使用rdtsc进行秒计算 [英] Seconds calculation using rdtsc

问题描述

这是用于计算CPU时间的代码，但它不正确，因为当我使用 gettimeofday 时，它为我提供了正确的时间(以毫秒为单位).我在一个处理器上运行我的进程，其时钟运行在800MHz.我对rdtsc的了解如下:

Here is the code to calculate CPU time but it's not correct because when I use gettimeofday it gives me correct time in ms. I am running my process on one processor and its clock runs at 800MHz. My knowledge about rdtsc is as follows:

• Rdtsc返回周期数

• 使用这些周期数，可以在给定时钟频率(800 MHZ)的情况下计算CPU时间

• Rdtsc returns number of cycles

• Using those # of cycles one can calculate the CPU time given the clock rate (800 MHZ)

unsigned long long a,b;
unsigned long cpuMask;
cpuMask = 2; // bind to cpu 1
if(!sched_setaffinity(0, sizeof(cpuMask), &cpuMask))
fprintf(stderr,"Running on one core!\n");
setpriority(PRIO_PROCESS, 0, 20);
struct timeval t1, t2;
double elapsedTime;
int i=0;
// start timer
gettimeofday(&t1, NULL);    
a = rdtsc();
sleep(20);      
//for(;i<1000000;i++);
      //fprintf(stderr,"%d\n",i);
gettimeofday(&t2, NULL);
b = rdtsc();
printf("a:%llu\n", a);
printf("b:%llu\n", b);
double val = ((b-a)/800000);        
fprintf(stderr,"Time 1st through rdtsc in msec:%f\n\nSubtraction:%llu\n\n", val,b-a);
elapsedTime = (t2.tv_sec - t1.tv_sec) * 1000.0;      // sec to ms
elapsedTime += (t2.tv_usec - t1.tv_usec) / 1000.0;   // us to ms
fprintf(stderr,"Time through gettimeofday in ms:%f\n\n", elapsedTime);

推荐答案

理论上，不能保证rdtsc与CPU周期有很强的关系，例如1个周期等于3个rdtsc单位.实际上，假设具有constant_tsc功能，在Intel CPU上rdtsc单位等于(1秒/max_frequency_of_cpu).因此，第一个问题是: max 频率是800MHz还是当前频率?

In theory, there is no guarantee that rdtsc would have strong relation to CPU cycles, e.g. 1 cycle may equal to 3 rdtsc units. In practice, rdtsc unit equals to (1 second / max_frequency_of_cpu) on Intel CPUs assuming constant_tsc feature is present. So, first question is: is 800MHz is max frequency or is it current frequency?

无论如何，clock_gettime(CLOCK_MONOTONIC_RAW, ...)是您最有可能要使用的.我的理解是，它准确地映射到时间戳记计数器，并且在引导OS时使用系统时钟进行了校准.

Anyway, clock_gettime(CLOCK_MONOTONIC_RAW, ...) is what most likely you want to use. My understanding is that it is mapped exactly to timestamp counter and is calibrated with system clock when OS is booted.

(是的，您的代码完全可以在我的i7-3635QM上正常工作.)

(And yes, your code works exactly as expected on my i7-3635QM).

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