1CSC 4103 - Operating SystemsFall 2009Tevfik Ko!arLouisiana State UniversitySeptember 8th, 2009Lecture - IVThreads2Roadmap• Threads– Why do we need them?– Threads vs Processes– Threading Examples– Threading Implementation & Multi-threading Models– Other Threading Issues• Thread cancellation• Signal handling• Thread pools• Thread specific data3Concurrent Programming• In certain cases, a single application may need to run several tasks at the same time 1123245345sequentialconcurrent4Motivation• Increase the performance by running more than one tasks at a time.– divide the program to n smaller pieces, and run it n times faster using n processors• To cope with independent physical devices.– do not wait for a blocked device, perform other operations at the backgroundSerial vs Parallel5QPleaseCOUNTERCOUNTER 1COUNTER 26Divide and Computex1 + x2 + x3 + x4 + x5 + x6 + x7 + x8How many operations with sequential programming?7Step 1: x1 + x2Step 2: x1 + x2 + x3Step 3: x1 + x2 + x3 + x4 Step 4: x1 + x2 + x3 + x4 + x5Step 5: x1 + x2 + x3 + x4 + x5 + x6Step 6: x1 + x2 + x3 + x4 + x5 + x6 + x7 Step 7: x1 + x2 + x3 + x4 + x5 + x6 + x7 + x87Divide and Computex1 + x2 + x3 + x4 + x5 + x6 + x7 + x8Step 1: parallelism = 4Step 2: parallelism = 2Step 3: parallelism = 18Gain from parallelismIn theory:• dividing a program into n smaller parts and running on n processors results in n time speedupIn practice:• This is not true, due to– Communication costs– Dependencies between different program parts• Eg. the addition example can run only in log(n) time not 1/n9Concurrent Programming• Implementation of concurrent tasks:– as separate programs– as a set of processes or threads created by a single program• Execution of concurrent tasks:– on a single processor (can be multiple cores)! Multithreaded programming– on several processors in close proximity! Parallel computing– on several processors distributed across a network! Distributed computing10Why Threads?• In certain cases, a single application may need to run several tasks at the same time– Creating a new process for each task is time consuming– Use a single process with multiple threads• faster• less overhead for creation, switching, and termination• share the same address spaceOwnership vs Execution11"A process embodies two independent concepts:1. resource ownership2. execution & scheduling1. Resource ownership#a process is allocated address space to hold the image, and is granted control of I/O devices and files#the O/S prevents interference among processes while they make use of resources (multiplexing)2. Execution & scheduling #a process follows an execution path through a program --> Thread#it has an execution state and is scheduled for dispatchingMulti-threading12"The execution part is a “thread”Pasta for six– boil 1 quart salty water – stir in the pasta– cook on medium until “al dente”– serveProgramProcessCPUinput datathread of execution"The execution part is a “thread” that can be multipliedother threadsame CPU workingon two things13Single and Multithreaded ProcessesNew Process Description Model14"Multithreading requires changes in the process description modelstackprocess control block (PCB)programcodedata#each thread of execution receives its own control block and stack$own execution state (“Running”, “Blocked”, etc.)$own copy of CPU registers$own execution history (stack)#the process keeps a global control block listing resources currently usedprocess control block (PCB)programcodedatathread 1 stackthread 1 control block (TCB 1)thread 2 stackthread 2 control block (TCB 2)New process imagePer-process vs per-thread items15"Per-process items and per-thread items in the control block structures#process identification data$numeric identifiers of the process, the parent process, the user, etc.#CPU state information$user-visible, control & status registers$stack pointers#process control information$scheduling: state, priority, awaited event$used memory and I/O, opened files, etc.$pointer to next PCB#process identification data + thread identifiers$numeric identifiers of the process, the parent process, the user, etc.#CPU state information$user-visible, control & status registers$stack pointers#process control information$scheduling: state, priority, awaited event$used memory and I/O, opened files, etc.$pointer to next PCB16Multi-process modelProcess Spawning:Process creation involves the following four main actions:• setting up the process control block, • allocation of an address space and• loading the program into the allocated address space and • passing on the process control block to the scheduler17Multi-thread modelThread Spawning:• Threads are created within and belonging to processes• All the threads created within one process share the resources of the process including the address space• Scheduling is performed on a per-thread basis. • The thread model is a finer grain scheduling model than the process model• Threads have a similar lifecycle as the processes and will be managed mainly in the same way as processes are18Threads vs Processes• A common terminology:– Heavyweight Process = Process– Lightweight Process = ThreadAdvantages (Thread vs. Process):• Much quicker to create a thread than a process– spawning a new thread only involves allocating a new stack and a new CPU state block • Much quicker to switch between threads than to switch between processes• Threads share data easilyDisadvantages (Thread vs. Process):• Processes are more flexible– They don’t have to run on the same processor• No security between threads: One thread can stomp on another thread's data• For threads which are supported by user thread package instead of the kernel:– If one thread blocks, all threads in task block.Thread Creation•pthread_create// creates a new thread executing start_routineint pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void*), void *arg);•pthread_join// suspends execution of the calling thread until the target// thread terminatesint pthread_join(pthread_t thread, void **value_ptr);19Thread Exampleint main(){ pthread_t thread1, thread2; /* thread variables */ pthread_create (&thread1, NULL, (void *) &print_message_function, (void*)”hello “);