CSE 451: Introduction to Operating Systems

 

Section 2, April/5/2001

Jochen Jäger

 

 

 

 

 

 

1.    Solutions to Unix tasks

2.    Project turn in procedure/policy

3.    Questions about lecture, project, etc

4.    Scheduler, processes & context switching

 

Basic Unix commands:

 

 

pwd:           print name of current/working directory

 

cd:               Change working directory (cd returns to homedir)

 

ls:                list directory contents

 

find:            search for files in a directory hierarchy

 

grep:           print lines matching a pattern

 

more:           file viewer, see also less

 

cat:              print files on stdout, see also: cat file1 >> file2

 

cp:               copy files and directories

 

man:           format and display the on-line manual pages

 

vi:               text editor

 

emacs:           text editor, see also xemacs

 

Tasks:

 

find a file called sched.h

 

ls sched.h

 

find a textstring in sched.h

 

grep define sched.h

 

find a textstring in all .h and .c files in the current directory

 

grep define *.[ch]

 

find a textstring in all .h and .c files in all subsequent dirs

 

          grep define `find . -name '*.[ch]'`

Attention this is not the same:

 find . -name '*.[ch]' | grep define

 

find all files recently modified

 

          ls -rt

 

 

find last 5 most recently files

 

          ls –rt | tail -5

 

find all files modified within the last 48 hours

 

          find . –ctime 2

 

 

Project turn-in:

Accepted formats:

-         Email: Plain Text, Word, PS, PDF

-         Hardcopy: name, project nr, date on cover sheet and paper clipped or stapled at left top corner

or name, project nr, date on every sheet

 

Turn-in time (both electronically as well as hardcopy):

end of sections

 

Late Policy:

we will take off 20% for each day (or portion thereof) it is late.

 

Cheating vs. Collaboration

The Gilligan's Island Rule:

-         that you are free to meet with fellow students(s) and discuss assignments with them.

-         Writing on a board or shared piece of paper is acceptable during the meeting; however, you should not take any written (electronic or otherwise) record away from the meeting.

-         This applies when the assignment is supposed to be an individual effort or whenever two teams discuss common problems they are each encountering (inter-group collaboration).

-         After the meeting, engage in a half hour of mind-numbing activity (like watching an episode of Gilligan's Island), before starting to work on the assignment. This will assure that you are able to reconstruct what you learned from the meeting, by yourself, using your own brain.

 

The Freedom of Information Rule:

To assure that all collaboration is on the level, you must always write the name(s) of your collaborators on your assignment.

 

Scheduling

 

Scheduler selects process which will run next

Linux uses pre-emptive scheduling

 

Time-slice: e.g. 200ms

Quantum: time for a process during one epoch

Epoch:  CPU scheduling cycle

 

The epoch ends when all runnable processes have exhausted their quantum; in this case, the scheduler algorithm recomputes the time-quantum durations of all processes and a new epoch begins.

 

struct task_struct {

…

        long counter; // amount of time (in jiffies) that this process is allowed to run for, it is decremented each clock tick.

        long nice;

        unsigned long policy;  // scheduling policy for this process: normal or real time (round robin or first in first out)

        struct mm_struct *mm;

        int has_cpu, processor;

        unsigned long cpus_allowed;

…

        struct task_struct *next_task, *prev_task;

        struct mm_struct *active_mm;

…

        pid_t pid;

        pid_t pgrp;

…

        unsigned long rt_priority;

…

        uid_t uid,euid,suid,fsuid;

        gid_t gid,egid,sgid,fsgid;

...

        struct user_struct *user;

…

};

 

 

The schedule( ) Function

 

Direct invocation: when the current process must be blocked right away because the resource it needs is not available

 

Lazy invocation: Setting the need_resched field of current to 1. Since a check on the value of this field is always made before resuming the execution of a User Mode process, schedule( )

will definitely be invoked at some close future time.

 

 

Kernel/sched.c

 

asmlinkage void schedule(void)

{

…

repeat_schedule:

        /*

         * Default process to select..

         */

        next = idle_task(this_cpu);

        c = -1000;

        if (prev->state == TASK_RUNNING)

                goto still_running;

 

still_running_back:

        list_for_each(tmp, &runqueue_head) {

                p = list_entry(tmp, struct task_struct, run_list);

              if (can_schedule(p, this_cpu)) {

                        int weight = goodness(p, this_cpu, prev->active_mm);

                        if (weight > c)

                                c = weight, next = p;

                }

        }

 

        /* Do we need to re-calculate counters? */

        if (!c)

                goto recalculate;

        sched_data->curr = next;

…

 

recalculate:

        {

                struct task_struct *p;

                spin_unlock_irq(&runqueue_lock);

                read_lock(&tasklist_lock);

                for_each_task(p)

                        p->counter = (p->counter >> 1) + NICE_TO_TICKS(p->nice);

                read_unlock(&tasklist_lock);

                spin_lock_irq(&runqueue_lock);

        }

        goto repeat_schedule;

 

still_running:

        c = goodness(prev, this_cpu, prev->active_mm);

        next = prev;

        goto still_running_back;

…

}