Nic*_*rca 9 c++ gcc c++11 gcc4.7
因此,准确的时间是对我很重要,我正在调查这3种类型的C++ 11,即指定的时钟system_clock,steady_clock和high_resolution_clock.我最初关注的是测试不同类型时钟的呼叫开销是否存在差异,以及检查每种类型时钟的分辨率.这是我的示例程序:
#include <chrono>
#include <cstdio>
using namespace std;
using namespace std::chrono;
int main(int argc, char **argv)
{
size_t N = 1e6;
if(2 == argc) {
sscanf(argv[1], "%zu", &N);
}
#if defined(hrc)
typedef high_resolution_clock clock;
#warning "High resolution clock"
#elif defined(sc)
typedef steady_clock clock;
#warning "Steady clock"
#elif defined(sys)
typedef system_clock clock;
#warning "System clock"
#endif
const double resolution = double(clock::period::num) / double(clock::period::den);
printf("clock::period: %lf us.\n", resolution*1e6);
printf("clock::is_steady: %s\n", clock::is_steady ? "yes" : "no");
printf("Calling clock::now() %zu times...\n", N);
// first, warm up
for(size_t i=0; i<100; ++i) {
time_point<clock> t = clock::now();
}
// loop N times
time_point<clock> start = clock::now();
for(size_t i=0; i<N; ++i) {
time_point<clock> t = clock::now();
}
time_point<clock> end = clock::now();
// display duration
duration<double> time_span = duration_cast<duration<double>>(end-start);
const double sec = time_span.count();
const double ns_it = sec*1e9/N;
printf("That took %lf seconds. That's %lf ns/iteration.\n", sec, ns_it);
return 0;
}
我用它编译它
$ g++-4.7 -std=c++11 -Dhrc chrono.cpp -o hrc_chrono
chrono.cpp:15:2: warning: #warning "High resolution clock" [-Wcpp]
$ g++-4.7 -std=c++11 -Dsys chrono.cpp -o sys_chrono
chrono.cpp:15:2: warning: #warning "System clock" [-Wcpp]
$ g++-4.7 -std=c++11 -Dsc chrono.cpp -o sc_chrono
chrono.cpp:15:2: warning: #warning "Steady clock" [-Wcpp]
我用G ++ 4.7.2编译,然后运行它
第一个令人惊讶的事情是3种类型的时钟显然是同义词.它们都具有相同的周期(1微秒),并且时间/呼叫几乎相同.如果它们全部相同,那么指定3种类型的时钟有什么意义呢？这只是因为G ++的实现chrono还不成熟吗？或者3.1.10内核可能只有一个用户可访问的时钟？
第二个惊喜,这是巨大的,是steady_clock :: is_steady == false.我相当肯定,根据定义,该属性应该是真的.是什么赋予了？？我该如何解决它(即实现稳定的时钟)？
如果你可以在其他平台/编译器上运行简单的程序,我会非常有兴趣知道结果.如果有人想知道,我的Core i7上大约25 ns /迭代,Tegra 2上大约1000 ns /迭代.
steady_clock 被支持GCC 4.7(作为4.7释放所示的文档:http://gcc.gnu.org/onlinedocs/gcc-4.7.2/libstdc++/manual/manual/status.html#status.iso.2011)并且steady_clock::is_steady是真的,但只有你用它构建GCC--enable-libstdcxx-time=rt
有关该配置选项的详细信息,请参阅/sf/answers/907327151/.
对于GCC 4.9,如果您的OS和C库支持POSIX单调时钟,它将自动启用clock_gettime(对于带有glibc 2.17或更高版本的GNU/Linux和适用于Solaris 10,IIRC的GNU/Linux)
以下是配置--enable-libstdcxx-time=rt在AMD Phenom II X4 905e,2.5GHz上的GCC 4.8的结果,但我认为它现在被限制为800MHz,运行Linux 3.6.11,glibc 2.15
$ ./hrc
clock::period: 0.001000 us.
clock::is_steady: no
Calling clock::now() 1000000 times...
That took 0.069646 seconds. That's 69.645928 ns/iteration.
$ ./sys
clock::period: 0.001000 us.
clock::is_steady: no
Calling clock::now() 1000000 times...
That took 0.062535 seconds. That's 62.534986 ns/iteration.
$ ./sc
clock::period: 0.001000 us.
clock::is_steady: yes
Calling clock::now() 1000000 times...
That took 0.065684 seconds. That's 65.683730 ns/iteration.
在运行Linux 3.4.0的ARMv7 Exynos5上没有 GCC 4.7 --enable-libstdcxx-time(所有三种时钟类型的结果都相同),glibc 2.16
clock::period: 1.000000 us.
clock::is_steady: no
Calling clock::now() 1000000 times...
That took 1.089904 seconds. That's 1089.904000 ns/iteration.
如果你可以在其他平台/编译器上运行简单的程序,我会非常有兴趣知道结果.
Mac OS X 10.8,clang ++/libc ++, - O3,2.8 GHz Core i5:
High resolution clock
clock::period: 0.001000 us.
clock::is_steady: yes
Calling clock::now() 1000000 times...
That took 0.021833 seconds. That's 21.832827 ns/iteration.
System clock
clock::period: 1.000000 us.
clock::is_steady: no
Calling clock::now() 1000000 times...
That took 0.041930 seconds. That's 41.930000 ns/iteration.
Steady clock
clock::period: 0.001000 us.
clock::is_steady: yes
Calling clock::now() 1000000 times...
That took 0.021478 seconds. That's 21.477953 ns/iteration.
steady_clock并且system_clock必须是不同的类型. steady_clock::is_steady必须是true. high_resolution_clock可以是或的别名steady_clock或别名system_clock. system_clock::rep必须是签名类型.