It is the job of the linux 'oom killer' to sacrifice one or more processes in order to free up memory for the system when all else fails. It will also kill any process sharing the same mm_struct as the selected process, for obvious reasons. *Any particular process leader may be immunized against the oom killer* if the value of its /proc/<pid>/oomadj is set to the constant OOM_DISABLE (currently defined as *-17*).

The badness heuristic assigns a value to each candidate task ranging from *0 (never kill) to 1000 (always kill)* to determine which process is targeted.  

The units are roughly a proportion along that range of allowed memory the process may allocate from based on an estimation of its current memory and swap use.

For example, if a task is using all allowed memory, its badness score will be 1000.  If it is using half of its allowed memory, its score will be 500.

The value of /proc/<pid>/oom_score_adj is added to the badness score before it is used to determine which task to kill.  

Acceptable values range from *-1000 (OOM_SCORE_ADJ_MIN) to +1000 (OOM_SCORE_ADJ_MAX)*.  

This allows userspace to polarize the preference for oom killing either by always preferring a certain task or completely disabling it.  

The lowest possible value, *-1000, is equivalent to disabling oom killing entirely for that task* since it will always report a badness score of 0.

Caveat: when a parent task is selected, the oom killer will sacrifice any first generation children with separate address spaces instead, if possible.  

This avoids servers and important system daemons from being killed and loses the minimal amount of work.

root@rspadim-Latitude:/proc/2398/task# ls

*2398*  *_2400_*  2402  2403  2404  2406  2413  2414  2415  2416  2417  2418  2419  2420  2423

root@rspadim-Latitude:/proc/2398# echo -n 200 > oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

200

200

root@rspadim-Latitude:/proc/2398# echo -n -200 > task/2400/oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

0

-200

root@rspadim-Latitude:/proc/2398# echo -n 200 > task/2400/oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

200

200

root@rspadim-Latitude:/proc/2398# echo -n -200 > task/2400/oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

0

-200

root@rspadim-Latitude:/proc/2398# echo -n 200 > oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

200

200

root@rspadim-Latitude:/proc/2398# echo -n -100 > oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

0

-100

root@rspadim-Latitude:/proc/2398# echo -n -150 > oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

0

-150

root@rspadim-Latitude:/proc/2398# echo -n -200 > oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

0

-200

root@rspadim-Latitude:/proc/2398# echo -n -500 > oom_score_adj; cat oom_score; cat task/2400/oom_score_adj 

0

-500

/*

 * /proc/<pid>/oom_score_adj set to OOM_SCORE_ADJ_MIN disables oom killing for

 * pid.

 */

#define OOM_SCORE_ADJ_MIN       (-1000)

#define OOM_SCORE_ADJ_MAX       1000

/*

 * /proc/<pid>/oom_adj set to -17 protects from the oom killer for legacy

 * purposes.

 */

#define OOM_DISABLE (-17)

/* inclusive */

#define OOM_ADJUST_MIN (-16)

#define OOM_ADJUST_MAX 15

// NOTE: This is not the only version of this function in the source:

// the setuid sandbox (in process_util_linux.c, in the sandbox source)

// also has its own C version.

bool AdjustOOMScore(ProcessId process, int score) {

  if (score < 0 || score > kMaxOomScore)

    return false;

  FilePath oom_path(internal::GetProcPidDir(process));

  // Attempt to write the newer oom_score_adj file first.

  FilePath oom_file = oom_path.AppendASCII("oom_score_adj");

  if (PathExists(oom_file)) {

    std::string score_str = IntToString(score);

    DVLOG(1) << "Adjusting oom_score_adj of " << process << " to "

             << score_str;

    int score_len = static_cast<int>(score_str.length());

    return (score_len == WriteFile(oom_file, score_str.c_str(), score_len));

  }

  // If the oom_score_adj file doesn't exist, then we write the old

  // style file and translate the oom_adj score to the range 0-15.

  oom_file = oom_path.AppendASCII("oom_adj");

  if (PathExists(oom_file)) {

    // Max score for the old oom_adj range.  Used for conversion of new

    // values to old values.

    const int kMaxOldOomScore = 15;

    int converted_score = score * kMaxOldOomScore / kMaxOomScore;

    std::string score_str = IntToString(converted_score);

    DVLOG(1) << "Adjusting oom_adj of " << process << " to " << score_str;

    int score_len = static_cast<int>(score_str.length());

    return (score_len == WriteFile(oom_file, score_str.c_str(), score_len));

  }

  return false;

}