Is getrusage not working for me? Why?

Question

I'm trying to measure the maximum resident set size and I found that you can do so with getrusage:

When I run this

#include <iostream>
#include<vector>
#include <sys/resource.h>

using namespace std;

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

    int who = RUSAGE_SELF;
    struct rusage usage;
    int ret = -1;

    vector<int> v(1024);

    ret = getrusage(who, &usage);
    if (ret == 0) cout << usage.ru_maxrss << endl;

    return 0;
}

I get the same value as I do when I comment the declaration of the vector.

Is there anything I'm doing wrong?

Thanks!

Is your compiler maybe optimising out the vector because it isn't being used? Try iterating the vector and pushing values into v. — Irelia, Jul 31 '19 at 14:41
I understand that this allows you to get the resource usage up to a certain point, so in theory I could use the vector after that and this should take the vector into account. But yes, I tried writing values on the vector. Thanks for your comment anyways. — Sebastián, Jul 31 '19 at 15:04
Have you tried modifying the 1024 to a larger value (e.g. 100,000)? perhaps 1024 is not enough to incur memory increase. — Tomer, Jul 31 '19 at 15:13
Why wouldn't it be enough? In theory it's the number of KB, so a vector of 1024 ints should add 4KB. Anyways, I tried with 1024000 and the increase in the output didn't make any sense to me :/ Thanks for your comment. — Sebastián, Aug 01 '19 at 17:44

score 2 · Answer 1 · answered Mar 28 '20 at 19:01

I have modified your code a bit to make sense for you for the data which is shown by the ru_maxrss element of rusage structure.

Here is my code

#include <vector>
#include <sys/resource.h>

using namespace std;

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

    int who = RUSAGE_SELF;
    struct rusage usage;
    int ret = -1;
    long int a,b;

getrusage(who, &usage);
a=usage.ru_maxrss;

cout << "Maximum resident set size before vector allocation   "<<a << endl;
    vector<int> v(10240000);

cout<<"Size of vector v in kilobytes (kB)  "<<(v.size()*sizeof(int))/(1024)<<endl;

    ret = getrusage(who, &usage);

    cout<<"Size of integer data type   "<<sizeof(int)<<endl;
    b=usage.ru_maxrss;

    if (ret == 0) 
cout << "Maximum resident set size after vector allocation  "<<b << endl;
 cout<<"Difference between resident set size before and after vector allocation  "<<b-a<<endl;

    return 0;
}

Each time getrusage() is called it allocates a different resident set size which is a few kB(approx 600-800) greater than the size allocated for the vector.

Some of the outputs of the above code are as follow

If I reduce the no. of elements in the vector to 1024000 the output is as follow

Is getrusage not working for me? Why?

1 Answers1