Description
Background
An embedded system is a computer system performing dedicated functions within a larger
mechanical or electrical system. Embedded systems range from portable devices such as
Google Glasses, to large stationary installations like traffic lights, factory controllers, and
complex systems like hybrid vehicles, and avionic. Typically, the software of an embedded
system consists of a set of tasks (threads) with timing constraints. Typical timing constraints
are release times and deadlines. A release time specifies the earliest time a task can start, and
a deadline is the latest time by which a task needs to finish. One major goal of embedded
system design is to find a feasible schedule for the task set such that all the timing constraints
are satisfied.
Task scheduler
We assume that the hardware platform of the target embedded systems is a single processor
with m identical cores, Core1, Core2, …, Corem. The task set V={v1, v2, …, vn} consists of n
independent, non-pre-emptible tasks. A non-pre-emptible task cannot be pre-empted by
another task during its execution, i.e., it runs to completion without being interrupted. Each
task vi (i=1, 2, …, n) has four attributes: a unique task name vi, an execution time ci, a release
time ri, and a deadline di (di>ri). All the execution times, the release times and deadlines are
non-negative integers. You need to design a task scheduler and implement it in C. Your task
scheduler uses EDF (Earliest Deadline First) heuristic to find a feasible schedule for a task set.
A schedule of a task set specifies when each task starts and on which core it is executed. A
feasible schedule is a schedule satisfying all the release time and deadline constraints.
The problem of finding a feasible schedule for this task scheduling problem is NP-complete. It
is widely believed that an NP-complete problem has no polynomial-time algorithm. However,
nobody can prove it.
First, we introduce two definitions: scheduling point and ready task.
• A scheduling point is a time point at which a task can be scheduled on a core, In other
words, a scheduling point is either the release time or the completion time of a task.
• A task vi (i=1, 2, …, n) is ready at a time t if t≥ri holds.
The EDF scheduling heuristic works as follows:
• At each scheduling point ti (ti≤ti+1, i=1, 2, …), among all the ready tasks, find a task
with the smallest deadline, and schedule it on an idle core such that its start time is
minimized. Ties are broken arbitrarily.
Since this task scheduling problem is NP-complete, the EDF heuristic is not guaranteed to find
a feasible schedule even if a feasible schedule exists.
Example One
Consider a set S1 of 6 independent tasks whose release times and deadlines are shown in Table
1. The target processor has two identical cores. A feasible schedule of the task set by using
EDF scheduling strategy is shown in Figure 1.
Table 1: A set S1 of 6 tasks with individual release times and deadlines
Task Execution
time
Release time Deadline
v1 4 0 4
v2 4 1 5
v3 5 3 10
v4 6 4 11
v5 4 6 13
v6 5 6 18
Core1 v1 v3 v5
Core2 v2 v4 v6
Time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Figure 1: A feasible schedule for S1
Example Two
Consider a set S2 of 6 independent tasks whose release times and deadlines are shown in Table
2. The target processor has two identical cores. A schedule of the task set by using EDF
scheduling strategy is shown in Figure 2. As we can see, in the schedule, v6 finishes at time 16
and thus misses its deadline. Therefore, the schedule is not feasible. However, a feasible
schedule, as shown in Figure 3, does exist.
Table 2: A set of tasks with individual release times and deadlines
Task Execution
time
Release time Deadline
v1 4 0 4
v2 4 1 5
v3 5 3 10
v4 6 4 11
v5 4 9 16
v6 5 10 15
Core1 v1 v3 v5
Core2 v2 v4 v6
Time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Figure 2: An infeasible schedule constructed by the EDF scheduling strategy
Core1 v1 v3 v6
Core2 v2 v4 v5
Time 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Figure 3: A feasible schedule for S2
The TaskScheduler function
You need to write a task scheduler function named TaskScheduler. A prototype of
TaskScheduler is shown as follows:
int TaskScheduler(char *file1, char *file2, int m) {};
This function gets a task set from a file named file1, constructs a feasible schedule for the task
set on a processor with m identical cores by using the EDF strategy, and writes the feasible
schedule to file2. If a feasible schedule is found, it returns 1. Otherwise, it returns 0.
Both file1 and file2 are text files. file1 contains a set of independent tasks each of which has a
name, an execution time, a release time and a deadline in that order. Task names, execution
times, release times and the deadlines are a string of digits between 0 and 9. All the release
times are non-negative integers, and all the execution times and the deadlines are natural
numbers. The format of file1 is as follows:
v1 c1 r1 d1 v2 c2 r2 d2 … vn cn rn dn
Two adjacent attributes (task name, execution time, release time and deadline) are separated
by one or more white space characters or a newline character. A sample file1 is shown here.
For simplicity, you may assume that all the task names in file1 are distinct. Therefore, you
don’t need to check for duplicates.
The TaskScheduler function needs to handle the following possible cases properly when
reading from file1 and writing to file2:
1.file1 does not exist. In this case, print “file1 does not exist” and the program
terminates.
2.file2 already exists. In this case, overwrite the old file2.
3. The task attributes (task name, execution time, release time and deadline) of file1 do
not follow the formats as shown before. In this case, print “input error when reading
the attribute of the task X” and the program terminates, where X is the name of the
task with an incorrect attribute.
file2 has the following format:
v1 p1 t1 v2 p2 t2 … vn pn tn
where each vi (i=1, 2, …, n) is the task name, pi is the name of the core where vi is scheduled,
and ti is the start time of the task vi in the schedule. In file2, all the tasks must be sorted in
non-decreasing order of start times. A sample file2 is shown here.
Your task scheduler needs to use binomial heap-based priority queues. A priority queue has a
header which stores the number of tasks in the heap and other implementation dependent
information. The data type of a heap header is defined as follows:
typedef struct BinomialHeap{
int size; // count of tasks in the heap
… // you need to add additional fields here
} BinomialHeap;
Each node in the heap stores the priority (key) and the attributes of a task. The attributes of a
task include its name, execution time, release time and deadline. For simplicity, we use an
integer to denote the name of task.
The data type for heap nodes is defined as follows:
typedef struct HeapNode {
int key; // key of this task
int TaskName; // task name
int Etime; // execution time of the task
int Rtime; // release time of the task
int Dline; // deadline of the task
… // you need to add additional fields here
} HeapNode;
Therefore, you also need to implement the following functions of a binomial heap-based
priority queue:
1. void Insert(BinomialHeap *T, int k, int n, int c, int r, int d). This function inserts a new
task into a heap T, where k, n, c, r and d are the key, name, execution time, release
time, and deadline of the task, respectively.
2. HeapNode *RemoveMin(BinomialHeap *T). This function removes a task with the
smallest key from the heap T and returns the node containing the task.
3. int Min(BinomialHeap *T). This function returns the smallest key of all the tasks in the
heap T without modifying the heap T.
The prototypes of all the above-mentioned function and some other basic functions are
included in the template file MyTaskScheduler.c. In addition to the above functions, you
may add your own helper functions and fields (variables) to the file MyTaskScheduler.c.
Note that you must implement a binomial heap-based priority queue. Any other priority
queues are not acceptable.
Time complexity requirement
You need to include the time complexity analysis of each function as comments in your
program. The time complexity of your scheduler is required to be no higher than O(n*log n),
where n is the number of tasks (Hints: use three binomial heap-based priority queues, one
priority queue with release times as keys, one priority queue with deadlines as keys and one
priority queue for the scheduling points of all the cores). There is no specific requirement on
space complexity. However, try your best to make your program space efficient.
When analysing the time complexity of your task scheduler, you can assume that the time
complexity of each function (insertion, deletion, removeMin and merge) on a binomial heap
is O(log n), where n is the number of tasks in the binomial heap.
How to submit your code?
a. Go to the Assignment page
b. Click on Assignment Specification
c. Click on Make Submission
d. Submit your MyTaskScheduler.c file that contains all the code.
Plagiarism
This is an individual assignment. Each student will have to develop their own solution without
help from other people. In particular, it is not permitted to exchange code or pseudocode.
You are not allowed to use code developed by persons other than yourself. All work
submitted for assessment must be entirely your own work. We regard unacknowledged
copying of material, in whole or part, as an extremely serious offence. For further information,
see the Course Information.
