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time_measure.md

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Measure CPU Time Stamp on A Ubuntu Server

System Configuration
  1. Edit /etc/default/grub as:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash intel_pstate=disable isolcpus=7 nohz_full=7 nosmep nosmap"

to disable Intel PState and to prevent system from using core 7. Then sudo update-grub && sudo reboot.

  1. Change BIOS settings:
  • Enable OS DBMP or OS control, and enable C_State.
  • Optionally disable hyper-threading on all cores.
  1. Set CPU Frequency:
  • Check if module acpi_cpuinfo is loaded: lsmod | grep acpi.
  • Check CPUFreq: lscpu | grep MHz.
  • An easy way using cpupower (sudo apt install linux-tools-common):
    • cpupower frequency-info shows available frequency settings.
    • cpupower frequency-set sets CPUs to run in the <freq> in KHz:
      • append --min <freq> or --max <freq> to set a range;
      • append --freq <freq> to set a specific frequency.
      • Example: cpupower frequency-set --freq 2600000.
  • A complex but more flexible way: 
    cd /sys/devices/system/cpu/
echo performance | tee cpu*/cpufreq/scaling_governor
// check available frequencies
cat cpu0/cpufreq/scaling_available_frequencies
// <freq> is in KHz, from availabel frequencies
echo <freq> | tee cpu*/cpufreq/scaling_max_freq
echo <freq> | tee cpu*/cpufreq/scaling_min_freq
// check if it works
lscpu | grep MHz
  1. Disable Hyper-threading on core 7: echo 0 > /sys/devices/system/cpu/cpu15/online.
  2. Set IRQ affinity mask to CPU 0: echo 1 | tee /proc/irq/*/smp_affinity.
  3. Run command on CPU 7: sudo taskset -c 7 <command>
Code to measure CPU Time Stamp

This code takes 64-bit measurements.

#include <time.h>

// either
#include <stdint.h>
// or use unsigned int / unsigned long int

// saves 64-bit CPU time stamp into `time`
inline void GetCPUTime(uint64_t& time) {
  unsigned int *val = (unsigned int *)time;
  asm volatile ("rdtscp" : "=a" (val[0]), "=d" (val[1])); // no mfence, lfence
}

uint64_t time_start, time_stop, time_elapsed;
GetCPUTime(time_start);
// ... target code ...
GetCPUTime(time_stop);
time_elapsed = time_stop - time_start; // roughly 20 if code is empty