Process.scala

object Process {

  type UserTime = Long
  type KernelTime = Long
  type StartTime = Long

  //Hertz (number of clock ticks per second) of your system.
  val Hz: Long = executeCmd("getconf CLK_TCK").map(_.toLong).getOrElse(100L)

  /**
    * A process is an instance of a computer program that is being executed.
    *
    * @param pid process Id.
    * @param name process name.
    * @param uTime CPU time spent in user code, measured in clock ticks.
    * @param sTime CPU time spent in kernel code, measured in clock ticks.
    * @param startTime Time when the process started, measured in clock ticks.
    */
  case class Process(pid:Int, name:String, uTime:UserTime, sTime:KernelTime, startTime:StartTime) extends Ordered[Process] {
    override def compare(that: Process): Int =
      java.lang.Double.compare((that.uTime + that.sTime) / that.startTime.toDouble, (this.uTime + this.sTime) / this.startTime.toDouble)
  }

  /**
    * A "jiffy" is a unit of CPU time.
    *
    * Exactly what it corresponds to in wall-clock time depends on the architecture and how your kernel is configured,
    * but the important thing is that /proc/stat tells you how many jiffies the CPU has executed in total
    * and /proc/<PID>/stat tells you how many jiffies have been executed by a single process.
    *
    * @param pid process id
    * @return (UserTime, KernelTime, StartTime) in jiffies
    */
  def jiffiesByProcess(pid: Long): (UserTime, KernelTime, StartTime) =
    firstLineOf(s"/proc/$pid/stat").map { line =>
      val values = line.split(" ")
      val uTime = Option(values(13).toLong).getOrElse(0L) * 1000L / Hz
      val sTime = Option(values(14).toLong).getOrElse(0L) * 1000L / Hz
      val startTime = Option(values(21).toLong).getOrElse(0L) * 1000L / Hz
      (uTime, sTime, startTime)
    }.getOrElse((0L, 0L, 0L))


  private def getProcessByPid(pid:Int):Option[Process] = {
    firstLineOf(s"/proc/$pid/stat").map { line =>
      val values = line.split(" ")
      //See man proc for how to parse /proc/[pid]/stat
      val name = values(1).replaceFirst("\\(", "").replace(")", "")
      val (uTime, sTime, startTime) = jiffiesByProcess(pid)
      Process(pid, name, uTime, sTime, System.currentTimeMillis() - startTime)
    }
  }

  private def firstLineOf(f: String): Option[String] = {
    val src = Source.fromFile(f)
    try src.getLines.find(_ => true) finally {
      src.close()
    }
  }

  private def executeCmd(cmd:String): Option[String] = {
    import sys.process._
    Try((cmd !!).trim).toOption
  }
}

object Pid {
  val Digits: Pattern = Pattern.compile("\\d+")

  /**
    * Gets an Seq of integers in the /proc directory with only numeric digit
    * filenames, corresponding to processes
    *
    * @return An Seq of integers that represents the process ids.
    */
  def getAll: Seq[Int] = Option(new File("/proc").listFiles(new FileFilter() {
    override def accept(file: File): Boolean =
      Digits.matcher(file.getName).matches()
  }).toList.map(_.getName.toInt)).getOrElse(Nil)
}

object Test extends App {
    while(true) {
      Thread.sleep(1000)
      println(Pid.getAll.map(p => Process.getProcessByPid(p)).sorted.take(10))
    }
}

//List(Some(Process(5549,Web,0,26673100,1513710984082)), Some(Process(31238,chrome,16024140,5713990,1513130505838)), Some(Process(24479,java,11861730,3099950,1512196478427)), Some(Process(1390,Xorg,6211480,4629680,1513710968296)), Some(Process(5057,firefox,6082530,2672330,1512697554152)), Some(Process(31317,chrome,5734670,2596150,1513130505249)), Some(Process(31778,chrome,5766340,513800,1513130498929)), Some(Process(1543,pulseaudio,2779660,2887010,1513710967307)), Some(Process(31807,chrome,4682150,616300,1513130494870)), Some(Process(1984,gnome-shell,3135200,423750,1512420219311)))