通过Clock和stable_clock测量的时间差 [英] Difference in time measured by clock and steady_clock

问题描述

我试图测量在我的代码中执行特定功能所花费的时间.最初，我使用如下的 clock()函数

I was trying to measure the time taken to execute a specific function in my code. Initially I used the clock() function as below

clock_t start = clock();
do_something();
clock_t end = clock();

printf("Time taken: %f ms\n", ((double) end - start)*1000/CLOCKS_PER_SEC);

稍后，我正在阅读有关 C ++ 11 中的 chrono 库的信息，并尝试使用 std :: chrono :: steady_clock 进行测量.代码>如下

Later I was reading about the chrono library in C++11 and tried to measure the same with a std::chrono::steady_clock as below

using namespace std::chrono;

auto start = steady_clock::now();
do_something();
auto end = steady_clock::now();
printf("Time taken: %lld ms\n", duration_cast<milliseconds>(end - start).count());

第一个代码段(使用 clock )测量的时间为 89.53 ms ，而 steady_clock 所测量的时间为 1140 ms.

The time measured by the first code snippet (using clock) was 89.53 ms and that measured by steady_clock was 1140 ms.

为什么两个时钟所测量的时间差异如此之大?

Why is there such a big difference in time measured by both the clocks?

推荐答案

clock 测量处理器时间，而 steady_clock 测量物理时间.因此，如果 do_something()被其他进程(例如检查邮件或其他东西)抢占了，则您会得到这样的区别.

clock measures processor time, whereas steady_clock measures physical time. So you can get differences like this if do_something() was preempted by other processes (such as checking mail or whatever).

Daniel H 在下面的评论中指出了一个要点，即如果 do_something()不受CPU限制.例如，如果它睡眠，阻塞锁定互斥锁，等待条件变量等.

Daniel H makes a great point below in the comments that this can also happen if do_something() isn't CPU bound. For example if it sleeps, blocks on locking a mutex, waits on a condition variable, etc.

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