OS signaling

SMP5: Scheduler with Signals
============================

This MP is a variation of SMP4.

In the last MP, we built a simulated OS process scheduler. The scheduler can
hold only a certain number of processes (workers) at one time. Once the process
has been accepted into the scheduler, the scheduler decides in what order the
processes execute. We implemented two scheduling algorithms: FIFO and Round
Robin.

In this MP, we are to simulate a time-sharing system by using signals and
timers. We will only implement the Round Robin algorithm. Instead of using
iterations to model the concept of “time slices” (as in the last MP), we use
interval timers.  The scheduler is installed with an interval timer. The timer
starts ticking when the scheduler picks a thread to use the CPU which in turn
signals the thread when its time slice is finished thus allowing the scheduler
to pick another thread and so on. When a thread has completely finished its work
it leaves the scheduler to allow a waiting thread to enter. Please note that in
this MP, only the timer and scheduler send signals. The threads passively handle
the signals without signaling back to the scheduler.

The program takes a number of arguments. Arg1 determines the number of jobs
(threads in our implementation) created; arg2 specifies the queue size of the
scheduler. Arg3 through argN gives the duration (the required time slices to
complete a job) of each job. Hence if we create 2 jobs, we should supply arg3
and arg4 for the required duration. You can assume that the autograder will
always supply the correct number of arguments and hence you do not have to
detect invalid input.

Here is an example of program output, once the program is complete:

% scheduler 3 2 3 2 3
Main: running 3 workers with queue size 2 for quanta:
 3 2 3
Main: detaching worker thread 3075926960.
Main: detaching worker thread 3065437104.
Main: detaching worker thread 3054947248.
Main: waiting for scheduler 3086416816.
Scheduler: waiting for workers.
Thread 3075926960: in scheduler queue.
Thread 3075926960: suspending.
Thread 3065437104: in scheduler queue.
Thread 3065437104: suspending.
Scheduler: scheduling.
Scheduler: resuming 3075926960.
Thread 3075926960: resuming.
Scheduler: suspending 3075926960.
Scheduler: scheduling.
Scheduler: resuming 3065437104.
Thread 3065437104: resuming.
Thread 3075926960: suspending.
Scheduler: suspending 3065437104.
Scheduler: scheduling.
Scheduler: resuming 3075926960.
Thread 3075926960: resuming.
Thread 3065437104: suspending.
Scheduler: suspending 3075926960.
Scheduler: scheduling.
Scheduler: resuming 3065437104.
Thread 3065437104: resuming.
Thread 3075926960: suspending.
Scheduler: suspending 3065437104.
Thread 3065437104: leaving scheduler queue.
Scheduler: scheduling.
Scheduler: resuming 3075926960.
Thread 3075926960: resuming.
Thread 3065437104: terminating.
Thread 3054947248: in scheduler queue.
Thread 3054947248: suspending.
Scheduler: suspending 3075926960.
Thread 3075926960: leaving scheduler queue.
Scheduler: scheduling.
Scheduler: resuming 3054947248.
Thread 3054947248: resuming.
Thread 3075926960: terminating.
Scheduler: suspending 3054947248.
Scheduler: scheduling.
Scheduler: resuming 3054947248.
Thread 3054947248: suspending.
Thread 3054947248: resuming.
Scheduler: suspending 3054947248.
Scheduler: scheduling.
Scheduler: resuming 3054947248.
Thread 3054947248: suspending.
Thread 3054947248: resuming.
Scheduler: suspending 3054947248.
Thread 3054947248: leaving scheduler queue.
Thread 3054947248: terminating.
The total wait time is 12.062254 seconds.
The total run time is 7.958618 seconds.
The average wait time is 4.020751 seconds.
The average run time is 2.652873 seconds.

The goal of this MP is to help you understand (1) how signals and timers work,
and (2) how to evaluate the performance of your program. You will first
implement the time-sharing system using timers and signals. Then, you will
evaluate the overall performance of your program by keeping track of how long
each thread is idle, running, etc.

The program will use these four signals:

SIGALRM: sent by the timer to the scheduler, to indicate another time
  quantum has passed.
SIGUSR1: sent by the scheduler to a worker, to tell it to suspend.
SIGUSR2: sent by the scheduler to a suspended worker, to tell it to resume.
SIGTERM: sent by the scheduler to a worker, to tell it to cancel.

You will need to set up the appropriate handlers and masks for these signals.

You will use these functions:
  clock_gettime
  pthread_sigmask
  pthread_kill
  sigaction
  sigaddset
  sigemptyset
  sigwait
  timer_settime
  timer_create

Also, make sure you understand how the POSIX:TMR interval timer works.

There are two ways you can test your code.  You can run the built-in grading
tests by running “scheduler -test -f0 rr”.  This runs 5 tests, each of which can
be run individually.  You can also test you program with specific parameters by
running “scheduler -test gen …” where the ellipsis contains the parameters you
would pass to scheduler.

Programming
===========

Part I: Modify the scheduler code (scheduler.c)
———————————————–

We use the scheduler thread to setup the timer and handle the scheduling for the
system.  The scheduler handles the SIGALRM events that come from the timer, and
sends out signals to the worker threads.

Step 1.

Modify the code in init_sched_queue() function in scheduler.c to initialize the
scheduler with a POSIX:TMR interval timer. Use CLOCK_REALTIME in timer_create().
The timer will be stored in the global variable “timer”, which will be started
in scheduler_run() (see Step 4 below).

Step 2.

Implement setup_sig_handlers().  Use sigaction() to install signal handlers for
SIGALRM, SIGUSR1, and SIGTERM.  SIGALRM should trigger timer_handler(), SIGUSR1
should trigger suspend_thread(), and SIGTERM should trigger cancel_thread().
Notice no handler is installed for SIGUSR2; this signal will be handled
differently, in step 8.

Step 3.

In the scheduler_run() function, start the timer.  Use timer_settime().  The
time quantum (1 second) is given in scheduler.h.  The timer should go off
repeatedly at regular intervals defined by the timer quantum.

In Round-Robin, whenever the timer goes off, the scheduler suspends the
currently running thread, and tells the next thread to resume its operations
using signals. These steps are listed in timer_handler(), which is called every
time the timer goes off.  In this implementation, the timer handler makes use of
suspend_worker() and resume_worker() to accomplush these steps.

Step 4.

Complete the suspend_worker() function.  First, update the info->quanta value.
This is the number of quanta that remain for this thread to execute.  It is
initialized to the value passed on the command line, and decreases as the thread
executes.  If there is any more work for this worker to do, send it a signal to
suspend, and update the scheduler queue.  Otherwise, cancel the thread.

Step 5.

Complete the cancel_worker() function by sending the appropriate signal to the
thread, telling it to kill itself.

Step 6.

Complete the resume_worker() function by sending the appropriate signal to the
thread, telling it to resume execution.

Part II: Modify the worker code (worker.c)
——————————————

In this section, you will modify the worker code to correctly handle the signals
from the scheduler that you implemented in the previous section.

You need to modify the thread functions so that it immediately suspends the
thread, waiting for a resume signal from the scheduler. You will need to use
sigwait() to force the thread to suspend itself and wait for a resume signal.
You need also to implement a signal handler in worker.c to catch and handle the
suspend signals.

Step 7.

Modify start_worker() to (1) block SIGUSR2 and SIGALRM, and (2) unblock SIGUSR1
and SIGTERM.

Step 8.

Implement suspend_thread(), the handler for the SIGUSR1 signal.  The
thread should block until it receives a resume (SIGUSR2) signal.

Part III: Modify the evaluation code (scheduler.c)
————————————————–

This program keeps track of run time, and wait time.  Each thread saves these
two values regarding its own execution in its thread_info_t.  Tracking these
values requires also knowing the last time the thread suspended or resumed.
Therefore, these two values are also kept in thread_info_t.  See scheduler.h.

In this section, you will implement the functions that calculate run time and
wait time.  All code that does this will be in scheduler.c.  When the program
is done, it will collect all these values, and print out the total and average
wait time and run time.  For your convenience, you are given a function
time_difference() to compute the difference between two times in microseconds.

Step 9.

Modify create_workers() to initialize the various time variables.

Step 10.

Implement update_run_time().  This is called by suspend_worker().

Step 11.

Implement update_wait_time().  This is called by resume_worker().

Questions
==========

Question 1.
Why do we block SIGUSR2 and SIGALRM in worker.c?  Why do we unblock SIGUSR1 and
SIGTERM in worker.c?

Question 2.
We use sigwait() and sigaction() in our code. Explain the difference between the
two. (Please explain from the aspect of thread behavior rather than syntax).

Question 3.
When we use POSIX:TMR interval timer, we are using relative time. What is the
alternative? Explain the difference between the two.

Question 4.
Look at start_worker() in worker.c, a worker thread is executing within an
infinite loop at the end. When does a worker thread terminate?

Question 5.
When does the scheduler finish?  Why does it not exit when the scheduler queue
is empty?

Question 6.
After a thread is scheduled to run, is it still in the sched_queue? When is it
removed from the head of the queue? When is it removed from the queue completely?

Question 7.
We’ve removed all other condition variables in SMP4, and replaced them with a
timer and signals. Why do we still use the semaphore queue_sem?

Question 8.
What’s the purpose of the global variable “completed” in scheduler.c? Why do we
compare “completed” with thread_count before we wait_for_queue() in
next_worker()?

Question 9.
We only implemented Round Robin in this SMP. If we want to implement a FIFO
scheduling algorithm and keep the modification as minimum, which function in
scheduler.c is the one that you should modify? Briefly describe how you would
modify this function.

Question 10.
In this implementation, the scheduler only changes threads when the time quantum
expires.  Briefly explain how you would use an additional signal to allow the
scheduler to change threads in the middle of a time quantum.  In what situations
would this be useful?