1. Multithreading

Why?

process가 가지고 있던 두가지 특징은 1) Unit of resource ownership 2) Unit of dispatching 이다. 이 둘을 separate하여 process가 1번, thread가 2번을 수행하도록 한다.

Basics

• running, ready, stopped 세가지 state을 가진다.
• 각 thread는 TCB와 stack을 가지고, process의 PCB와 memory를 공유한다.

Pros

• Effective concurrent programming
• Resource sharing
• Economy - cheap to implement compared to process
• Agility in responses
  ex) web server with worker threads

2. Pthreads Programming Model

Pthreads : POSIX standard for threads

pthread_create() // create a new thread
pthread_exit() // terminate the calling thread
pthread_join() // wait for a specific thread to exit
pthread_yield() // release CPU

int main(void)
{
	pthread_t threads[NUM_THREADS]; 
    int thread_args[NUM_THREADS]; 
    int rc, i;
    
    /* create all threads */
   	for (i=0; i<NUM_THREADS; ++i) {
		thread_args[i] = i;
		printf("In main: creating thread %d\n", i);
		rc = pthread_create(&threads[i], NULL, ThreadCode, (void *)&thread_args[i]); 		
        assert(0 == rc);
	}

	/* wait for all threads to complete */
	for (i=0; i<NUM_THREADS; ++i) {
		rc = pthread_join(threads[i], NULL); 
    	assert(0 == rc);
	}
    
	exit(EXIT_SUCCESS); 
}

3. Implementation

User-Level Threads

user level에서 thread간 scheduling

Pros) thread switching할 때 mode switching 필요 x → overhead 줄어듬
Cons) HW interrupt가 발생했을 때 어느 thread에 전달할 지 kernel이 알 수 없어 preemptive scheduling 불가능
하나의 thread가 block되면 전체 process가 block됨

Kernel-Level Threads

kernel level에서 thread간 scheduling
thread 단위로 blocking이라는 장점이 있지만 mode switching 필요하다는 단점

UL/KL Threads

lightweight process(LWP)가 접착제 역할