在一个带有constant_tsc和nonstop_tsc的cpu上，为什么我的时间会漂移？

I am running this test on a cpu with constant_tsc and nonstop_tsc

我正在使用constant_tsc和nonstop_tsc在cpu上运行此测试

$ grep -m 1 ^flags /proc/cpuinfo | sed 's/ /\n/g' | egrep "constant_tsc|nonstop_tsc"
constant_tsc
nonstop_tsc

Step 1: Calculate the tick rate of the tsc:

第1步:计算tsc的滴答率:

I calculate _ticks_per_ns as the median over a number of observations. I use rdtscp to ensure in-order execution.

我将_ticks_per_ns计算为多个观测值的中位数。我使用rdtscp来确保按顺序执行。

static const int trials = 13;
std::array<double, trials> rates;

for (int i = 0; i < trials; ++i)
{
    timespec beg_ts, end_ts;
    uint64_t beg_tsc, end_tsc;

    clock_gettime(CLOCK_MONOTONIC, &beg_ts);
    beg_tsc = rdtscp();

    uint64_t elapsed_ns;
    do
    {
        clock_gettime(CLOCK_MONOTONIC, &end_ts);
        end_tsc = rdtscp();

        elapsed_ns = to_ns(end_ts - beg_ts); // calculates ns between two timespecs
    }
    while (elapsed_ns < 10 * 1e6); // busy spin for 10ms

    rates[i] = (double)(end_tsc - beg_tsc) / (double)elapsed_ns;
}

std::nth_element(rates.begin(), rates.begin() + trials/2, rates.end());

_ticks_per_ns = rates[trials/2];

Step 2: Calculate starting wall clock time and tsc

第2步:计算起始挂钟时间和tsc

uint64_t beg, end;
timespec ts;

// loop to ensure we aren't interrupted between the two tsc reads
while (1)
{
    beg = rdtscp();
    clock_gettime(CLOCK_REALTIME, &ts);
    end = rdtscp();

    if ((end - beg) <= 2000) // max ticks per clock call
        break;
}

_start_tsc        = end;
_start_clock_time = to_ns(ts); // converts timespec to ns since epoch

Step 3: Create a function which can return wall clock time from the tsc

步骤3:创建一个可以从tsc返回挂钟时间的功能

uint64_t tsc_to_ns(uint64_t tsc)
{
    int64_t diff = tsc - _start_tsc;
    return _start_clock_time + (diff / _ticks_per_ns);
}

Step 4: Run in a loop, printing wallclock time from clock_gettime and from rdtscp

第4步:循环运行,从clock_gettime和rdtscp打印wallclock时间

// lock the test to a single core
cpu_set_t mask;
CPU_ZERO(&mask);
CPU_SET(6, &mask);
sched_setaffinity(0, sizeof(cpu_set_t), &mask);

while (1)
{
    timespec utc_now;
    clock_gettime(CLOCK_REALTIME, &utc_now);
    uint64_t utc_ns = to_ns(utc_now);
    uint64_t tsc_ns = tsc_to_ns(rdtscp());

    uint64_t ns_diff = tsc_ns - utc_ns;

    std::cout << "clock_gettime " << ns_to_str(utc_ns) << '\n';
    std::cout << "tsc_time      " << ns_to_str(tsc_ns) << " diff=" << ns_diff << "ns\n";

    sleep(10);
}

Output:

clock_gettime 11:55:34.824419837
tsc_time      11:55:34.824419840 diff=3ns
clock_gettime 11:55:44.826260245
tsc_time      11:55:44.826260736 diff=491ns
clock_gettime 11:55:54.826516358
tsc_time      11:55:54.826517248 diff=890ns
clock_gettime 11:56:04.826683578
tsc_time      11:56:04.826684672 diff=1094ns
clock_gettime 11:56:14.826853056
tsc_time      11:56:14.826854656 diff=1600ns
clock_gettime 11:56:24.827013478
tsc_time      11:56:24.827015424 diff=1946ns

Questions:

It is quickly evident that the times calculated in these two ways rapidly drift apart.

很明显,以这两种方式计算的时间会迅速分开。

I'm assuming that with constant_tsc and nonstop_tsc that the tsc rate is constant.

我假设使用constant_tsc和nonstop_tsc,tsc速率是常量。

• Is this the on board clock that is drifting? Surely it doesn't drift at this rate?

  这是漂流的车载时钟吗?当然它不会以这个速度漂移?

• What is the cause of this drift?

  这种漂移的原因是什么?

• Is there anything I can do to keep them in sync (other than very frequently recalculating _start_tsc and _start_clock_time in step 2)?

  有什么办法可以让它们保持同步(除非经常在步骤2中重新计算_start_tsc和_start_clock_time)吗?

3 个解决方案

#1

The reason for the drift seen in the OP, at least on my machine, is that the TSC ticks per ns drifts away from its original value of _ticks_per_ns. The following results were from this machine:

在OP中看到漂移的原因,至少在我的机器上,是每秒的TSC滴答偏离其原始值_ticks_per_ns。以下结果来自这台机器:

don@HAL:~/UNIX/OS/3EZPcs/Ch06$ uname -a
Linux HAL 4.4.0-81-generic #104-Ubuntu SMP Wed Jun 14 08:17:06 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
don@HAL:~/UNIX/OS/3EZPcs/Ch06$  cat /sys/devices/system/clocksource/clocksource0/current_clocksource
tsc

cat /proc/cpuinfo shows constant_tsc and nonstop_tsc flags.

cat / proc / cpuinfo显示了constant_tsc和nonstop_tsc标志。

viewRates.cc can be run to see the current TSC Ticks per ns on a machine:

可以运行viewRates.cc来查看计算机上每ns的当前TSC Ticks:

rdtscp.h:

static inline unsigned long rdtscp_start(void) {
  unsigned long var;
  unsigned int hi, lo;

  __asm volatile ("cpuid\n\t"
          "rdtsc\n\t" : "=a" (lo), "=d" (hi)
          :: "%rbx", "%rcx");

  var = ((unsigned long)hi << 32) | lo;
  return (var);
}

static inline unsigned long rdtscp_end(void) {
  unsigned long var;
  unsigned int hi, lo;

  __asm volatile ("rdtscp\n\t"
          "mov %%edx, %1\n\t"
          "mov %%eax, %0\n\t"
          "cpuid\n\t"  : "=r" (lo), "=r" (hi)
          :: "%rax", "%rbx", "%rcx", "%rdx");

  var = ((unsigned long)hi << 32) | lo;
  return (var);
  }

/*see https://www.intel.com/content/www/us/en/embedded/training/ia-32-ia-64-benchmark-code-execution-paper.html
 */

viewRates.cc:

#include <time.h>
#include <unistd.h>
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include "rdtscp.h"
using std::cout;  using std::cerr;  using std::endl;

#define CLOCK CLOCK_REALTIME

uint64_t to_ns(const timespec &ts);   // Converts a struct timespec to ns (since epoch).
void view_ticks_per_ns(int runs =10, int sleep =10);

int main(int argc, char **argv) {
  int runs = 10, sleep = 10;
  if (argc != 1 && argc != 3) {
    cerr << "Usage: " << argv[0] << " [ RUNS SLEEP ] \n";
    exit(1);
  } else if (argc == 3) {
    runs = std::atoi(argv[1]);
    sleep = std::atoi(argv[2]);
  }

  view_ticks_per_ns(runs, sleep); 
}

  void view_ticks_per_ns(int RUNS, int SLEEP) {
// Prints out stream of RUNS tsc ticks per ns, each calculated over a SLEEP secs interval.
  timespec clock_start, clock_end;
  unsigned long tsc1, tsc2, tsc_start, tsc_end;
  unsigned long elapsed_ns, elapsed_ticks;
  double rate; // ticks per ns from each run.

  clock_getres(CLOCK, &clock_start);
  cout <<  "Clock resolution: " << to_ns(clock_start) << "ns\n\n";

  cout << " tsc ticks      " << "ns      " << " tsc ticks per ns\n";
  for (int i = 0; i < RUNS; ++i) {
    tsc1 = rdtscp_start();
    clock_gettime(CLOCK, &clock_start);
    tsc2 = rdtscp_end();                      
    tsc_start = (tsc1 + tsc2) / 2;

    sleep(SLEEP);

    tsc1 = rdtscp_start();
    clock_gettime(CLOCK, &clock_end);
    tsc2 = rdtscp_end();                     
    tsc_end = (tsc1 + tsc2) / 2;

    elapsed_ticks = tsc_end - tsc_start;
    elapsed_ns = to_ns(clock_end) - to_ns(clock_start);
    rate = static_cast<double>(elapsed_ticks) / elapsed_ns;

    cout << elapsed_ticks << " " << elapsed_ns << " " << std::setprecision(12) << rate << endl;
  } 
}

linearExtrapolator.cc can be run to re-create the experiment of the OP:

可以运行linearExtrapolator.cc来重新创建OP的实验:

linearExtrapolator.cc:

#include <time.h>
#include <unistd.h>
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <array>
#include "rdtscp.h"

using std::cout;  using std::endl;  using std::array;

#define CLOCK CLOCK_REALTIME

uint64_t to_ns(const timespec &ts);   // Converts a struct timespec to ns (since epoch).
void set_ticks_per_ns(bool set_rate); // Display or set tsc ticks per ns, _ticks_per_ns.
void get_start();             // Sets the 'start' time point: _start_tsc[in ticks] and _start_clock_time[in ns].
uint64_t tsc_to_ns(uint64_t tsc);     // Convert tsc ticks since _start_tsc to ns (since epoch) linearly using
                                      // _ticks_per_ns with origin(0) at the 'start' point set by get_start().

uint64_t _start_tsc, _start_clock_time; // The 'start' time point as both tsc tick number, start_tsc, and as
                                        // clock_gettime ns since epoch as _start_clock_time.
double _ticks_per_ns;                   // Calibrated in set_ticks_per_ns()

int main() {
  set_ticks_per_ns(true); // Set _ticks_per_ns as the initial TSC ticks per ns.

  uint64_t tsc1, tsc2, tsc_now, tsc_ns, utc_ns;
  int64_t ns_diff;
  bool first_pass{true};
  for (int i = 0; i < 10; ++i) {
    timespec utc_now;
    if (first_pass) {
      get_start(); //Get start time in both ns since epoch (_start_clock_time), and tsc tick number(_start_tsc)
      cout << "_start_clock_time: " <<  _start_clock_time << ", _start_tsc: " << _start_tsc << endl;
      utc_ns = _start_clock_time;
      tsc_ns = tsc_to_ns(_start_tsc);   // == _start_clock_time by definition.
      tsc_now = _start_tsc;
      first_pass = false;
    } else {
      tsc1 = rdtscp_start();
      clock_gettime(CLOCK, &utc_now);
      tsc2 = rdtscp_end();
      tsc_now = (tsc1 + tsc2) / 2;
      tsc_ns = tsc_to_ns(tsc_now);
      utc_ns = to_ns(utc_now);
    }

    ns_diff = tsc_ns - (int64_t)utc_ns;

    cout << "elapsed ns: " << utc_ns - _start_clock_time << ", elapsed ticks: " << tsc_now - _start_tsc 
     << ", ns_diff: " << ns_diff << '\n' << endl;

    set_ticks_per_ns(false);  // Display current TSC ticks per ns (does not alter original _ticks_per_ns).
  }
}

void set_ticks_per_ns(bool set_rate) {
  constexpr int RUNS {1}, SLEEP{10};
  timespec clock_start, clock_end;
  uint64_t tsc1, tsc2, tsc_start, tsc_end;
  uint64_t elapsed_ns[RUNS], elapsed_ticks[RUNS];
  array<double, RUNS> rates; // ticks per ns from each run.

  if (set_rate) {
    clock_getres(CLOCK, &clock_start);
    cout <<  "Clock resolution: " << to_ns(clock_start) << "ns\n";
  }

  for (int i = 0; i < RUNS; ++i) {
    tsc1 = rdtscp_start();
    clock_gettime(CLOCK, &clock_start);
    tsc2 = rdtscp_end();                      
    tsc_start = (tsc1 + tsc2) / 2;

    sleep(SLEEP);

    tsc1 = rdtscp_start();
    clock_gettime(CLOCK, &clock_end);
    tsc2 = rdtscp_end();                     
    tsc_end = (tsc1 + tsc2) / 2;

    elapsed_ticks[i] = tsc_end - tsc_start;
    elapsed_ns[i] = to_ns(clock_end) - to_ns(clock_start);
    rates[i] = static_cast<double>(elapsed_ticks[i]) / elapsed_ns[i];
  }

  cout << " tsc ticks      " << "ns     " << "tsc ticks per ns" << endl;
  for (int i = 0; i < RUNS; ++i)
    cout << elapsed_ticks[i] << " " << elapsed_ns[i] << " " << std::setprecision(12) << rates[i] << endl;

  if (set_rate)
    _ticks_per_ns = rates[RUNS-1];
}

constexpr uint64_t BILLION {1000000000};

uint64_t to_ns(const timespec &ts) {
  return ts.tv_sec * BILLION + ts.tv_nsec;
}

void get_start() { // Get start time both in tsc ticks as _start_tsc, and in ns since epoch as _start_clock_time
  timespec ts;
  uint64_t beg, end;

// loop to ensure we aren't interrupted between the two tsc reads
  while (1) {
    beg = rdtscp_start();
    clock_gettime(CLOCK, &ts);
    end = rdtscp_end();   
    if ((end - beg) <= 2000) // max ticks per clock call
      break;
  }

  _start_tsc = (end + beg) / 2;
  _start_clock_time = to_ns(ts); // converts timespec to ns since epoch
}

uint64_t tsc_to_ns(uint64_t tsc) { // Convert tsc ticks into absolute ns:
  // Absolute ns is defined by this linear extrapolation from the start point where
  //_start_tsc[in ticks] corresponds to _start_clock_time[in ns].
  uint64_t diff = tsc - _start_tsc;
  return _start_clock_time + static_cast<uint64_t>(diff / _ticks_per_ns);
}

Here is output from a run of viewRates immediately followed by linearExtrapolator:

这是一系列viewRates的输出,紧接着是linearExtrapolator:

don@HAL:~/UNIX/OS/3EZPcs/Ch06$ ./viewRates 
Clock resolution: 1ns

 tsc ticks      ns       tsc ticks per ns
28070466526 10000176697 2.8069970538
28070500272 10000194599 2.80699540335
28070489661 10000196097 2.80699392179
28070404159 10000170879 2.80699245029
28070464811 10000197285 2.80699110338
28070445753 10000195177 2.80698978932
28070430538 10000194298 2.80698851457
28070427907 10000197673 2.80698730414
28070409903 10000195492 2.80698611597
28070398177 10000195328 2.80698498942
don@HAL:~/UNIX/OS/3EZPcs/Ch06$ ./linearExtrapolator
Clock resolution: 1ns
 tsc ticks      ns     tsc ticks per ns
28070385587 10000197480 2.8069831264
_start_clock_time: 1497966724156422794, _start_tsc: 4758879747559
elapsed ns: 0, elapsed ticks: 0, ns_diff: 0

 tsc ticks      ns     tsc ticks per ns
28070364084 10000193633 2.80698205596
elapsed ns: 10000247486, elapsed ticks: 28070516229, ns_diff: -3465

 tsc ticks      ns     tsc ticks per ns
28070358445 10000195130 2.80698107188
elapsed ns: 20000496849, elapsed ticks: 56141027929, ns_diff: -10419

 tsc ticks      ns     tsc ticks per ns
28070350693 10000195646 2.80698015186
elapsed ns: 30000747550, elapsed ticks: 84211534141, ns_diff: -20667

 tsc ticks      ns     tsc ticks per ns
28070324772 10000189692 2.80697923105
elapsed ns: 40000982325, elapsed ticks: 112281986547, ns_diff: -34158

 tsc ticks      ns     tsc ticks per ns
28070340494 10000198352 2.80697837242
elapsed ns: 50001225563, elapsed ticks: 140352454025, ns_diff: -50742

 tsc ticks      ns     tsc ticks per ns
28070325598 10000196057 2.80697752704
elapsed ns: 60001465937, elapsed ticks: 168422905017, ns_diff: -70335

^C

The viewRates output shows that the TSC ticks per ns are decreasing fairly rapidly with time corresponding to one of those steep drops in the plot above. The linearExtrapolator output shows, as in the OP, the difference between the elapsed ns as reported by clock_gettime(), and the elapsed ns obtained by converting the elapsed TSC ticks to elapsed ns using _ticks_per_ns == 2.8069831264 obtained at start time. Rather than a sleep(10); between each print out of elapsed ns, elapsed ticks, ns_diff, I re-run the TSC ticks per ns calculation using a 10s window; this prints out the current tsc ticks per ns ratio. It can be seen that the trend of decreasing TSC ticks per ns observed from the viewRates output is continuing throughout the run of linearExtrapolator.

viewRates输出显示每秒的TSC滴答数相当快地减少,其中时间对应于上图中的一个急剧下降。如在OP中那样,linearExtrapolator输出显示clock_gettime()报告的经过的ns之间的差异,以及通过使用在开始时获得的_ticks_per_ns == 2.8069831264将经过的TSC刻度转换为经过的ns而获得的经过的ns。而不是睡觉(10);在每个打印经过的ns之间,经过的滴答,ns_diff,我使用10s窗口重新运行每ns计算的TSC滴答;这打印出当前的每个ns比率的tsc滴答。可以看出,从viewRates输出中观察到的每ns的TSC滴答减少的趋势在整个线性增强器的运行中持续。

Dividing an elapsed ticks by _ticks_per_ns and subtracting the corresponding elapsed ns gives the ns_diff, e.g.: (84211534141 / 2.8069831264) - 30000747550 = -20667. But this is not 0 mainly due the drift in TSC ticks per ns. If we had used a value of 2.80698015186 ticks per ns obtained from the last 10s interval, the result would be: (84211534141 / 2.80698015186) - 30000747550 = 11125. The additional error accumulated during that last 10s interval, -20667 - -10419 = -10248, nearly disappears when the correct TSC ticks per ns value is used for that interval: (84211534141 - 56141027929) / 2.80698015186 - (30000747550 - 20000496849) = 349.

将经过的滴答除以_ticks_per_ns并减去相应的经过的ns得到ns_diff,例如:(84211534141 / 2.8069831264) - 30000747550 = -20667。但这不是0,主要是由于每秒TSC滴答的漂移。如果我们使用从过去10s间隔获得的每ns的2.80698015186个滴答值,结果将是:(84211534141 / 2.80698015186) - 30000747550 = 11125.在过去10s间隔期间累积的额外误差,-20667 - -10419 = - 1024当在每个ns值的正确TSC滴答用于该间隔时几乎消失:(84211534141 - 56141027929)/ 2.80698015186 - (30000747550 - 20000496849)= 349。

If the linearExtrapolator had been run at a time when the TSC ticks per ns had been constant, the accuracy would be limited by how well the (constant) _ticks_per_ns had been determined, and then it would pay to take, e.g., a median of several estimates. If the _ticks_per_ns was off by a fixed 40 parts per billion, a constant drift of about 400ns every 10 seconds would be expected, so ns_diff would grow/shrink by 400 each 10 seconds.

如果在每ns的TSC滴答一直不变的情况下运行linearExtrapolator,那么准确度将受到(常数)_ticks_per_ns的确定程度的限制,然后它会花费多少,例如,几个中位数估计。如果_ticks_per_ns以固定的十亿分之40的速度关闭,那么预计每10秒会有大约400ns的恒定漂移,因此ns_diff每10秒会增长/缩小400。

genTimeSeriesofRates.cc can be used to generate data for a plot like above: genTimeSeriesofRates.cc:

genTimeSeriesofRates.cc可用于生成如上图所示的数据:genTimeSeriesofRates.cc:

#include <time.h>
#include <unistd.h>
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <array>
#include "rdtscp.h"

using std::cout;  using std::cerr;  using std::endl;  using std::array;

double get_ticks_per_ns(long &ticks, long &ns); // Get median tsc ticks per ns, ticks and ns.
long ts_to_ns(const timespec &ts);

#define CLOCK CLOCK_REALTIME            // clock_gettime() clock to use.
#define TIMESTEP 10
#define NSTEPS  10000
#define RUNS 5            // Number of RUNS and SLEEP interval used for each sample in get_ticks_per_ns().
#define SLEEP 1

int main() {
  timespec ts;
  clock_getres(CLOCK, &ts);
  cerr << "CLOCK resolution: " << ts_to_ns(ts) << "ns\n";

  clock_gettime(CLOCK, &ts);
  int start_time = ts.tv_sec;

  double ticks_per_ns;
  int running_elapsed_time = 0; //approx secs since start_time to center of the sampling done by get_ticks_per_ns()
  long ticks, ns;
  for (int timestep = 0; timestep < NSTEPS; ++timestep) {
    clock_gettime(CLOCK, &ts);
    ticks_per_ns = get_ticks_per_ns(ticks, ns);
    running_elapsed_time = ts.tv_sec - start_time + RUNS * SLEEP / 2;

    cout << running_elapsed_time << ' ' << ticks << ' ' << ns << ' ' 
     << std::setprecision(12) << ticks_per_ns << endl;

    sleep(10);
  }
}

double get_ticks_per_ns(long &ticks, long &ns) {
  // get the median over RUNS runs of elapsed tsc ticks, CLOCK ns, and their ratio over a SLEEP secs time interval 
  timespec clock_start, clock_end;
  long tsc_start, tsc_end;
  array<long, RUNS> elapsed_ns, elapsed_ticks;
  array<double, RUNS> rates; // arrays from each run from which to get medians.

  for (int i = 0; i < RUNS; ++i) {
    clock_gettime(CLOCK, &clock_start);
    tsc_start = rdtscp_end(); // minimizes time between clock_start and tsc_start.
    sleep(SLEEP);
    clock_gettime(CLOCK, &clock_end);
    tsc_end = rdtscp_end();

    elapsed_ticks[i] = tsc_end - tsc_start;
    elapsed_ns[i] = ts_to_ns(clock_end) - ts_to_ns(clock_start);
    rates[i] = static_cast<double>(elapsed_ticks[i]) / elapsed_ns[i];
  }

  // get medians:
  std::nth_element(elapsed_ns.begin(), elapsed_ns.begin() + RUNS/2, elapsed_ns.end());
  std::nth_element(elapsed_ticks.begin(), elapsed_ticks.begin() + RUNS/2, elapsed_ticks.end());
  std::nth_element(rates.begin(), rates.begin() + RUNS/2, rates.end());
  ticks = elapsed_ticks[RUNS/2];
  ns = elapsed_ns[RUNS/2];

  return rates[RUNS/2];
}

constexpr long BILLION {1000000000};

long ts_to_ns(const timespec &ts) {
  return ts.tv_sec * BILLION + ts.tv_nsec;
}

#2

Is this the on board clock that is drifting? Surely it doesn't drift at this rate?
No, they shouldn't drift

这是漂流的车载时钟吗?当然它不会以这个速度漂移?不,他们不应该漂移

What is the cause of this drift?
NTP service or similar that runs your OS. They affects clock_gettime(CLOCK_REALTIME, ...);

这种漂移的原因是什么? NTP服务或运行您的操作系统的类似服务。它们会影响clock_gettime(CLOCK_REALTIME,...);

Is there anything I can do to keep them in sync (other than very frequently recalculating _start_tsc and _start_clock_time in step 2)? Yes you can ease the problem.

有什么办法可以让它们保持同步(除非经常在步骤2中重新计算_start_tsc和_start_clock_time)吗?是的,你可以缓解这个问题。

1 You can try to use CLOCK_MONOTONIC instead of CLOCK_REALTIME.

1您可以尝试使用CLOCK_MONOTONIC而不是CLOCK_REALTIME。

2 You can calculate the difference as a linear function from the time and apply it to compensate the drifting. But it will not be very reliable because time services doesn't adjust the time as linear function. But it will give you some more accuracy. Periodically you can do readjustment.

2您可以根据时间将差值计算为线性函数,并应用它来补偿漂移。但它不会非常可靠,因为时间服务不会将时间调整为线性函数。但它会给你更准确的一些。您可以定期进行重新调整。

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Some drifting you can get because you calculate _ticks_per_ns not accurately. You can check it by running you program several times. If results are not reproducible, it is mean that you calculate your _ticks_per_ns incorrectly. It is better to use statistics method then just an average value.

你可以得到一些漂移,因为你没有准确地计算_ticks_per_ns。您可以通过多次运行程序来检查它。如果结果不可重现,则表示您错误地计算了_ticks_per_ns。最好使用统计方法,然后只使用平均值。

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Also please note, _ticks_per_ns you are calculating by using CLOCK_MONOTONIC, which is related to TSC.

另请注意,_ticks_per_ns是使用CLOCK_MONOTONIC计算的,它与TSC有关。

Next you are using CLOCK_REALTIME. It provides the system time. If your system has NTP or similar service, the time will be adjusted.

接下来,您正在使用CLOCK_REALTIME。它提供系统时间。如果您的系统具有NTP或类似服务,则将调整时间。

Your difference is around 2 micro seconds per minute. It is 0.002 * 24*60 = 2.9 milli seconds a day. It is a great accuracy for CPU clock. 3 ms a day it is a 1 second a year.

你的差异大约是每分钟2微秒。每天0.002 * 24 * 60 = 2.9毫秒。它对CPU时钟来说非常准确。每天3毫秒,一年1秒。

#3

The relationship between the TSC and something like CLOCK_MONOTONIC will not be exactly unchanging. Even though you "calibrate" the TSC against CLOCK_MONOTONIC, the calibration will be out of date almost as soon as it is finished!

TSC和CLOCK_MONOTONIC之类的关系不会完全不变。即使您使用CLOCK_MONOTONIC“校准”TSC,校准也会在完成后立即过时!

The reasons they won't stay in sync long term:

他们不会长期保持同步的原因:

1. CLOCK_MONOTONIC is affected by NTP clock rate adjustments. NTP will constantly check network time and subtly slow down or speed up the system clock to match network time. This results in some kind of oscillating pattern in the true CLOCK_MONOTONIC frequency, and so your calibration will always be slightly off, especially the next time NTP applies a rate adjustment. You could compare against CLOCK_MONOTONIC_RAW to eliminate this effect.

CLOCK_MONOTONIC受NTP时钟频率调整的影响。 NTP将不断检查网络时间,并巧妙地减慢或加快系统时钟以匹配网络时间。这会在真正的CLOCK_MONOTONIC频率中产生某种振荡模式,因此您的校准将始终略微偏离,尤其是下次NTP应用速率调整时。您可以与CLOCK_MONOTONIC_RAW进行比较以消除此影响。

2. CLOCK_MONOTONIC and TSC are almost certainly based on totally different underlying oscillators. It is often say that modern OSes use the TSC for time-keeping, but this is only to apply a small "local" offset to some other underlying slow-running clock to provide a very precise time (e.g., the "slow time" might be updated every timer tick, and then the TSC is used to interpolate between timer ticks). It is the slow underlying clock (something like the HPET or APIC clocks) that determines the longer-term behavior of CLOCK_MONOTONIC. The TSC itself, however is an independent free running clock, deriving its frequency from a different oscillator, on a different place on the chipset/motherboard and will different natural fluctuations (in particular, different response to temperature changes).

CLOCK_MONOTONIC和TSC几乎肯定是基于完全不同的底层振荡器。通常说现代操作系统使用TSC进行计时,但这只是将一个小的“本地”偏移量应用于其他一些潜在的慢速运行时钟,以提供非常精确的时间(例如,“慢速时间”可能每次计时器滴答都会更新,然后TSC用于在计时器滴答之间进行插值)。慢速基础时钟(类似于HPET或APIC时钟)决定了CLOCK_MONOTONIC的长期行为。然而,TSC本身是一个独立的*运行时钟,它从不同的振荡器,在芯片组/主板上的不同位置获得其频率,并且将产生不同的自然波动(特别是对温度变化的不同响应)。

It is (2) that is more fundamental out of the two above: it means that even without any kind of NTP adjustments (or if you use a clock that is not subject to them), you'll see drift over time if the underlying clocks are based on different physical oscillators.

它是(2)在上面两个中更为基础:它意味着即使没有任何类型的NTP调整(或者如果你使用不受它们约束的时钟),你会发现如果潜在的时间有所不同时钟基于不同的物理振荡器。