uss.js

Cu.import("resource://gre/modules/Task.jsm");

GetUSS("self").then(s => alert(s +"kB"))


// Works with any PID. "self" is an alias to current process.
function GetUSS(PID) {
  const BUF_SIZE = 32 * 1024;
  let path = "/proc/" + PID + "/smaps";
  let decoder = new TextDecoder();

  return Task.spawn(function* () {
    let file = yield OS.File.open(path);
    let text = "";
    // There would be an easier way to do that
    // if we would be reading a regular file.
    // smaps' size is 0 at first until we start
    // reading, so we need to read piece by piece.
    for(;;) {
      let bytes = yield file.read(BUF_SIZE);
      if(bytes.byteLength == 0) {
        break;
      }
      text += decoder.decode(bytes);
    }
    let lines = text.split("\n");
    let total = 0;
    for (let line of lines) {
      if (line.indexOf("Private_Dirty") == 0 || line.indexOf("Private_Clean") == 0) {
        total += parseInt(line.match(/Private_.*:\s+([\d]+)/)[1]);
      }
    }
    return total;
  })
}