atomic memory 原子内存拷贝

ゝ一世哀愁。 2023-01-13 04:21 169阅读 0赞

下面的方法更快而且对于cache的压力更小

// Simpler structure
    struct record { 
        int spin = 0;
        char data[64];
    };
    
    
    
    struct record *node;
    
    while (node->spin || ! __sync_bool_compare_and_swap(&node->spin , 0 , 1)); // spin
    memcpy(destination,source,NoOfBytes);
    node->spin = 0;

不确定的使用CAS替代 node->spin是不是能提高一点效率。

下面对于对于使用mutex 和 spin lock进行讨论

#include <stdio.h>
    #include <pthread.h>
    #include <unistd.h>
    #include <stdlib.h>
    #include <sched.h>
    #include <linux/unistd.h>
    #include <sys/syscall.h>
    #include <errno.h>
    
    #define INC_TO 1000000 // one million...
    
    int global_int = 0;
    
    pid_t gettid( void )
    { 
      return syscall( __NR_gettid );
    }
    
    void *thread_routine( void *arg )
    { 
      int i;
      int proc_num = (int)(long)arg;
      cpu_set_t set;
    
      CPU_ZERO( &set );
      CPU_SET( proc_num, &set );
    
      // sched_setaffinity 将每一个线程绑定到一个核
      if (sched_setaffinity( gettid(), sizeof( cpu_set_t ), &set ))
      { 
        perror( "sched_setaffinity" );
        return NULL;
      }
    
      for (i = 0; i < INC_TO; i++)
      { 
        global_int++;
        //__asm__ __volatile__("inc %0" :"=m" (global_int) :"m"(global_int));
        // __asm__ __volatile__("incl %0;" : "=m"(global_int) : );
      }
    
      return NULL;
    }
    
    int main()
    { 
      int procs = 0;
      int i;
      pthread_t *thrs;
    
      // Getting number of CPUs
      procs = (int)sysconf( _SC_NPROCESSORS_ONLN );
      if (procs < 0)
      { 
        perror( "sysconf" );
        return -1;
      }
    
      thrs = malloc( sizeof( pthread_t ) * procs );
      if (thrs == NULL)
      { 
        perror( "malloc" );
        return -1;
      }
    
      printf( "Starting %d threads...\n", procs );
    
      for (i = 0; i < procs; i++)
      { 
        if (pthread_create( &thrs[i], NULL, thread_routine, (void *)(long)i ))
        { 
          perror( "pthread_create" );
          procs = i;
          break;
        }
      }
    
      for (i = 0; i < procs; i++)
        pthread_join( thrs[i], NULL );
    
      free( thrs );
    
      printf( "After doing all the math, global_int value is: %d\n", global_int );
      printf( "Expected value is: %d\n", INC_TO * procs );
    
      return 0;
    }

Makefile

TARGET =	pg
    OBJS =		main.o
    
    CC =		gcc
    CFLAGS =	-Wall -D_GNU_SOURCE
    LDFLAGS =	-pthread
    
    all: $(TARGET)
    
    $(TARGET): $(OBJS)
    	$(CC) $(OBJS) $(LDFLAGS) -o $(TARGET)
    
    %.o: %.c
    	$(CC) $(CFLAGS) $< -c -o $@
    
    clean:
    	rm -f $(TARGET)
    	rm -f $(OBJS)
    
    call: clean all

执行 make all && ./pg  
输出

Starting 40 threads...
    After doing all the math, global_int value is: 2942375
    Expected value is: 40000000

可以看到及时对于int32字节的加法也不是原子操作,这是因为过程涉及到取值，加法，赋值.