使用简单的计时器类创建秒表功能,然后捕获每个线程中的时间。另外,创建系统线程比使用std::async
慢,后者可以返回值并传播异常,这些异常使用线程导致程序终止,除非被线程捕获。
#include <thread>
#include <iostream>
#include <atomic>
#include <chrono>
#include <future>
// stopwatch. Returns time in seconds
class timer {
public:
std::chrono::time_point<std::chrono::high_resolution_clock> lastTime;
timer() : lastTime(std::chrono::high_resolution_clock::now()) {}
inline double elapsed() {
std::chrono::time_point<std::chrono::high_resolution_clock> thisTime=std::chrono::high_resolution_clock::now();
double deltaTime = std::chrono::duration<double>(thisTime-lastTime).count();
lastTime = thisTime;
return deltaTime;
}
};
// for exposition clarity, generally avoid global varaibles.
const int count = 1000000;
double timerResult1;
double timerResult2;
void f1() {
volatile int i = 0; // volatile eliminates optimization removal
timer stopwatch;
while (i++ < count);
timerResult1=stopwatch.elapsed();
}
void f2() {
volatile int i = 0; // volatile eliminates optimization removal
timer stopwatch;
while (i++ < count);
timerResult2=stopwatch.elapsed();
}
int main()
{
std::cout.precision(6); std::cout << std::fixed;
f1(); std::cout << "f1 execution time " << timerResult1 << std::endl;
timer stopwatch;
{
std::thread thread1(f1);
std::thread thread2(f2);
thread1.join();
thread2.join();
}
double elapsed = stopwatch.elapsed();
std::cout << "f1 with f2 execution time " << elapsed << std::endl;
std::cout << "thread f1 execution time " << timerResult1 << std::endl;
std::cout << "thread f1 execution time " << timerResult2 << std::endl;
{
stopwatch.elapsed(); // reset stopwatch
auto future1 = std::async(std::launch::async, f1); // spins a thread and descturctor automatically joins
auto future2 = std::async(std::launch::async, f2);
}
elapsed = stopwatch.elapsed();
std::cout << "async f1 with f2 execution time " << elapsed << std::endl;
std::cout << "async thread f1 execution time " << timerResult1 << std::endl;
std::cout << "async thread f1 execution time " << timerResult2 << std::endl;
}
我的机器上创建线程增加每个线程大约0.3毫秒,而异步是每个线程只有大约0.05毫秒,因为它是一个线程池来实现。 https://msdn.microsoft.com/en-us/:
f1 execution time 0.002076
f1 with f2 execution time 0.002791
thread f1 execution time 0.002018
thread f1 execution time 0.002035
async f1 with f2 execution time 0.002131
async thread f1 execution time 0.002028
async thread f1 execution time 0.002018
[编辑]本来不正确的语句的前面(削减和过去的错误)
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