Out of memory, but swap available

In /proc/meminfo you find:

CommitLimit:    1551056920 kB
Committed_AS:   1549560424 kB

So you are at the commit limit.

If you have disabled overcommiting of memory (to avoid the OOM-killer) by:

echo 2 > /proc/sys/vm/overcommit_memory

Then the commit limit is computed as:

2   -   Don't overcommit. The total address space commit
        for the system is not permitted to exceed swap + a
        configurable amount (default is 50%) of physical RAM.
        Depending on the amount you use, in most situations
        this means a process will not be killed while accessing
        pages but will receive errors on memory allocation as
        appropriate.

(From: https://www.kernel.org/doc/Documentation/vm/overcommit-accounting)

You can use the full memory by:

echo 100 > /proc/sys/vm/overcommit_ratio

Then you will get out-of-memory when physical RAM and swap is all reserved.

The name overcommit_ratio is in this case a bit misleading: You are not overcommitting anything.

Even with this setup you may see out-of-memory before swap is exhausted. malloc.c:

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

void main(int argc, char **argv) {
  long bytes, sleep_sec;
  if(argc != 3) {
    printf("Usage: malloc bytes sleep_secn");
    exit(1);
  }
  sscanf(argv[1],"%ld",&bytes);
  sscanf(argv[2],"%ld",&sleep_sec);
  printf("Bytes: %ld Sleep: %ldn",bytes,sleep_sec);
  if(malloc(bytes)) {
    sleep(sleep_sec);
  } else {
    printf("Out of memoryn");
    exit(1);
  }
}

Compile as:

gcc -o malloc malloc.c

Run as (reserve 1 GB for 10 seconds):

./malloc 1073741824 10

If you run this you may see OOM even though there is swap free:

# Plenty of ram+swap free before we start
$ free -m
              total        used        free      shared  buff/cache   available
Mem:         515966        2824      512361          16         780      511234
Swap:       1256720           0     1256720

# Reserve 1.8 TB
$ ./malloc 1800000000000 100 &
Bytes: 1800000000000 Sleep: 100

# It looks as if there is plenty of ram+swap free
$ free -m
              total        used        free      shared  buff/cache   available
Mem:         515966        2824      512361          16         780      511234
Swap:       1256720           0     1256720

# But there isn't: It is all reserved (just not used yet)
$ cat /proc/meminfo |grep omm
CommitLimit:    1815231560 kB
Committed_AS:   1761680484 kB

# Thus this fails (as you would expect)
$ ./malloc 180000000000 100
Bytes: 180000000000 Sleep: 100
Out of memory

So while free in practice often will do The Right Thing, looking at CommitLimit and Committed_AS seems to be more bullet-proof.