[英]Does (p+x)-x always result in p for pointer p and integer x in gcc linux x86-64 C++
[英]Atomically incrementing an integer in shared memory for multiple processes on linux x86-64 with gcc
问题什么是增加计数器并在计数器达到给定值后发出信号的好方法(即,在下面通过信号通知函数等待块直到满 )? 这很像要求信号量。 所涉及的进程正在通过共享内存( /dev/shm
)进行通信,而我目前正在尝试避免使用库(例如Boost)。
初始解决方案
声明一个包含SignalingIncrementingCounter的结构。 该结构在共享内存中分配,并且单个进程在其他进程开始之前使用此结构设置共享内存。 SignalingIncrementingCounter包含以下三个字段:
一个普通的int
,代表计数器的值。
注意:由于采用了MESI缓存协议,我们可以保证,如果一个cpu内核修改了该值,则从其他缓存中读取该值后,更新后的值将反映在其他缓存中。
一个pthread互斥体,以保护整数计数器的读取和递增
一个pthread条件变量,用于在整数达到所需值时发出信号
其他解决方案
int
,我还尝试使用std::atomic<int>
。 我尝试仅将该字段定义为SignalingIncrementingCounter类的成员,并且还尝试在运行时使用new放置将其分配到结构中。 似乎两者都没有比int
更好。 以下应该工作。
实施
我包含了大多数代码,但是为了简洁起见,我省略了部分代码。
signalling_incrementing_counter.h
#include <atomic>
struct SignalingIncrementingCounter {
public:
void init(const int upper_limit_);
void reset_to_empty();
void increment(); // only valid when counting up
void block_until_full(const char * comment = {""});
private:
int upper_limit;
volatile int value;
pthread_mutex_t mutex;
pthread_cond_t cv;
};
signalling_incrementing_counter.cpp
#include <pthread.h>
#include <stdexcept>
#include "signaling_incrementing_counter.h"
void SignalingIncrementingCounter::init(const int upper_limit_) {
upper_limit = upper_limit_;
{
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
int retval = pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
if (retval) {
throw std::runtime_error("Error while setting sharedp field for mutex");
}
pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);
pthread_mutex_init(&mutex, &attr);
pthread_mutexattr_destroy(&attr);
}
{
pthread_condattr_t attr;
pthread_condattr_init(&attr);
pthread_condattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_cond_init(&cv, &attr);
pthread_condattr_destroy(&attr);
}
value = 0;
}
void SignalingIncrementingCounter::reset_to_empty() {
pthread_mutex_lock(&mutex);
value = 0;
// No need to signal, because in my use-case, there is no function that unblocks when the value changes to 0
pthread_mutex_unlock(&mutex);
}
void SignalingIncrementingCounter::increment() {
pthread_mutex_lock(&mutex);
fprintf(stderr, "incrementing\n");
++value;
if (value >= upper_limit) {
pthread_cond_broadcast(&cv);
}
pthread_mutex_unlock(&mutex);
}
void SignalingIncrementingCounter::block_until_full(const char * comment) {
struct timespec max_wait = {0, 0};
pthread_mutex_lock(&mutex);
while (value < upper_limit) {
int val = value;
printf("blocking until full, value is %i, for %s\n", val, comment);
clock_gettime(CLOCK_REALTIME, &max_wait);
max_wait.tv_sec += 5; // wait 5 seconds
const int timed_wait_rv = pthread_cond_timedwait(&cv, &mutex, &max_wait);
if (timed_wait_rv)
{
switch(timed_wait_rv) {
case ETIMEDOUT:
break;
default:
throw std::runtime_error("Unexpected error encountered. Investigate.");
}
}
}
pthread_mutex_unlock(&mutex);
}
使用int
或std::atomic
工程。
关于std::atomic
接口的std::atomic
是,它与int
“ interface”可以很好地协作。 因此,代码几乎完全相同。 您可以通过添加#define USE_INT_IN_SHARED_MEMORY_FOR_SIGNALING_COUNTER true
在下面的每个实现之间切换。
我不确定在共享内存中静态创建std::atomic
,因此我使用new布局来分配它。 我的猜测是,依靠静态分配会起作用,但从技术上讲,这可能是未定义的行为。 弄清楚这超出了我的问题范围,但是对此主题发表评论将是非常受欢迎的。
signalling_incrementing_counter.h
#include <atomic>
#include "gpu_base_constants.h"
struct SignalingIncrementingCounter {
public:
/**
* We will either count up or count down to the given limit. Once the limit is reached, whatever is waiting on this counter will be signaled and allowed to proceed.
*/
void init(const int upper_limit_);
void reset_to_empty();
void increment(); // only valid when counting up
void block_until_full(const char * comment = {""});
// We don't have a use-case for the block_until_non_full
private:
int upper_limit;
#if USE_INT_IN_SHARED_MEMORY_FOR_SIGNALING_COUNTER
volatile int value;
#else // USE_INT_IN_SHARED_MEMORY_FOR_SIGNALING_COUNTER
std::atomic<int> value;
std::atomic<int> * value_ptr;
#endif // USE_INT_IN_SHARED_MEMORY_FOR_SIGNALING_COUNTER
pthread_mutex_t mutex;
pthread_cond_t cv;
};
signalling_incrementing_counter.cpp
#include <pthread.h>
#include <stdexcept>
#include "signaling_incrementing_counter.h"
void SignalingIncrementingCounter::init(const int upper_limit_) {
upper_limit = upper_limit_;
#if !GPU_USE_INT_IN_SHARED_MEMORY_FOR_SIGNALING_COUNTER
value_ptr = new(&value) std::atomic<int>(0);
#endif // GPU_USE_INT_IN_SHARED_MEMORY_FOR_SIGNALING_COUNTER
{
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
int retval = pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
if (retval) {
throw std::runtime_error("Error while setting sharedp field for mutex");
}
pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);
pthread_mutex_init(&mutex, &attr);
pthread_mutexattr_destroy(&attr);
}
{
pthread_condattr_t attr;
pthread_condattr_init(&attr);
pthread_condattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_cond_init(&cv, &attr);
pthread_condattr_destroy(&attr);
}
reset_to_empty(); // should be done at end, since mutex functions are called
}
void SignalingIncrementingCounter::reset_to_empty() {
int mutex_rv = pthread_mutex_lock(&mutex);
if (mutex_rv) {
throw std::runtime_error("Unexpected error encountered while grabbing lock. Investigate.");
}
value = 0;
// No need to signal, because there is no function that unblocks when the value changes to 0
pthread_mutex_unlock(&mutex);
}
void SignalingIncrementingCounter::increment() {
fprintf(stderr, "incrementing\n");
int mutex_rv = pthread_mutex_lock(&mutex);
if (mutex_rv) {
throw std::runtime_error("Unexpected error encountered while grabbing lock. Investigate.");
}
++value;
fprintf(stderr, "incremented\n");
if (value >= upper_limit) {
pthread_cond_broadcast(&cv);
}
pthread_mutex_unlock(&mutex);
}
void SignalingIncrementingCounter::block_until_full(const char * comment) {
struct timespec max_wait = {0, 0};
int mutex_rv = pthread_mutex_lock(&mutex);
if (mutex_rv) {
throw std::runtime_error("Unexpected error encountered while grabbing lock. Investigate.");
}
while (value < upper_limit) {
int val = value;
printf("blocking during increment until full, value is %i, for %s\n", val, comment);
/*const int gettime_rv =*/ clock_gettime(CLOCK_REALTIME, &max_wait);
max_wait.tv_sec += 5;
const int timed_wait_rv = pthread_cond_timedwait(&cv, &mutex, &max_wait);
if (timed_wait_rv)
{
switch(timed_wait_rv) {
case ETIMEDOUT:
break;
default:
pthread_mutex_unlock(&mutex);
throw std::runtime_error("Unexpected error encountered. Investigate.");
}
}
}
pthread_mutex_unlock(&mutex);
}
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