Linux OOM killer does not work

Question

I would like to test if the kernel OOM killer work fine on my embedded Linux or not. I used an application test to fill all memory and see if OOM will kill my application if the system run in out of memory condition.

The test program I used:

#include <stdio.h>
#include <stdlib.h>

#define MEGABYTE 1024*1024

int main(int argc, char *argv[])
{
        void *myblock = NULL;
        int count = 0;

        while(1)
        {
                myblock = (void *) malloc(MEGABYTE);
                if (!myblock) break;
                memset(myblock,1, MEGABYTE);
                printf("Currently allocating %d MB\n",++count);

        }
        exit(0);

}

Results:

I always get :

MyApplication triggered out of memory codition (oom killer not called): gfp_mask=0x1200d2, order=0, oomkilladj=0

I try to change /etc/sysctl by adding :

vm.oom_kill_allocating_task=1
vm.panic_on_oom=0
vm.overcommit_memory=0
how can I make OOM works fine on my system 

Kernel version :2.6.30 #7 SMP PREEMPT

Answer 1

The Linux “OOM killer” is a solution to the overcommit problem .

If you just “fill all memory”, then overcommit will not show up. The malloc call will eventually return a null pointer, the convention to indicate that the memory request cannot be fulfilled.

In order to cause an overcommit-related problem, you must allocate too much memory without writing to it , and then decide to write to all of it, so that the system finds itself forced to honor promises it made without having the capacity to fulfill them.

EDIT after source code was provided:

To be completely precise, in order to trigger a problem with overcommit and force the Linux OOM killer to take action, you should have several processes that in a first phase all reserve memory with malloc() (but do not write to it yet). Then have all of them write to the memory they have reserved at the same time. This will force Linux to honor the memory promises outside of any memory allocation, and it will have no choice but to kill a process that wasn't allocating (since none of them will be allocating at that moment).

Answer 2

Also, if you still want to see how or when OOM-killer works. I would suggest you to add fork() before while loop. That will create many processes, and eventually one of them OOM-killer will kill.