When Linux Runs Out of Memory

Perhaps you rarely face it, but once you do, you surely know what's wrong: lack of free memory, or Out of Memory (OOM). The results are typical: you can no longer allocate more memory and the kernel kills a task (usually the current running one). Heavy swapping usually accompanies this situation, so both screen and disk activity reflect this.
At the bottom of this problem lie other questions: how much memory do you want to allocate? How much does the operating system (OS) allocate for you? The basic reason of OOM is simple: you've asked for more than the available virtual memory space. I say "virtual" because RAM isn't the only place counted as free memory; any swap areas apply.

Exploring OOM

To begin exploring OOM, first type and run this code snippet that allocates huge blocks of memory:

#include 
#include 

#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;
printf("Currently allocating %d MB\n", ++count);
}

exit(0);
}

Compile the program, run it, and wait for a moment. Sooner or later it will go OOM. 
Now compile the next program, which allocates huge blocks and fills them with 1: