuctios MT safe meas that a fuctio ca be called from multiple threads cocurretly. The fuctio ca be a C library fuctio, a system call, etc. To be MT safe, a fuctio must lock ay shared data it uses. call oly other MT safe fuctios. use the correct error umber (erro). Be aware that erro is process-wide. * Note: It is OK to use a MT usafe fuctio i a MT program, but just do t call it cocurretly. Forkig processes There are two sematics i defiig fork(): Oly the callig thread is replicated (fork1()). 23 POSIX uses this oe. All threads ad LWPs are replicated (forkall()). Be cautious about touchig ay locks that might be held by threads that do ot exist i the child process. Oe may arrive at a deadlock. Suggestio: Have the child process call exec() immediately after the fork1() call to avoid a potetial deadlock. Also, POSIX defies pthread_atfork() to help solve the deadlock-i-the-child problem. Threads are still evolvig. Do t be surprised to kow some thigs workig differetly i the ewer versios of UNIX later. Some relevat web sites