Shared-Memory Programming with Threads

Shared-Memory Programmingwith Threads Adapted and edited by Aleksey Zimin from http://navet.ics.hawaii.edu/~casanova/courses/ics632_fall07/slides/ics632_threads.ppt http://users.actcom.co.il/~choo/lupg/tutorials/multi-process/multi-process.html#process_creation_fork_syscall

Parallel Applications • Modern computers have multiple CPU cores (and/or multiple CPUs) on board • We have to be able to utilize the computing power by parallelizing our tasks

CPU Information • Linux computer: /proc/cpuinfo • Cat /proc/cpuinfo example: processor : 0 vendor_id : AuthenticAMD cpu family : 15 model : 65 model name : Dual-Core AMD Opteron(tm) Processor 8220 stepping : 3 cpu MHz : 2800.000 cache size : 1024 KB physical id : 0 siblings : 2 core id : 0 cpu cores : 2 fpu : yes fpu_exception : yes cpuid level : 1 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow pni cx16 lahf _lm cmp_legacy svm extapic cr8_legacy bogomips : 5625.16 TLB size : 1024 4K pages clflush size : 64 cache_alignment : 64 address sizes : 40 bits physical, 48 bits virtual power management: ts fid vid ttp tm stc

Processes in UNIX • UNIX is natively parallel operating system • A process is an instance of running a program • Each process has a unique process id • Shell command “ps” gives the list of all running processes

Using the shell commands • In any UNIX shell, “&” will run the command in background. • The command will run in its own shell, which is a child of the current shell [alekseyz@genome10]$ run_command.sh & • “wait” command will wait for all child processes in the current shell to finish

Example of & and wait • In bash: #!/bin/bash let NUM_CPUS=`cat /proc/cpuinfo |grep processor|tail -1|awk '{print $NF+1}'` let counter=1; let cpu_counter=1; echo "Total processes to run:"$1 echo "Simultaneously running:"$NUM_CPUS while [[ $counter -le $1 ]];do while [[ $cpu_counter -le $NUM_CPUS && $counter -le $1 ]];do ./echo_sleep_echo.sh & let counter=$counter+1 let cpu_counter=$cpu_counter+1; done let cpu_counter=1; wait done --------------------------------------------------------------------------- #!/bin/bash echo "Sleeping 10 seconds in shell "$$ sleep 10 echo "Done"

Using fork() and wait() in C • The fork() system call is the basic way to create a new process. fork() is used to produce child shell. • Returns twice(!!!!) • fork() causes the current process to be split into two processes - a parent process, and a child process. • All of the memory pages used by the original process get duplicated during the fork() call, so both parent and child process see the exact same memory image.

fork() continued • When fork() returns in the parent process, its return value is the process ID (PID) of the child process. • When it returns inside the child process, its return value is '0'. • If for some reason fork() failed (not enough memory, too many processes, etc.), no new process is created, and the return value of the call is '-1'. • Both child process and parent process continue from the same place in the code where the fork() call was used.

Child processes • When a child process exits, it sends a signal to its parent process, which needs to acknowledge it's child's death. During this time the child process is in a state called zombie. • When a process exits, if it had any children, they become orphans. An orphan process is automatically inherited by the init process, and becomes a child of this init process. • When the parent process is not properly coded, the child remains in the zombie state forever. Such processes can be noticed by running the “ps” command, and seeing processes having the string "<defunct>" as their command name.

Simple fork() and wait() example #include <stdio.h> #include <unistd.h> /* defines fork(), and pid_t. */ #include <sys/wait.h> /* defines the wait() system call. */ int main(){ pid_t child_pid; int child_status; child_pid = fork(); switch (child_pid) { case -1: perror("fork"); exit(1); case 0: printf("I am the child, Hello world\n"); sleep(10); exit(0); default: printf("I am the parent, waiting for the child process %d to exit... \n",child_pid); wait(&child_status); printf("I am the parent, child process %d exited with status %d\n",child_pid,child_status); } }

InterProcess communication • One can prescribe what each child does in the fork() call • It is helpful if parent could communicate with child (e.g. report progress, get data) • Easiest way for parent and child to communicate is through pipe

Using pipes • Anonymous pipe: A pipe is a one-way mechanism that allows two related processes (i.e. one is an ancestor of the other) to send a byte stream from one of them to the other one. • The order in which data is written to the pipe, is the same order as that in which data is read from the pipe. • The system assures that data won't get lost in the middle, unless one of the processes (the sender or the receiver) exits prematurely.

pipe() • The pipe() system call is used to create a read-write pipe. • pipe() takes as an argument an array of 2 integers that will be used to save the two file descriptors used to access the pipe. The first to read from the pipe, and the second to write to the pipe.

Using pipe() /* first, define an array to store the two file descriptors */ int pipes[2]; /* now, create the pipe */ int rc = pipe(pipes); if (rc == -1) { /* pipe() failed */ perror("pipe"); exit(1); }

pipe() example -- main int main() { int data_pipe[2]; /* an array to store the file descriptors of the pipe. */ int pid; int rc; rc = pipe(data_pipe); if (rc == -1) { perror("pipe"); exit(1); } pid = fork(); switch (pid) { case -1: perror("fork"); exit(1); case 0: do_child(data_pipe); default: do_parent(data_pipe); } }

pipe() example -- parent void do_parent(int data_pipe[]) { int c; /* data received from the user. */ int rc; /* first, close the un-needed read-part of the pipe. */ close(data_pipe[0]); while ((c = getchar()) > 0) { rc = write(data_pipe[1], &c, 1); if (rc == -1) { perror("Parent:write");close(data_pipe[1]);exit(1); } } close(data_pipe[1]); exit(0); }

pipe() example -- child void do_child(int data_pipe[]) { int c; /* data received from the parent. */ int rc; /* first, close the un-needed write-part of the pipe. */ close(data_pipe[1]); while ((rc = read(data_pipe[0], &c, 1)) > 0) { putchar(c); } exit(0); }

Processes vs. threads Each process owns: Unit of resources ownership Allocated with virtual address space + control of other resources such as I/O, files…. Unit of dispatching Execution path and state, dispatching priority. Controlled by OS Each thread owns: Unit of dispatching Threads share resources

Comments Traditional program is one thread per process. The main thread starts with main() Only one thread or program counter (PC) is allowed to execute the code segment To add a new PC, you need to fork() to have another PC to execute in another process address space.

Key benefits of multithreading Less time to create a thread than a process Less time to terminate a thread than a process Less time to switch a thread Enhance efficiency in communication: no need for kernel to intervene Smaller chance of driving you crazy while writing code / debugging

Shared memory programming • The “easiest” form of parallel programming • Can be used to parallelize a sequential code in an incremental way: • take a sequential code • parallelize a small section • check that it works • check that it speeds things up a bit • move on to another section

Thread • A thread is a stream of instructions that can be scheduled as an independent unit. • A process is created by an operating system • contains information about resources • process id, file descriptors, ... • contains information on the execution state • program counter, stack, ...

Schedulable Part of a Process • The concept of a thread requires that we make a separation between these two kinds of information in a process • resources available to the entire process • program instructions, global data, working directory • schedulable entities • program counters and stacks. • A thread is an entity within a process which consists of the schedulable part of the process.

Process is still there,what’s new for thread? Thread shares with process Virtual address space (holding process image), access to memory and resources of its process, shared with all other threads in that process Protected access to CPU, files, and I/O resources Thread has its own Thread execution state Saved thread context (an independent PC within a process) Execution stack Per-thread static storage for local variables

Possible combination of thread and processes One process one thread One process multiple thread Multiple processes multiple Threads per process Multiple processes One thread per process

Parallelism with Threads • Create threads within a process • Each thread does something (hopefully) useful • Threads may be working truly concurrently • Multi-processor • Multi-core • Or just pseudo-concurrently • Single-proc, single-core

Example • Say I want to compute the sum of two arrays • I can just create N threads, each of which sums 1/Nth of both arrays and then combine their results • I can also create N threads that each increment some sum variable element-by-element, but then I’ve got to make sure they don’t step on each other’s toes • The first version is a bit less “shared-memory”, but is probably more efficient

Multi-threading issues • There are really two main issues when writing multi-threaded code: • Issue #1: Load Balancing • Make sure that no processors/cores is left idle when it could be doing useful work • Issue #2: Correct access to shared variables • Implemented via mutual exclusion: create sections of code that only a single thread can be in at a time • Called “critical sections” • Classical variable update example • Done via “locks” and “unlocks” • Warning: locks are NOT on variables, but on sections of code

User-level threads (DOS, Windows 95) • User-level threads: Many-to-one thread mapping • Implemented by user-level runtime libraries • Create, schedule, synchronize threads at user-level • OS is not aware of user-level threads • OS thinks each process contains only a single thread of control

User-level threads • Advantages • Does not require OS support; Portable • Can tune scheduling policy to meet application demands • Lower overhead thread operations since no system calls • Disadvantages • Cannot leverage multiprocessors • Entire process blocks when one thread blocks

Kernel-level threads • Kernel-level threads: One-to-one thread mapping • OS provides each user-level thread with a kernel thread • Each kernel thread scheduled independently • Thread operations (creation, scheduling, synchronization) performed by OS

Kernel-level threads • Advantages • Each kernel-level thread can run in parallel on a multiprocessor • When one thread blocks, other threads from process can be scheduled • Disadvantages • Higher overhead for thread operations • OS must scale well with increasing number of threads

Threads in Practice • Pthreads • Popular C library • Flexible • Will discuss these • OpenMP • Java Threads

Pthreads • A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization • The API specifies the standard behavior • Implementation choices are up to developers • Implementations vary, some better than some others • Common in all UNIX operating systems • Some people have written it for Win32 • The most portable threading library out there • What do threads look like in UNIX?

Using the Pthread Library • Pthread library typically uses kernel-threads • Programs must include the file pthread.h • Programs must be linked with the pthread library (-lpthread) • The API contains functions to • create threads • control threads • manage threads • synchronize threads

pthread_self() • Returns the thread identifier for the calling thread • At any point in its instruction stream a thread can figure out which thread it is • Convenient to be able to write code that says: “If you’re thread 1 do this, otherwise to that” #include <pthread.h> pthread_tpthread_self(void);

pthread_create() • Creates a new thread of control #include <pthread.h> int pthread_create ( pthread_t *thread, pthread_attr_t *attr, void * (*start_routine) (void *), void *arg); • Returns 0 to indicate success, otherwise returns error code • thread: output argument that will contain the thread id of the new thread • attr: input argument that specifies the attributes of the thread to be created (NULL = default attributes) • start_routine: function to use as the start of the new thread must have prototype: void * foo(void*) • arg: argument to pass to the new thread routine • If the thread routine requires multiple arguments, they must be passed bundled up in an array or a structure

pthread_create() example • Want to create a thread to compute the sum of the elements of an array void *do_work(void *arg); • Needs three arguments • the array, its size, where to store the sum • we need to bundle them in a structure struct arguments { long int *array; long int size; long int *sum; }

pthread_create() example int main(void) { long int array[ARRAY_SIZE], sum, i; pthread_t worker_thread; struct arguments *arg; for(i=0;i<ARRAY_SIZE;i++) array[i]=1; arg = calloc(1,sizeof(struct arguments)); arg->array = array; arg->size=ARRAY_SIZE; arg->sum = ∑ if (pthread_create(&worker_thread, NULL, do_work, (void *)arg)) { fprintf(stderr,"Error while creating thread"); exit(1); } ... exit(0); }

pthread_create() example void *do_work(void *arg){ long int i, size; long int *array; long int *sum; size = ((struct arguments *)arg)->size; array = ((struct arguments *)arg)->array; sum = ((struct arguments *)arg)->sum; *sum = 0; for (i=0;i<size;i++) *sum += array[i]; return NULL; }

Comments about the example • The “parent thread” continues its normal execution after creating the “child thread” • Memory is shared by the parent and the child (the array, the location of the sum) • Nothing prevents from the parent doing something to it while the child is still executing which may lead to a wrong computation • The bundling and unbundling of arguments is a bit tedious, but nothing compared to what’s needed with shared memory segments and processes

pthread_exit() • Terminates the calling thread #include <pthread.h> void pthread_exit( void *retval); • The return value is made available to another thread calling a pthread_join() (see later) • The previous example had the thread just return from function do_work() • In this case the call to pthread_exit() is implicit • The return value of the function serves as the argument to the (implicitly called) pthread_exit().

pthread_join() • Causes the calling thread to wait for another thread to terminate #include <pthread.h> int pthread_join( pthread_t thread, void **value_ptr); • thread: input parameter, id of the thread to wait on • value_ptr: output parameter, value given to pthread_exit() by the terminating thread (which happens to always be a void *) • returns 0 to indicate success, error code otherwise • multiple simultaneous calls for the same thread are not allowed

pthread_kill() • Causes the termination of a thread #include <pthread.h> int pthread_kill( pthread_t thread, int sig); • thread: input parameter, id of the thread to terminate • sig: signal number • returns 0 to indicate success, error code otherwise

pthread_join() example int main(void) { long int array[100]; long int sum; pthread_t worker_thread; struct arguments *arg; arg = (struct arguments *)calloc(1,sizeof(struct arguments)); arg->array = array; arg->size=100; arg->sum = ∑ if (pthread_create(&worker_thread, NULL, do_work, (void *)arg)) { fprintf(stderr,”Error while creating thread\n”); exit(1); } ... if (pthread_join(worker_thread, NULL)) { fprintf(stderr,”Error while waiting for thread\n”); exit(1); } }

Synchronizing pthreads • As we’ve seen earlier, we need a system to implement locks to create mutual exclusion for variable access, via critical sections • Lock creation int pthread_mutex_init( pthread_mutex_t *mutex, const pthread_mutexattr_t *attr); • returns 0 on success, an error code otherwise • mutex: output parameter, lock • attr: input, lock attributes • NULL: default • There are functions to set the attribute (look at the man pages if you’re interested)

Synchronizing pthreads • Locking a lock • If the lock is already locked, then the calling thread is blocked • If the lock is not locked, the the calling thread acquires it int pthread_mutex_lock( pthread_mutex_t *mutex); • returns 0 on success, an error code otherwise • mutex: input parameter, lock • Just checking • Returns instead of locking int pthread_mutex_trylock( pthread_mutex_t *mutex); • returns 0 on success, EBUSY is the lock is locked, an error code otherwise • mutex: input parameter, lock

Synchronizing pthreads • Releasing a lock int pthread_mutex_unlock( pthread_mutex_t *mutex); • returns 0 on success, an error code otherwise • mutex: input parameter, lock • With locking, trylocking, and unlocking, one can avoid all race conditions and protect access to shared variables

Mutex Example: ... pthread_mutex_t mutex; pthread_mutex_init(&mutex, NULL); ... pthread_mutex_lock(&mutex); count++; pthread_mutex_unlock(&mutex); Critical Section • To “lock” variable count, just put a pthread_mutex_lock() and pthread_mutex_unlock() around all sections of the code that write to variable count • Again, you’re really locking code, not variables

Cleaning up memory • Releasing memory for a mutex attribute int pthread_mutex_destroy( pthread_mutex_t *mutex); • Releasing memory for a mutex int pthread_mutexattr_destroy( pthread_mutexattr_t *mutex);

Shared-Memory Programming with Threads