Using C program to allocate memory using malloc on Linux to test OOM killer behavior

Sometimes we want to manually allocate memory on Linux system to test OOM killer's behavior.

Theory:

In OS with below kernel parameters:

1 2	vm.overcommit_memory = 2 vm.overcommit_ratio = 50

which means, OS will at most allocate virtual memory to:  SWAP size +50% RAM size.

For example, if OS has 48G RAM with 48G SWAP as shown below:

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[root]# free              total       used       free     shared    buffers     cached Mem:      49316552     841556   48474996          0     150432     466384 -/+ buffers/cache:     224740   49091812 Swap:     50307508          0   50307508

Which means, OS will at most allocate 48G+50%*48G = 72G virtual memory.

Experiment:

Let's do a simple test to prove that.
1.  Prepare "mem.c" C program which can allocate memory using function malloc:

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#include <stdio.h> #include <stdlib.h> int main(){  int *ptr;  unsigned long i,n;  printf("Enter number of int(4 byte) you want to allocate:");  scanf("%lu",&n);    printf("Allocating %lu bytes......\n",n*sizeof(int));  ptr=(int*)malloc(n*sizeof(int));  if (ptr==NULL){    printf("ERROR!Memory not allocated!");    exit(0);  }  printf("Filling int into memory.....\n");  for (i = 0; i < n; i++){     ptr[i] = 1;  }  printf("Sleep 10 seconds......\n");  sleep(10);  printf("Free memory.\n");  free(ptr);  return 0; }

2.  Compile it:

1	gcc mem.c -o mem

3.  If allocating 40G memory,  all of them are from RAM.

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[root]# ./mem Enter number of int(4 byte) you want to allocate:10000000000 Allocating 40000000000 bytes...... Filling int into memory..... Sleep 10 seconds...... Free memory.

Before :

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[root]# free              total       used       free     shared    buffers     cached Mem:      49316552     841556   48474996          0     150432     466384 -/+ buffers/cache:     224740   49091812 Swap:     50307508          0   50307508

After :

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[root]# free              total       used       free     shared    buffers     cached Mem:      49316552   39978952    9337600          0     150432     466408 -/+ buffers/cache:   39362112    9954440 Swap:     50307508          0   50307508

4.  If allocating 72G memory, 50% SWAP and all RAM are used up.
Note, previously we mentioned that the total virtual memory size is SWAP size +50% RAM size, that is only for calculating the size. In fact, RAM is firstly used before starting to use SWAP.

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[root]# ./mem Enter number of int(4 byte) you want to allocate:18000000000 Allocating 72000000000 bytes...... Filling int into memory..... Sleep 10 seconds...... Free memory.

After:

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[root]# free              total       used       free     shared    buffers     cached Mem:      49316552   49179688     136864          0        700      21856 -/+ buffers/cache:   49157132     159420 Swap:     50307508   21593608   28713900

5.  If allocating 80G memory,OOM killer shows up.

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[root]# ./mem Enter number of int(4 byte) you want to allocate:20000000000 Allocating 80000000000 bytes...... ERROR!Memory not allocated!

So one suggestion after adding more physical memory, is to add more SWAP also.