笔记014 - HW6: Threads and Locking

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题意

提供一个指针数组struct entry *table[NBUCKET]，启动多个thread按照key % NBUCKET的结果往指针数组的某一个项的链表插入entry。由于并发过程中没有进行锁控制，如果多个thread同时操作同一个链表，可能会导致某个线程插入失败，例如：

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t-A: calls insert() on bucket 1 (key = 6, 6 % NBUCKETS = 1)
t-B: calls insert() on bucket 1 (key = 21, 21 % NBUCKETS = 1)
...
...
t-A: executes e->next = n
t-B: executes e->next = n
t-A: executes *p = e
t-B: executes *p = e

t-A在执行e->next = n之后、执行*p = e之前，t-B执行了e->next = n，从而导致t-A没有成功插入。

解决思路

可参考的函数使用：

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互斥锁的使用：
pthread_mutex_t lock;     // declare a lock
pthread_mutex_init(&lock, NULL);   // initialize the lock
pthread_mutex_lock(&lock);  // acquire lock
pthread_mutex_unlock(&lock);  // release lock

在put函数中添加锁控制。

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static 
void put(int key, int value)
{
  pthread_mutex_lock(&lock);  // acquire lock

  int i = key % NBUCKET;
  insert(key, value, &table[i], table[i]);

  pthread_mutex_unlock(&lock);  // release lock
}

编译并运行：

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$ gcc -g -O2 ph.c -pthread
$ ./a.out 2

源代码

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <assert.h>
#include <pthread.h>
#include <sys/time.h>

#define SOL
#define NBUCKET 5
#define NKEYS 100000

struct entry {
  int key;
  int value;
  struct entry *next;
};
struct entry *table[NBUCKET];
int keys[NKEYS];
int nthread = 1;
volatile int done;

pthread_mutex_t lock;     // declare a lock

double
now()
{
 struct timeval tv;
 gettimeofday(&tv, 0);
 return tv.tv_sec + tv.tv_usec / 1000000.0;
}

static void
print(void)
{
  int i;
  struct entry *e;
  for (i = 0; i < NBUCKET; i++) {
    printf("%d: ", i);
    for (e = table[i]; e != 0; e = e->next) {
      printf("%d ", e->key);
    }
    printf("\n");
  }
}

static void 
insert(int key, int value, struct entry **p, struct entry *n)
{
  struct entry *e = malloc(sizeof(struct entry));
  e->key = key;
  e->value = value;
  e->next = n;
  *p = e;
}

static 
void put(int key, int value)
{
  pthread_mutex_lock(&lock);  // acquire lock

  int i = key % NBUCKET;
  insert(key, value, &table[i], table[i]);

  pthread_mutex_unlock(&lock);  // release lock
}

static struct entry*
get(int key)
{
  struct entry *e = 0;
  for (e = table[key % NBUCKET]; e != 0; e = e->next) {
    if (e->key == key) break;
  }
  return e;
}

static void *
thread(void *xa)
{
  long n = (long) xa;
  int i;
  int b = NKEYS/nthread;
  int k = 0;
  double t1, t0;

  //  printf("b = %d\n", b);
  t0 = now();
  for (i = 0; i < b; i++) {
    // printf("%d: put %d\n", n, b*n+i);
    put(keys[b*n + i], n);
  }
  t1 = now();
  printf("%ld: put time = %f\n", n, t1-t0);

  // Should use pthread_barrier, but MacOS doesn't support it ...
  __sync_fetch_and_add(&done, 1);
  while (done < nthread) ;

  t0 = now();
  for (i = 0; i < NKEYS; i++) {
    struct entry *e = get(keys[i]);
    if (e == 0) k++;
  }
  t1 = now();
  printf("%ld: lookup time = %f\n", n, t1-t0);
  printf("%ld: %d keys missing\n", n, k);
  return NULL;
}

int
main(int argc, char *argv[])
{
  pthread_mutex_init(&lock, NULL);   // initialize the lock

  pthread_t *tha;
  void *value;
  long i;
  double t1, t0;

  if (argc < 2) {
    fprintf(stderr, "%s: %s nthread\n", argv[0], argv[0]);
    exit(-1);
  }
  nthread = atoi(argv[1]);
  tha = malloc(sizeof(pthread_t) * nthread);
  srandom(0);
  assert(NKEYS % nthread == 0);
  for (i = 0; i < NKEYS; i++) {
    keys[i] = random();
  }
  t0 = now();
  for(i = 0; i < nthread; i++) {
    assert(pthread_create(&tha[i], NULL, thread, (void *) i) == 0);
  }
  for(i = 0; i < nthread; i++) {
    assert(pthread_join(tha[i], &value) == 0);
  }
  t1 = now();
  printf("completion time = %f\n", t1-t0);
}
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