Deprication of now/0 and time precision issue in windows

[c-bik]

In windows erlang uses GetSystemTime to get current system time:

> erlang:system_info(os_system_time_source).                                                                          
[{function,'GetSystemTime'},
 {resolution,100},
 {parallel,yes},
 {time,1482414054673408}]

This has the following limitation
   Remarks
   The resolution of the GetTickCount function is limited to the resolution of the system timer,
   which is typically in the range of 10 milliseconds to 16 milliseconds. The resolution of the
   GetTickCount function is not affected by adjustments made by the GetSystemTimeAdjustment
   function.

As an alternative, GetSystemTimePreciseAsFileTime can be used for Windows (NIF), which gives:
   The GetSystemTimePreciseAsFileTime function retrieves the current
   system date and time with the highest possible level of precision (<1us).
   The retrieved information is in Coordinated Universal Time (UTC) format.

Current Resolution : 10 millisecond

[acautin]

in OS X I got:

erlang:system_info(os_system_time_source).
[{function,gettimeofday},
 {resolution,1000000},
 {parallel,yes},
 {time,1447671428706194000}]

Resolution : 1 micro second

[c-bik]

In CentOS release 6.6 (Final), 2.6.32-504.1.3.el6.x86_64 #1 SMP I got:

Erlang/OTP 18 [erts-7.1] [source-2882b0c] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]

Eshell V7.1  (abort with ^G)
1> erlang:system_info(os_system_time_source).
[{function,clock_gettime},
 {clock_id,'CLOCK_REALTIME'},
 {resolution,1000000000},
 {parallel,yes},
 {time,1447672387678884590}]

Resolution : 1 nano second

[c-bik]

@acautin MAC To be same as Linux in clock resolution.
From https://gist.github.com/jbenet/1087739

/* 
author: jbenet
os x, compile with: gcc -o testo test.c 
linux, compile with: gcc -o testo test.c -lrt
*/

#include <time.h>
#include <sys/time.h>
#include <stdio.h>

#ifdef __MACH__
#include <mach/clock.h>
#include <mach/mach.h>
#endif


void current_utc_time(struct timespec *ts) {

#ifdef __MACH__ // OS X does not have clock_gettime, use clock_get_time
  clock_serv_t cclock;
  mach_timespec_t mts;
  host_get_clock_service(mach_host_self(), CALENDAR_CLOCK, &cclock);
  clock_get_time(cclock, &mts);
  mach_port_deallocate(mach_task_self(), cclock);
  ts->tv_sec = mts.tv_sec;
  ts->tv_nsec = mts.tv_nsec;
#else
  clock_gettime(CLOCK_REALTIME, ts);
#endif

}


int main(int argc, char **argv) {

  struct timespec ts;
  current_utc_time(&ts);

  printf("s:  %lu\n", ts.tv_sec);
  printf("ns: %lu\n", ts.tv_nsec);
  return 0;

}

[shamis]

fixed in #151