how to measure exact cpucycles of a task in C++ linux

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Dear All,
I am working on C++ in Linux and I want to find the exact number of cpucycles used my task for execution.But whenever I run my code I get different value of cpucycles.How I can find the exact cpucycles used by my task.Also how I can convert cpucycles to actual execution time.I have also appended my code below.

Thanks in advance
Regards,
Ishtiaq

#include<stdio.h>

static __inline__ unsigned long long rdtsc(void)
{
  unsigned long long int x;
     __asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
     return x;
}


int main() {
    	unsigned long long cycles = rdtsc(); //1
    	cycles = rdtsc() - cycles;           //2
	
	 
    printf("Time is %d\n", (unsigned)cycles);

    return 0;
}

Posted 10-Jun-11 4:45am

smishtiaqhussain

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OriginalGriff · Accepted Answer · 2011-06-10T04:59:00

You can't find out the exact number of cycles for a number of reasons:
1) The timer isn't fast enough - the minimum interval between two timer "ticks" is greater than the processor cycle time.
2) The execution time will vary depending on what other tasks are running
3) The execution time will vary depending on where your code is, and what it is doing: if it is in the CPU cache(s) entirely, it can run as a very high speed!

TRK3 · Accepted Answer · 2011-06-10T09:31:00

Solution 2

see:

http://www.ecrypt.eu.org/ebats/cpucycles.html[^]

If your task is short, then then best thing to do is run it multiple times and take the average elapsed cpucycles.

Posted 10-Jun-11 9:31am

TRK3

Comments

smishtiaqhussain 13-Jun-11 7:47am

Thanks a lot for your guidance.

LloydA111 · Accepted Answer · 2011-06-10T09:57:00

It returns different values each time because of what else the system is doing. For example, the OS may be servicing an interrupt, and during this time your process will be suspended for an extremely small amount of time until the OS continues the execution of your program, but this small amount of time (depending on processor speed) could equal many cycles.

To achieve more accurate results, you could write a bootable program which will test your code's speed, this way there will be no OS affecting your code.

how to measure exact cpucycles of a task in C++ linux

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