📄 real-time experiment #6 preemptive priority based task scheduling.htm
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<H1>Embry-Riddle Real-Time Laboratory Experiment<BR>Experiment #6<BR>Preemptive
Priority Based Task Scheduling </H1></CENTER>
<HR SIZE=3>
<H2>Introduction</H2>Task scheduling is the assignment of starting and ending
times to a set of tasks, subject to certain constraints. Constraints are
typically either time constraints or resource constraints. On a time-sharing
operating system, running each active process in turn for its share of time (its
"timeslice"), thus creating the illusion that multiple processes are running
simultaneously on a single processor.
<P></P><EM>Wind</EM> task scheduling uses a priority based preemptive scheduling
algorithm as default, but it can also accommodate round-robin scheduling.
<P></P>
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<H2>Objectives</H2>The following are the primary objectives of this experiment:
<UL>
<LI>To demonstrate the use of VxWorks preemptive priority based task
scheduling facilities. </LI></UL>
<P></P>
<HR SIZE=3>
<H2>Description</H2>
<UL>
<LI><B>Preemptive Priority Based Scheduling</B>
<P></P>With a preemptive priority based scheduler, each task has a priority
and the kernel insures that the CPU is allocated to the highest priority task
that is ready to run. This scheduling method is <EM>preemptive</EM> in that if
a task that has a higher priority than the current task becomes ready to run,
the kernel immediately saves the current tasks's context and switches to the
context of the higher priority task.
<P></P>The <EM>Wind</EM> kernel has 256 priority levels(0-255). Priority 0 is
the highest and priority 255 is the lowest. Tasks are assigned a priority when
created; however, while executing, a task can change its priority using
<EM>taskPrioritySet()</EM>. </LI></UL>
<P></P><B>1. Example: Preemptive Priority Based Scheduling</B>
<P></P>One of the arguments to <EM>taskSpawn()</EM> is the priority at which the
task is to execute:
<P></P><EM>id</EM> = taskSpawn(<EM>name, <B>priority</B>, options, stacksize,
function, arg1,.. , arg10);</EM>
<P></P>By varying the priority(0-255) of the task spawned, you can affect the
priority of the task. <EM>Priority</EM> 0 is the highest and priority 255 is the
lowest.The Note the priority of a task is relative to the priorities of other
tasks. In other words, the task priority number itself has no particular
significance by itself.
<P></P>In addition a task's priority can be changed after its spawned using the
following routine:
<UL>
<LI><EM>taskPrioritySet(int tid, int newPriority)</EM>: Change the priority of
a task. </LI></UL>
<P></P>In the example below, there are three tasks with different
priorities(HIGH,MID,LOW). The result of running the program is that the task
with the highest priority, "taskThree" will run to completion first, followed by
the next highest priority task, "taskTwo", and the finally the task with the
lowest priority which is "taskOne."
<P></P><PRE>------------------------------------------------------------------------------------
/* includes */
#include "vxWorks.h"
#include "taskLib.h"
#include "logLib.h"
/* function prototypes */
void taskOne(void);
void taskTwo(void);
void taskThree(void);
/* globals */
#define ITER1 100
#define ITER2 1
#define LONG_TIME 1000000
#define HIGH 100 /* high priority */
#define MID 101 /* medium priority */
#define LOW 102 /* low priority */
void sched(void) /* function to create the two tasks */
{
int taskIdOne, taskIdTwo, taskIdThree;
printf("\n\n\n\n\n");
/* spawn the three tasks */
if((taskIdOne = taskSpawn("task2",LOW,0x100,20000,(FUNCPTR)taskOne,0,0,0,0,0,0,0,
0,0,0)) == ERROR)
printf("taskSpawn taskOne failed\n");
if((taskIdTwo = taskSpawn("task2",MID,0x100,20000,(FUNCPTR)taskTwo,0,0,0,0,0,0,0,
0,0,0)) == ERROR)
printf("taskSpawn taskTwo failed\n");
if((taskIdThree = taskSpawn("task3",HIGH,0x100,20000,(FUNCPTR)taskThree,0,0,0,0,0,0,0,
0,0,0)) == ERROR)
printf("taskSpawn taskThree failed\n");
}
void taskOne(void)
{
int i,j;
for (i=0; i < ITER1; i++)
{
for (j=0; j < ITER2; j++)
logMsg("\n",0,0,0,0,0,0);
for (j=0; j < LONG_TIME; j++);
}
}
void taskTwo(void)
{
int i,j;
for (i=0; i < ITER1; i++)
{
for (j=0; j < ITER2; j++)
logMsg("\n",0,0,0,0,0,0);
for (j=0; j < LONG_TIME; j++);
}
}
void taskThree(void)
{
int i,j;
for (i=0; i < ITER1; i++)
{
for (j=0; j < ITER2; j++)
logMsg("\n",0,0,0,0,0,0);
for (j=0; j < LONG_TIME; j++);
}
}
------------------------------------------------------------------------------------
</PRE>
<P></P>
<HR SIZE=3>
<H2>Procedures</H2>1. Copy the source code in the example and compile it.
<P></P>2. Load the object file onto the target machine.
<P></P>3. Execute the following command on the WindSh terminal: "logFdSet 1".
This will direct the <EM>logMsg()</EM> output to the virtual console.
<P></P>4. Run the examples by executing the main routine("sched") of the example
on WindSh terminal.
<P></P>Note: Make sure you have redirected I/O, otherwise you won't see the
results of the <EM>logMsg()</EM> commands.
<HR SIZE=3>
<H2>Follow On Experiment</H2>Experiment 1. Modify the program such that the
order of execution is "taskOne" first, then "taskTwo", and then "taskThree."
<P></P>Experiment 2. Modify the program so that "taskOne" has the highest
priority and "taskOne" and "taskTwo" are running at the same priority. Is there
a difference in the output when compared with the output of Experiment 1 of the
Follow On Experiment in this section.
<P></P>
<HR SIZE=3>
<H2>Additional Information</H2>Refer to VxWorks User's Manual and Reference
Manual.
<P></P>
<HR SIZE=3>
<CENTER>
<H4><A
href="http://www.rt.db.erau.edu/experiments/vx/toc/TableOfContents.html">Return
to Primary Table of Contents </A></H4></CENTER>
<HR SIZE=3>
<CENTER>Last Updated: 21 March 1997<BR><EM>Created by: Dan Eyassu</EM><BR><A
href="mailto:eyassud@db.erau.edu">eyassud@db.erau.edu</A><BR></CENTER></BODY></HTML>
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