ThreadsThreads

Manohara PallewattaManohara Pallewatta

CSIM/SATCSIM/SAT

OverviewOverview• Concurrency and parallelismConcurrency and parallelism

• Processes and threadsProcesses and threads

• Thread creationThread creation

• Bound and Unbound threadsBound and Unbound threads

• SynchronizationSynchronization– Mutex locksMutex locks

– Condition variables (Example 1: Condition variables (Example 1: Producer/Consumer problem)Producer/Consumer problem)

– Read Write locksRead Write locks

– Semaphores (Example 2: Linked lists)Semaphores (Example 2: Linked lists)

Concurrency and Concurrency and ParallelismParallelism

• Concurrency - Interwoven execution Concurrency - Interwoven execution of two or more tasks.of two or more tasks.

• Parallelism - Simultaneous execution Parallelism - Simultaneous execution of two or more tasks.of two or more tasks.

• In a single CPU machine concurrency In a single CPU machine concurrency can exist but not parallelism.can exist but not parallelism.

Processes and ThreadsProcesses and Threads

• Traditional UNIX Traditional UNIX modelmodel– fork() task creationfork() task creation

– exec task exec task executionexecution

– wait waiting wait waiting

– exit finishexit finish

• Threads modelThreads model– thr_create task thr_create task

creation creation and and

execution. execution.

– thr_join waitingthr_join waiting

– thr_exit exit thr_exit exit threadthread

Why threads ?Why threads ?

• The cost of creating a thread is The cost of creating a thread is much less than the cost of creating much less than the cost of creating a process. a process.

Types of threadsTypes of threads

•PthreadsPthreads •OS specific OS specific threadsthreads

•Solaris threadsSolaris threads

A simple multi-threaded A simple multi-threaded programprogram

#define _REENTRANT#define _REENTRANT#include <stdio.h>#include <stdio.h>#include <thread.h>#include <thread.h>

void *func(void);void *func(void);

main()main(){{thread_t t_id;thread_t t_id;int exit_value;int exit_value;

thr_create(0,0,tprint,(void *)NULL,0,&t_id);thr_create(0,0,tprint,(void *)NULL,0,&t_id);thr_join(t_id,0,&exit_value);thr_join(t_id,0,&exit_value);}}

void *tprint(void) {void *tprint(void) {printf(“Printing from a thread”);printf(“Printing from a thread”);thr_exit(0);thr_exit(0);}}

How many processes will How many processes will this program create ?.this program create ?.

main()main(){{ fork();fork(); fork();fork(); fork();fork();}}

The answer is 8.The answer is 8.main()main(){{ fork();fork(); fork();fork(); fork();fork();}}

1 fork();1 fork(); fork();fork(); fork();fork();

1 fork();1 fork(); fork();fork();

2 fork();2 fork(); fork();fork();

3 fork();3 fork(); 2 fork();2 fork(); 4 fork();4 fork();1 fork();1 fork();

11 55 33 66 8822 77 44

How many threads this How many threads this program create ?program create ?

main()main(){{ thr_create();thr_create(); thr_create();thr_create(); thr_create();thr_create();}}

The answer is 4.The answer is 4.

main()main(){{ thr_create(..);thr_create(..);

thr_create(..);thr_create(..);

thr_create(..);thr_create(..);

thr_join(..);thr_join(..);}}

Thread 1Thread 1

Thread 2Thread 2

Thread 3Thread 3

Basic thread functionsBasic thread functions

#include <thread.h>

int thr_create(void *stack_base, size_t stack_size, void *(*start_routine)(void *), void *arg, long flags, thread_t *new_thread);

stack_base and stack_sizeUsed only when the default stack is inadequate. Can be 0for most operations.

flagsFor binding threads, Creation of LWPs.THR_NEW_LWP THR_BOUND

Basic thread functions Basic thread functions (cont.)(cont.)

#include <thread.h>

int thr_join(thread_t wait_for, thread_t *departed, void **status);

Blocks the calling thread until the thread specified by wait_for terminates. If wait_for is (thread_t)0, then thr_join() waits for any undetached thread in the process to terminate.

Basic thread functions Basic thread functions (cont.)(cont.)

#include <thread.h>

void thr_exit(void *status);

Terminates the calling thread.

Bound and Unbound Bound and Unbound threadsthreads

• Light Weight Process A virtual CPU that executes code and system calls.

The kernel schedules LWPs on the CPU resourses.

The threads library schedules threads in a process on the pool of LWPs.

Bound and Unbound Bound and Unbound threads (cont.)threads (cont.)

• Unbound threads Threads which are scheduled on the pool of

available LWPs.

• Bound threads Threads which are permanently bound to a LWP.

Synchronization ObjectsSynchronization Objects

• Mutex locks

• Condition Variables

• Read/Write locks

• Semaphores

#include <synch.h> int mutex_init(mutex_t *mp, int type, void * arg);

type

USYNC_PROCESS The mutex can be used to synchronize threads in this

process and other processes. Only one process should initialize the mutex. arg is ignored.

USYNC_THREAD The mutex can be used to synchronize threads in this

process, only. arg is ignored.

• Mutex locksMutex locks

#include <synch.h> ( included by threads.h )

int mutex_destroy(mutex_t *mp); mutex_destroy() destroys any state associated with the

mutex pointed to by mp. int mutex_lock(mutex_t *mp);

int mutex_trylock(mutex_t *mp); Attempts to lock the mutex pointed to by mp. If the mutex is already locked it returns with an error.

int mutex_unlock(mutex_t *mp);

• Mutex locks (cont.)Mutex locks (cont.)

mutex_t count_mutex;int count;

void increment_count(){

mutex_lock(&count_mutex);count = count + 1;mutex_unlock(&count_mutex);

}

• Mutex locks (cont.)Mutex locks (cont.) Restricting access to an integer while Restricting access to an integer while being incrementedbeing incremented

Thread 1..mutex_lock(&m1);mutex_lock(&m2);..mutex_unlock(&m2);mutex_unlock(&m1);.

• Mutex locks (cont.)Mutex locks (cont.) Deadlock situationsDeadlock situations

m1 m2

Thread 2..mutex_lock(&m2);mutex_lock(&m1);..mutex_unlock(&m1);mutex_unlock(&m2);.

• Mutex locks (cont.)Mutex locks (cont.) Conditional locking(A solution for Conditional locking(A solution for deadlocks)deadlocks)

Thread 2.for(;;;) {mutex_lock(&m2);if (mutex_trylock(&m1)==0)

break;mutex_unlock(&m2);}.mutex_unlock(&m1);mutex_unlock(&m2);.

m1 m2

Thread 1..mutex_lock(&m1);mutex_lock(&m2);..mutex_unlock(&m2);mutex_unlock(&m1);.

• Mutex locks (cont.)Mutex locks (cont.) Another solution for deadlocks. Another solution for deadlocks.

• Access locks in the same order. Access locks in the same order.

• Condition VariablesCondition Variables #include <synch.h>

int cond_init(cond_t *cvp, int type, int arg);

type

USYNC_PROCESS The mutex can be used to synchronize threads in this

process and other processes. Only one process should initialize the mutex. arg is ignored.

USYNC_THREAD The mutex can be used to synchronize threads in this

process, only. arg is ignored.

• Condition Variables Condition Variables (cont.)(cont.)

#include <synch.h>

int cond_wait(cond_t *cvp, mutex_t *mp);

.

.

mutex_lock(&m);while(!(condition))

cond_wait(condition,&m);mutex_unlock(&m);

.

.

• Condition variables are used with Condition variables are used with mutex locks. mutex locks.

• Condition Variables Condition Variables (cont.)(cont.)

int sum;cond_t sum_is_pos;mutex_t m;..mutex_init(&m,.....);cond_init(&sum_is_pos,...);

Thread 2 (dec).mutex_lock(&m);while (sum==0)cond_wait(&sum_is_pos,&m);

sum--;mutex_unlock(&m);

Thread 1 (inc).

mutex_lock(&m);sum++;cond_signal(&sum_is_pos)mutex_unlock(&m);.

• Rule: sum >= 0. Rule: sum >= 0.

• Condition Variables Condition Variables (cont.)(cont.)

#include <synch.h> int cond_signal(cond_t *cvp);

Unblocks one thread that is blocked on the condition variable pointed to by cvp.

int cond_broadcast(cond_t *cvp); Unblocks all threads that are blocked on the condition variable pointed to by cvp.

int cond_destroy(cond_t *cvp);

• Condition Variables Condition Variables (cont.)(cont.)

Note:

If no threads are blocked on the condition variable then cond_signal() and cond_broadcast() have no effect.

cond_signal() and cond_broadcast() should be called under the protection of the same mutex that is used with the condition variable being signaled. Otherwise the condition variable may be signaled between the test of the associated condition and blocking in cond_wait(). This can cause an infinite wait.

• Producer/Consumer Producer/Consumer exampleexample

Prod 1

value 1

Prod 2

value 2

Prod 3

value 3

Cons 1

value 1

Cons 2

value 2

Cons 3

value 3

sum

lock

• 0<sum<1000<sum<100

• Mutex lock should be acquired Mutex lock should be acquired before accessing sum. before accessing sum.

• Read/Write locksRead/Write locks

• Allow simultaneous read access by Allow simultaneous read access by many threads while restricting the many threads while restricting the write access only to one thread.write access only to one thread.

• When the write access is granted When the write access is granted reading is not allowed (ie. blocked).reading is not allowed (ie. blocked).

• Read/Write locks (cont.)Read/Write locks (cont.)

#include <synch.h>

int rwlock_init(rwlock_t *rwlp, int type, void * arg);

USYNC_PROCESS The readers/writer lock can be used to synchronize threads in this process and other processes. Only one process should initialize the readers/writer lock. arg is ignored.

USYNC_THREAD The readers/writer lock can be used to synchronize threads in this process, only. arg is ignored.

• Read/Write locks (cont.)Read/Write locks (cont.)int rw_rdlock(rwlock_t *rwlp); Acquires a read lock on the readers/writer lock pointed to by rwlp. If the readers/writer lock is already locked for writing, the calling thread blocks until the write lock is released. More than one thread may hold a read lock on a readers/writer lock at any one time.

int rw_wrlock(rwlock_t *rwlp); Acquires a write lock on the readers/writer lock pointed to by rwlp. If the readers/writer lock is already locked for reading or writing, the calling thread blocks until all the read locks and write locks are released. Only one thread may hold a write lock on a readers/writer lock at any one time.

• Read/Write locks (cont.)Read/Write locks (cont.)

int rw_tryrdlock(rwlock_t *rwlp); Attempts to acquire a read lock on the readers/writer lock pointed to by rwlp. If the readers/writer lock is already locked for writing, it returns an error. Otherwise the read lock is acquired.

int rw_trywrlock(rwlock_t *rwlp); Attempts to acquire a write lock on the readers/writer lock pointed to by rwlp. If the readers/writer lock is already locked for reading or writ- ing, it returns an error.

• Read/Write locks (cont.)Read/Write locks (cont.)

int rw_unlock(rwlock_t *rwlp);

Unlocks a readers/writer lock pointed to by rwlp. The readers/writer lock must be locked and the cal- ling thread must hold the lock either for reading or writ- ing.

int rwlock_destroy(rwlock_t *rwlp);

• Read/Write locks (cont.)Read/Write locks (cont.)

float balance;rwlock_t account_lock;.rwlock_init(&account_lock,...);

Thread 2.deposit(100.0);

Thread 1 .balance=check_balance();

float check_balance(){float bal; rw_rdlock(&account_lock);bal=balance;rw_unlock(&account_lock);return(bal);}

void deposit(float amount){ rw_wrlock(&account_lock);balance += amount;rw_unlock(&account_lock);

}

• SemaphoresSemaphores

•Wait Operation - Atomically decrease the semaphore count.

•Signal Operation - Atomically increase the semaphore count.

•sema_wait and sema_post are the threads’ equivalent of wait and signal operations.

•There is also a sema_trywait which is a conditional form of the wait operation.

• Semaphores (cont.)Semaphores (cont.) int sema_init(sema_t *sp, unsigned int count, int type, void * arg);

USYNC_PROCESS The semaphore can be used to synchronize threads in this process and other processes. Only one process should ini- tialize the semaphore. arg is ignored.

USYNC_THREAD The semaphore can be used to synchronize threads in this process, only. arg is ignored.

• Semaphores (cont.)Semaphores (cont.)int sema_wait(sema_t *sp);

Blocks the calling thread until the count in the semaphore pointed to by sp becomes greater than zero and then atomically decrements it.

int sema_trywait(sema_t *sp);

Atomically decrements the count in the semaphore pointed to by sp if the count is greater than zero. Otherwise it returns an error.

• Semaphores (cont.)Semaphores (cont.)int sema_post(sema_t *sp);

Atomically increments the count semaphore pointed to by sp. If there are any threads blocked on the semaphore, one is unblocked.

int sema_destroy(sema_t *sp);

• Double-linked listsDouble-linked lists

prev

next queue.head

prev

nextprev

next

• Enqueue operationEnqueue operation

elem

• Double-linked lists (cont.)Double-linked lists (cont.)

prev

next queue.tail

prev

nextprev

next

• Dequeue operationDequeue operation

elem

• Double-linked lists (cont.)Double-linked lists (cont.)

queue.tail

queue.head

prev

next

• Null listNull list

prev

next

• List exampleList example

• Two queues, free queue and Two queues, free queue and processed queue implemented as processed queue implemented as double-linked lists.double-linked lists.

• Each element has a mutex lock and a Each element has a mutex lock and a data area.data area.

• Queue heads and tails are elements, Queue heads and tails are elements, complete with mutex locks.complete with mutex locks.

• List example (cont.)List example (cont.)

• Input threads will dequeue from the free Input threads will dequeue from the free queue and enqueue in the processed queue and enqueue in the processed queue.queue.

• Output threads will dequeue from the Output threads will dequeue from the processed queue and enqueue in the free processed queue and enqueue in the free queue.queue.

• Free queue is filled up at the beginning Free queue is filled up at the beginning while the processed queue is empty.while the processed queue is empty.

• List example (cont.)List example (cont.)

Free queue

head tail

Inputthread

Processed queue

tail head

Inputthread

Outputthread

Outputthread