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Top Forums Programming Memory and cache access time discrepancy Post 302928311 by Vaibhavs1985 on Wednesday 10th of December 2014 07:05:08 PM
Old 12-10-2014
Memory and cache access time discrepancy

Code:
#include<stdio.h>
#include<stdlib.h>
#include<sys/time.h>
#include<time.h>
#include "rdtsc.h"
#define SIZE 4*64*1024

int main()
{
unsigned long long a,b;
int arr[SIZE]={0};
int i;
register int r;


a=rdtsc();
r=arr[0];
b=rdtsc();

printf("1st element Access Cycles = %llu\n",b-a);

a=rdtsc();
r=arr[1];
b=rdtsc();
printf("2nd Element Access Cycles = %llu\n",b-a);

}

In the above code I am trying to determine the number of cycles it takes to fetch the first element of an array from memory, and then the next cached element. When I execute this snippet, I am getting almost identical number of cycles for both accesses ~81 cycles. Can anybody explain me why this is happening. By all means, the first access should be very costly, but the access to next sequential element which has been brought into the cache should be much lesser.

Thanks.
 

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CURRENT(3)								 1								CURRENT(3)

current - Return the current element in an array

SYNOPSIS
mixed current (array &$array) DESCRIPTION
Every array has an internal pointer to its "current" element, which is initialized to the first element inserted into the array. PARAMETERS
o $array - The array. RETURN VALUES
The current(3) function simply returns the value of the array element that's currently being pointed to by the internal pointer. It does not move the pointer in any way. If the internal pointer points beyond the end of the elements list or the array is empty, current(3) returns FALSE. Warning This function may return Boolean FALSE, but may also return a non-Boolean value which evaluates to FALSE. Please read the section on Booleans for more information. Use the === operator for testing the return value of this function. EXAMPLES
Example #1 Example use of current(3) and friends <?php $transport = array('foot', 'bike', 'car', 'plane'); $mode = current($transport); // $mode = 'foot'; $mode = next($transport); // $mode = 'bike'; $mode = current($transport); // $mode = 'bike'; $mode = prev($transport); // $mode = 'foot'; $mode = end($transport); // $mode = 'plane'; $mode = current($transport); // $mode = 'plane'; $arr = array(); var_dump(current($arr)); // bool(false) $arr = array(array()); var_dump(current($arr)); // array(0) { } ?> NOTES
Note You won't be able to distinguish the end of an array from a boolean FALSE element. To properly traverse an array which may contain FALSE elements, see the each(3) function. SEE ALSO
end(3), key(3), each(3), prev(3), reset(3), next(3). PHP Documentation Group CURRENT(3)
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