基于java中cas实现的探索

目录

• 1.背景简介
• 2. java源码追踪
• 3. hotspot jvm源码追踪
• 4. 手写一个cas实现

• 1. 通过汇编手写一个cas方法
• 2. 多线程条件下测试自行实现的cas方法
• 3. cas与互斥锁方式的对比
• 4. 结论

import java.util.concurrent.atomic.AtomicInteger;
public class CASTest {
    private static AtomicInteger countAI = new AtomicInteger(0);
    private static int count = 0;
    private static final int THREAD_COUNT = 8;
    public static void main(String[] args) throws InterruptedException {
        long start = System.currentTimeMillis();
        Thread[] threads = new Thread[THREAD_COUNT];
        for(int i = 0; i < THREAD_COUNT; i++) {
            threads[i] = new Thread(){
                @Override
                public void run() {
                    for(int i = 0; i < 10000000; i++) {
//                        测试1：使用同步代码块方法，耗时：2927ms
                        synAdd();
//                        测试2：使用atomicInterger方式, 耗时：1860ms
//                        atomicAdd();
                    }
                }
            };
        }
        for(int i = 0; i < THREAD_COUNT; i++) {
            threads[i].start();
        }
        for(int i = 0; i < THREAD_COUNT; i++) {
            threads[i].join();
        }
        System.out.println("finish...耗时：" + (System.currentTimeMillis()-start) + "ms");
    }
    private static synchronized void synAdd() {
        count++;
    }
    private static void atomicAdd() {
        countAI.getAndAdd(1);
    }
}

getAndAddInt:1034, Unsafe (sun.misc)
getAndAdd:177, AtomicInteger (java.util.concurrent.atomic)
atomicAdd:45, CASTest (com.youai.cas)
access$000:5, CASTest (com.youai.cas)
run:21, CASTest$1 (com.youai.cas)

    public final int getAndAddInt(Object o, long offset, int delta) {
        int v;
        do {
            v = getIntVolatile(o, offset);
        } while (!compareAndSwapInt(o, offset, v, v + delta));
        return v;
    }

/**
     * Atomically update Java variable to <tt>x</tt> if it is currently
     * holding <tt>expected</tt>.
     * @return <tt>true</tt> if successful
     */
    public final native boolean compareAndSwapInt(Object o, long offset,
                                                  int expected,
                                                  int x);

UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapInt(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jint e, jint x))
  UnsafeWrapper("Unsafe_CompareAndSwapInt");
  oop p = JNIHandles::resolve(obj);
  jint* addr = (jint *) index_oop_from_field_offset_long(p, offset);
  return (jint)(Atomic::cmpxchg(x, addr, e)) == e; //此处调用了Atomic::cmpxchg方法
UNSAFE_END

inline jint     Atomic::cmpxchg    (jint     exchange_value, volatile jint*     dest, jint     compare_value) {
  int mp = os::is_MP();
  __asm__ volatile (LOCK_IF_MP(%4) "cmpxchgl %1,(%3)"
                    : "=a" (exchange_value)
                    : "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
                    : "cc", "memory");
  return exchange_value;
}

	.file	"cmpandset.c"
	.text
	.globl	xchange
	.type	xchange, @function
xchange:
.LFB0:
	.cfi_startproc
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	.cfi_def_cfa_register 6
	mov %esi, %eax
	lock cmpxchgl %edx, (%rdi)
	sete %al
	movzbl %al, %eax
.L3:
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	xchange, .-xchange
	.ident	"GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
	.section	.note.GNU-stack,"",@progbits

#include <stdio.h>
#include <pthread.h>
#include <sys/time.h>
#define THREAD_CNT 8
extern int xchange(int *ptr, int expect, int dest);
int a = 0;
void cmp_add(int* cnt, int adder);
long current_ms() {
    struct timeval cur_time;
    gettimeofday(&cur_time, NULL);
    return cur_time.tv_sec * 1000 + cur_time.tv_usec / 1000;
}
void * sum(void *arg) {
    for(int i = 0; i < 10000000; i++) {
        cmp_add(&a, 1);
    }
}
int main(int argc, char const *argv[])
{
    long start = current_ms();
    int result = xchange(&a, 13, 13);
    printf("result=%d\n", result);
    pthread_t tids[THREAD_CNT];
    for(int i = 0; i < THREAD_CNT; i++) {
        pthread_create(&tids[i], NULL, sum, NULL);
    }
    // 等待
    for(int i = 0; i < THREAD_CNT; i++) {
        pthread_join(tids[i], NULL);
    }
    printf("result=%d, 耗时:%ldms\n", a, (current_ms() - start));
    return 0;
}
void cmp_add(int* cnt, int adder) {
    int tmp = 0;
    do {
        tmp = *cnt;
    } while(xchange(cnt, tmp, tmp+adder) == 0);
}

#include <stdio.h>
#include <pthread.h>
#include <sys/time.h>
#include <semaphore.h>
#define THREAD_CNT 8
int a = 0;
sem_t add_mutex;
long current_ms() {
    struct timeval cur_time;
    gettimeofday(&cur_time, NULL);
    return cur_time.tv_sec * 1000 + cur_time.tv_usec / 1000;
}
void * sum(void *arg) {
    for(int i = 0; i < 10000000; i++) {
        sem_wait(&add_mutex);
        a++;
        sem_post(&add_mutex);
    }
}
int main(int argc, char const *argv[])
{
    long start = current_ms();
    sem_init(&add_mutex, 0, 1);
    pthread_t tids[THREAD_CNT];
    for(int i = 0; i < THREAD_CNT; i++) {
        pthread_create(&tids[i], NULL, sum, NULL);
    }
    // 等待
    for(int i = 0; i < THREAD_CNT; i++) {
        pthread_join(tids[i], NULL);
    }
    printf("result=%d, 耗时:%ldms\n", a, (current_ms() - start));
    sem_destroy(&add_mutex);
    return 0;
}