Week 5.a
CS6640
02/03 2023
https://naizhengtan.github.io/26spring/

1. Interrupt handler in egos-2k+
2. Review: OS scheduling
3. Processes in egos-2k+
4. Kernel scheduler
---

Admin:
 - Lab2 done
 - Lab3 released

1. Interrupt handler in egos-2k+

  [show slides]
  * Timer interrupt from last time
    (1) register handlers
    (2) enable timer interrupt
    (3) set timer
    We will see where egos-2k+ does these.

  CPU provides a set of registers:
    'mtvec',  'mstatus', 'mie', 'mtime', 'mtimecmp'

  EGOS: Timer handling
    [see earth/cpu_intr.c and earth/cpu_timer.c]

  * Timer Interrupt is one way to trap into kernel.
    Q: how many ways to trap to kernel?

    OS basics: there are three ways to trap into kernel
      (1) interrupt
      (2) exception
      (3) syscall

  Revisit RISC-V exception handling
    [see handout, panel 1]

  Q: with gdb and these exception CSRs, how to tell where the egos-2k+'s
  handler is?

  Exploration: where is the interrupt handler in egos-2k+?
    * in gdb, by "p/x $mtvec"; we will see mtvec = 0x80000020
    * by checking the memory layout, we found it is in egos's code
    * by checking build/debug/earth.lst, we see the handler named "trap_entry"
      in file grass/kernel.s

  * CSRs:
    briefly introduce CSRs
    [handout panel 1]


2. Review: OS scheduling

    High-level problem: operating system has to decide which process
    (or thread) to run.

    A. When scheduling decisions happen:

        [show process transition diagram]


                                                 exit (iv)
  [PROC_LOADING]                                +-------->[PROC_UNUSED]
     |                        (iii) interrupt   |
     +->[PROC_READY/PROC_RUNNABLE]  <------ [PROC_RUNNING]
                  ^                 ------>     |
      syscall msg  \              proc_yeild()  |
   or wake up (ii)  \                           |
                     \                         /
                     [PROC_PENDING_SYSCALL/ <-/ syscall or sleep (i)
                      PROC_SLEEPING]

    scheduling decisions take place when a process:

      (i) Switches from running to waiting state 
      (ii) Switches from waiting to ready 
      (iii) Switches from running to ready state 
      (iv) Exits 

    preemptive scheduling: at all four points

    nonpreemptive scheduling: at points (i), (iv), only (this is the
    definition of nonpreemptive scheduling)


    B. what are the metrics and criteria?

    timeline ----v---------v-------v------v-----v---->
              arrival  1st exec  yield   2nd  completion

    --turnaround time
    time for each process to complete (completion - arrival)

    --waiting/response/output time. variations on this definition,
    but they all capture the time spent waiting for something to
    happen rather than the time from launch to exit.

        --time between when job enters system and starts executing;
        following the class's textbook, we will call this "response time"
        (1st execution - arrival)
        [some texts call this "waiting time"]

        --time from request to first response (e.g., key press to
        character echo)
        we will call this "output time" 
        [some texts call this "response time"]

    --CPU time: how much CPU time the process actually consumed
       (this is the time when pc points to the code of this process)


3. Processes in egos-2k+

   A. Process control block in egos-2k+
      [see handout]

   B. Process life cycle in egos-2k+
      [see handout]

   C. Calculating scheduling metrics

    Q: how to calculate turnaround time?

    Q: how to calculate response time?

    Q: how to calculate CPU time?


4. Kernel scheduler

   A. Now
      [read grass/scheduler.c]

      Try "loop &" and "ls", which is super slow.

      Q: why this is the case?
      Because the loop which runs for a long time has the same priority as
      system services and short-lived jobs, so it takes at least one QUANTUM
      to hear back from system service.

   B. Multi-level feedback queue
      [a brief intro; see OSTEP Ch8]