java volatile
· 5 min read
背景
了解java的volaite 内容
volatile
volatile 在jls有几个描述:
- keyword
- filed modified
volatile目的
volatile详细描述在: 8.3.1.4 volatile Fields
The Java programming language allows threads to access shared variables (§17.1).
As a rule, to ensure that shared variables are consistently and reliably updated, a
thread should ensure that it has exclusive use of such variables by obtaining a lock
that, conventionally, enforces mutual exclusion for those shared variables.
The Java programming language provides a second mechanism, volatile fields,
that is more convenient than locking for some purposes
翻译:
在java中,可以由不同的线程共享一个变量.
线程必须通过 获得锁来保证共享的变量可以被可靠和一致的更新.
简单来说,就是使用mutual的排他性来获取这个特性
java除了通过锁,还有另外一个机制来访问和更新共享变量,这就是volatile fields.
在某些情况下volatile field会比锁更加方便
visibility, ordering and atomicity
同步有三个问题:
- 可见性
- 排序
- 原子性
jsr-133 里面有三个不正常同步会出现的问题 ，原文如下：
If a program is not correctly synchronized, then three types of problems can appear:
visibility, ordering and atomicity.
orderging
下面是jsr133 里面给的例子：
分别有两个线程：
threadOne 和线程threadTwo
class BadlyOrdered {
boolean a = false;
boolean b = false;
void threadOne() {
a = true;
b = true;
}
boolean threadTwo() {
boolean r1 = b; // sees true
boolean r2 = a; // sees false
return r1 && !r2; // returns true
}
}
在网上找了个c++ 版本的cpu指令重排的例子来源
命名为 reorder.cpp
#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
// Set either of these to 1 to prevent CPU reordering
#define USE_CPU_FENCE 0
#define USE_SINGLE_HW_THREAD 0 // Supported on Linux, but not Cygwin or PS3
#if USE_SINGLE_HW_THREAD
#include <sched.h>
#endif
//-------------------------------------
// MersenneTwister
// A thread-safe random number generator with good randomness
// in a small number of instructions. We'll use it to introduce
// random timing delays.
//-------------------------------------
#define MT_IA 397
#define MT_LEN 624
class MersenneTwister
{
unsigned int m_buffer[MT_LEN];
int m_index;
public:
MersenneTwister(unsigned int seed);
// Declare noinline so that the function call acts as a compiler barrier:
unsigned int integer() __attribute__((noinline));
};
MersenneTwister::MersenneTwister(unsigned int seed)
{
// Initialize by filling with the seed, then iterating
// the algorithm a bunch of times to shuffle things up.
for (int i = 0; i < MT_LEN; i++)
m_buffer[i] = seed;
m_index = 0;
for (int i = 0; i < MT_LEN * 100; i++)
integer();
}
unsigned int MersenneTwister::integer()
{
// Indices
int i = m_index;
int i2 = m_index + 1; if (i2 >= MT_LEN) i2 = 0; // wrap-around
int j = m_index + MT_IA; if (j >= MT_LEN) j -= MT_LEN; // wrap-around
// Twist
unsigned int s = (m_buffer[i] & 0x80000000) | (m_buffer[i2] & 0x7fffffff);
unsigned int r = m_buffer[j] ^ (s >> 1) ^ ((s & 1) * 0x9908B0DF);
m_buffer[m_index] = r;
m_index = i2;
// Swizzle
r ^= (r >> 11);
r ^= (r << 7) & 0x9d2c5680UL;
r ^= (r << 15) & 0xefc60000UL;
r ^= (r >> 18);
return r;
}
//-------------------------------------
// Main program, as decribed in the post
//-------------------------------------
sem_t beginSema1;
sem_t beginSema2;
sem_t endSema;
int X, Y;
int r1, r2;
void *thread1Func(void *param)
{
MersenneTwister random(1);
for (;;)
{
sem_wait(&beginSema1); // Wait for signal
while (random.integer() % 8 != 0) {} // Random delay
// ----- THE TRANSACTION! -----
X = 1;
#if USE_CPU_FENCE
asm volatile("mfence" ::: "memory"); // Prevent CPU reordering
#else
asm volatile("" ::: "memory"); // Prevent compiler reordering
#endif
r1 = Y;
sem_post(&endSema); // Notify transaction complete
}
return NULL; // Never returns
};
void *thread2Func(void *param)
{
MersenneTwister random(2);
for (;;)
{
sem_wait(&beginSema2); // Wait for signal
while (random.integer() % 8 != 0) {} // Random delay
// ----- THE TRANSACTION! -----
Y = 1;
#if USE_CPU_FENCE
asm volatile("mfence" ::: "memory"); // Prevent CPU reordering
#else
asm volatile("" ::: "memory"); // Prevent compiler reordering
#endif
r2 = X;
sem_post(&endSema); // Notify transaction complete
}
return NULL; // Never returns
};
int main()
{
// Initialize the semaphores
sem_init(&beginSema1, 0, 0);
sem_init(&beginSema2, 0, 0);
sem_init(&endSema, 0, 0);
// Spawn the threads
pthread_t thread1, thread2;
pthread_create(&thread1, NULL, thread1Func, NULL);
pthread_create(&thread2, NULL, thread2Func, NULL);
#if USE_SINGLE_HW_THREAD
// Force thread affinities to the same cpu core.
cpu_set_t cpus;
CPU_ZERO(&cpus);
CPU_SET(0, &cpus);
pthread_setaffinity_np(thread1, sizeof(cpu_set_t), &cpus);
pthread_setaffinity_np(thread2, sizeof(cpu_set_t), &cpus);
#endif
// Repeat the experiment ad infinitum
int detected = 0;
for (int iterations = 1; ; iterations++)
{
// Reset X and Y
X = 0;
Y = 0;
// Signal both threads
sem_post(&beginSema1);
sem_post(&beginSema2);
// Wait for both threads
sem_wait(&endSema);
sem_wait(&endSema);
// Check if there was a simultaneous reorder
if (r1 == 0 && r2 == 0)
{
detected++;
printf("%d reorders detected after %d iterations\n", detected, iterations);
}
}
return 0; // Never returns
}
## 然后编译
gcc -O2 reorder.cpp -o reorder
## 执行
./reorder