linux-kernel - Intel Virtualization and clock

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Date:   Thu, 9 Nov 2017 07:47:46 -0600
From:   "Mr. Berkley Shands" <bshands@...gy.com>
To:     linux-kernel <linux-kernel@...r.kernel.org>
Subject: Intel Virtualization and clock_gettime reproducer

This affects every kernel I tried, 4.9.[54|35|24], 3.18.27, RH 3.10.0-693*.
The test machine is a Dell R740 with 2X8168 and 384GB.

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

inline long GetPerf()
{
    int _hi, _lo;
    __asm__ __volatile__ ("rdtsc":"=a" (_lo), "=d"(_hi));
    return ((long) _hi << 32) | _lo;
}

int main()
{

    struct timespec ts;
    int i, count;
    long start, end, delta, min, max, total, avg;

    count = 100;
    min = 0x7fffffff;
    max = 0;
    total = 0;

    for (i = 0; i < count; i++)
    {

       start = GetPerf();

       clock_gettime(CLOCK_REALTIME, &ts);

       end = GetPerf();

       delta = end - start;

       if (delta > max) max = delta;
       if (delta < min) min = delta;
       total += delta;
    }

    avg = total / count;

    printf("min: %ld\n", min);

--

//E. F. Berkley Shands, MSc
Senior Software Architect/Engineer
Exegy Systems Engineering//

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