Threads 1 Processes versus Threads 2 Why Threads

Threads 1

Processes versus Threads 2

Why Threads? Processes do not share resources very well Why? Process context switching cost is very high Why? Thread: a light-weighted process A sequence of execution 3

Threads: Lightweight Processes Environment (resource) execution 4

Real Life Example? o Process q q o “system programming” course Different from “internet engineering” Thread q q q homework, Reading, Self-assessment quiz Each is a different “execution” But all share o o o q Content Textbook Personnel (TAs, instructors) Affect each other 5

Review: threads vs. processes (created with fork) Property Processes created with fork Threads of a process Ordinary function calls variables get copies of all variables share global variables IDs get new process IDs share the same process ID but have unique thread ID share the same process ID (and thread ID) Communication Must explicitly communicate, e. g. pipes or use small integer return value May communicate with return value or shared variables if done carefully May communicate with return value or shared variables (don't have to be careful) Parallelism (one CPU) Concurrent Sequential Parallelism (multiple CPUs) May be executed simultaneously Kernel threads may be executed simultaneously Sequential 6

Thread components o A thread has its own program counter and stack, but shares a number of resources with its process and other threads of the process: q q q o address space: code and global variables open files semaphores signals timers process ID Thread specific resource: q q q Thread ID Program counter Register set Stack space Signal mask (later…) 7

Thread vs. Process Each thread execute separately Threads in the same process share resources No protection among threads!! 8

Storage for Threads 9

Thread Model : Context Switch o o Extensive sharing makes CPU switching among peer threads and creation of threads inexpensive compared to processes Thread context switch still requires q q o Register set switch But no memory management related work!!! Why need to switch from one thread (process) to another? q q Some thread (process) may block for I/O Many threads (processes) share limited #CPUs 10

Thread State o Threads states are q q o Ready Blocked Running Terminated Why these states? q Threads share CPU o q On single processor machine only one thread can run at a time Threads can block waiting for a system call to be completed 11

Creating a Thread o When a new thread is created it runs concurrently with the creating process. o When creating a thread you indicate which function the thread should execute. 12

Normal function call 13

Threaded function call 14

Benefits of Threads o Responsiveness q o Resource sharing q o Sharing of memory, files and other resources of the process to which the threads belong Economy q o Multi-threading allows applications to run even if part of it is blocked Much more costly and time consuming to create and manage processes than threads Utilization of multiprocessor architectures q Each thread can run in parallel on a different processor 15

Thread Creation vs. Process Creation Time in seconds for 50000 fork or thread creations 16

Threaded Function Call Detail o A function that is used as a thread must have a special format. o It takes a single parameter and returns a single parameter. o Can point to a structure, so in effect, the function can use any number of parameters. http: //www. llnl. gov/computing/tutorials/pthreads/ 17

An Example The thread function casts and unpacks the first argument: void * my. Worker. Function(void *arg) { int fd 1 = ((int *)arg) [0]; int fd 2 = ((int *)arg) [1]; } 18

Pthread Operations POSIX function description pthread_create a thread pthread_detach set thread to release resources pthread_equal test two thread IDs for equality pthread_exit a thread without exiting process pthread_kill send a signal to a thread pthread_join wait for a thread pthread_self find out own thread ID 19

pthread_* return values Unlike most POSIX functions, they do not set errno but the value returned when an error occurs has the value that errno would have. None of the POSIX thread functions ever return the error EINTR. 20

Example program #include <phtread. h> #include <thread. h> #include <stdio. h> void *threadex(void *); int main() { pthread_t tid; /* stores the new thread ID */ pthread_create(&tid, NULL, threadex, NULL); /*create a new thread*/ pthread_join(tid, NULL); /*main thread waits for other thread to terminate */ return 0; /* main thread exits */ } void *threadex(void *arg) /*thread routine*/ { int i; for (i=0; i<5; i++) fprintf(stderr, `Hello, world! n''); return NULL; } 21

Thread Usage: word processor What if it is single-threaded? 22

Thread Usage: Web Server 23

Web Server Rough outline of code for previous slide (a) Dispatcher thread (b) Worker thread 24

Designing Threaded Programs Thread candidates? Discrete, independent tasks which can execute concurrently E. g. if routine 1 and routine 2 can be interchanged, interleaved and/or overlapped in real time, they are candidates for threading 25

Tasks Suitable for threading o o o Block for potentially long waits Use many CPU cycles Must respond to asynchronous events Are of lesser or greater importance than other tasks Are able to be performed in parallel with other tasks 26

Common Multi-thread Software Architectures o Manager/worker q q o Pipeline: q q o a single thread, the manager assigns work to other threads, the workers. Typically, the manager handles all input and parcels out work to the other tasks a task is broken into a series of suboperations, each of which is handled in series, but concurrently, by a different thread. An automobile assembly line best describes this model Peer q similar to the manager/worker model, but after the main thread creates other threads, it participates in the work 27

A Challenge: Making Single-Threaded Code Multithreaded Conflicts between threads over the use of a global variable 28

A solution: Private Global Variables 29

Thread Packages o Kernel thread packages q o Implemented and supported at kernel level User-level thread packages q Implemented at user level 30

User-level Thread 31

User Level Threads o User-level threads without direct O/S support q q Threads are invisible to the kernel Simpler kernel implementation Can only use one processor at a time Implementation dependent o Some threads can block other threads o (or) Requires a special library of system calls to prevent blocking 32

User-level Threads o Advantages q Fast Context Switching: o o User level threads are implemented using user level thread libraries, hence no call to OS and no interrupts to kernel One key difference with processes: n n o q when a thread is finished running for the moment, it can call thread_yield. This instruction § (a) saves the thread information in the thread table itself, and § (b) calls the thread scheduler to pick another thread to run. The procedure that saves the local thread state and the scheduler are local procedures, hence no trap to kernel, no context switch, no memory switch, and this makes the thread scheduling very fast. Customized Scheduling 33

Kernel-level Threads o o Kernel can schedule threads in addition to processes. Multiple threads of a process can run simultaneously on multiple CPUs. Synchronization more efficient than for processes (but less than for user-level threads). Kernel-level threads can make blocking I/O calls without blocking other threads of same process 34

Kernel-Level Thread 35

Trade-offs (review)? o Kernel thread packages q q o Each thread can make blocking I/O calls Can run concurrently on multiple processors Threads in User-level q q Fast context switch Customized scheduling 36

Implementing Threads in User Space (old Linux) A user-level threads package 37

Hybrid Implementations (Solaris) Multiplexing user-level threads onto kernellevel threads 38

What’s POSIX Got To Do With It? o o Each OS had it’s own thread library and style That made writing multithreaded programs difficult because: q you had to learn a new API with each new OS q you had to modify your code with each port to a new OS POSIX (IEEE 1003. 1 c-1995) provided a standard known as Pthreads Unix International (UI) threads (Solaris threads) are available on Solaris (which also supports POSIX threads) 39

Pthreads--- POSIX Threads o It is a standard API q q o Supported by most vendors General concepts applicable to other thread APIs (java threads, NT threads, etc). Low level functions 40

Creating a thread with pthread o A thread is created with int pthread_create( pthread_t *restrict thread, const pthread_attr_t *restrict attr, void *(*start_routine)(void *), void *restrict arg); o o o The creating process (or thread) must provide a location for storage of the thread id. The third parameter is just the name of the function for the thread to run. The last parameter is a pointer to the arguments. 41

Restrict Keyword o o One of the new features in the recently approved C standard C 99 This qualifier can be applied to a data pointer to indicate that During the scope of that pointer declaration, all data accessed through it will be accessed only through that pointer but not through any other pointer. q It enables the compiler to perform certain optimizations based on the premise that a given object cannot be changed through another pointer http: //www. cellperformance. com/mike_acton/2006/05/demystifyin g_the_restrict_keyw. html q o 42

The Thread ID pthread_t pthread_self(void) o Each thread has an id of type pthread_t. q q o o On most systems this is just an integer (like a process ID) But it does not have to be A thread can get its ID with pthread_self Compare two threads q int pthread_equal(thread_t t 1, pthread_t t 2) 43

Exiting and Cancellation o Question: q o A process can terminate when: q q o If a thread calls exit(), what about other threads in the same process? it calls exit directly one of its threads calls exit it returns from main() it receives a termination signal In any of these cases, all threads of the process terminate. 44

Exiting o When a thread is done, it can return from its first function (the one used by pthread_create) or it can call pthread_exit void pthread_exit(void *value_ptr); 45

Cancel that thread! o One thread can request that another exit with pthread_cancel int pthread_cancel(pthread_t thread); o o The pthread_cancel returns after making the request. A successful return does not mean that the target thread has terminated or even that it eventually will terminate as a result of the request 46

Thread Attributes o o o Create an attribute object (initialize it with default properties) Modify the properties of the attribute object Create a thread using the attribute object The object can be changed or reused without affecting the thread The attribute object affects the thread only at the time of thread creation 47

Attribute Initialization and Deletion Initialize or destroy an attribute with: int pthread_attr_destroy(pthread_attr_t *attr); int pthread_attr_init(pthread_attr_t *attr); 48

Example: Create a detached thread int e, fd; pthread_attr_t tattr; pthread_t tid; if(e = pthread_attr_init(&tattr)) fprintf(stderr, "Failed to create attribute object: %sn", strerror(e)); else if(e = pthread_attr_setdetachstate(&tattr, PTHREAD_CREATE_DETACHED)) fprintf(stderr, "Failed to set attribute state to detached: %sn", strerror(e)); else if(e = pthread_create(&tid, &tattr, data, &fd)) fprintf(stderr, "Failed to create thread: %sn", strerror(e)); 49

The thread stack You can set a location and size for the thread stack. int pthread_attr_setstack( pthread_attr_t *attr, void *stackaddr, size_t stacksize ) (there’s a getstack function too) Some systems allow you to set a guard for the stack so that an overflow into the guard area can generate a SIGSEGV signal. int pthread_attr_getguardsize(const pthread_attr_t *restrict attr, size_t *restrict guardsize); int pthread_attr_setguardsize(pthread_attr_t *attr, size_t guardsize); 50

Settable properties of thread attributes property function attribute objects pthread_attr_destroy pthread_attr_init detach state pthread_attr_getdetachstate pthread_attr_setdetachstate stack pthread_attr_getguardsize pthread_attr_setguardsize pthread_attr_getstack pthread_attr_setstack scheduling pthread_attr_getinheritsched pthread_attr_setinheritsched pthread_attr_getschedparam pthread_attr_setschedparam pthread_attr_getschedpolicy pthread_attr_setschedpolicy pthread_attr_getscope pthread_attr_setscope 51

Thread Detach & Join o Call pthread_join() or pthread_detach() for every thread that is created joinable q o so that the system can reclaim all resources associated with the thread Failure to join or to detach threads memory and other resource leaks until the process ends 52

Detaching a Thread int pthread_detach(pthread_t threadid); o Indicate that system resources for the specified thread should be reclaimed when the thread ends q q o If the thread is already ended, resources are reclaimed immediately This routine does not cause thread to end Threads are detached q q q after a pthread_detach() call after a pthread_join() call if a thread terminates and the PTHREAD_CREATE_DETACHED attribute was set on creation 53

How to make a thread detached void *processfd(void *arg); int error; int fd pthread_t tid; if (error = pthread_create(&tid, NULL, processfd, &fd)) { fprintf(stderr, "Failed to create thread: %sn", strerror(error)); } else if (error = pthread_detach(tid)){ fprintf(stderr, "Failed to detach thread: %sn", strerror(error)); } 54

How a thread can detach itself void *runnger(void *arg) { … if (!pthread_detach( pthread_self()) ) return NULL; } … 55

“Wating” on a Thread: pthread_join() int pthread_join(pthread_t thread, void** retval); o o o pthread_join() is a blocking call on non -detached threads It indicates that the caller wishes to block until the thread being joined exits You cannot join on a detached thread, only non-detached threads 56

Pthread_join int error; int *exitcodep; pthread_t tid; if (error = pthread_join(tid, &exitcodep)){ fprintf(stderr, "Failed to join thread: %sn", strerror(error)); } else { fprintf(stderr, "The exit code was %dn", *exitcodep); } 57

Thread scheduling int pthread_attr_getscope(const pthread_attr_t *restrict attr, int *restrict contentionscope); int pthread_attr_setscope(pthread_attr_t *attr, int contentionscope); o The contentionscope can be: PTHREAD_SCOPE_PROCESS o PTHREAD_SCOPE_SYSTEM. The scope determines whether the thread competes with other threads of the process or with other processes in the system. o o 58

Create a thread that contends with other processes pthread_attr_init(&tattr)) pthread_attr_setscope(&tattr, PTHREAD_SCOPE_SYSTEM)) thread_create(&tid, &tattr, myfunction, &myptr)) 59