Table Of Content+FAULT TOLERANCE IN REAL TIME MULTIPROCESSORS - EMBEDDED
SYSTEMS
A. Christy Persya, Sr.Lecturer, T.R.Gopalakrishnan Nair Ph.D.
Department of Information Science and Engineering Director, Research and Industry,
The Oxford College of Engineering DS Institutions
Bangalore, India Bangalore, India
[email protected] [email protected]
ABSTRACT
All real time tasks which are termed as critical tasks by nature have to complete its
execution before its deadline, even in presence of faults. The most popularly used real time
task assignment algorithms are First Fit (FF), Best Fit (BF), Bin Packing (BP).The common
task scheduling algorithms are Rate Monotonic (RM), Earliest Deadline First (EDF) etc.All the
current approaches deal with either fault tolerance or criticality in real time. In this paper we
have proposed an integrated approach with a new algorithm, called SASA (Sorting And
Sequential Assignment) which maps the real time task assignment with task schedule and fault
tolerance.
Keywords: Real Time, Fault Tolerance, Multiprocessor, SASA.
1. INTRODUCTION Real time systems are
characterized by three main features: 1)
An embedded system is a special response time 2) Fault Tolerance 3)Task
purpose computer system designed to Scheduling. Response time is the
perform one or a few dedicated functions operation correctness of a real-time
with real-time computing. Real time system which depends not only on its
computing means that all the task have logical results, but also on the time at
to finish its execution before its which these results become available.
deadline. Examples of such computing Fault tolerance, means all the tasks
applications are systems that control admitted to the system completes its
trucks, trains, aircraft, satellites and execution even in the presence of faults.
industrial process control systems. A Task scheduling, is the process to decide
task is a collection of related jobs (or on when and which processor the given
instances) cooperating to execute a tasks should be executed [5].This paper
function. The tasks in the system can be has three main
represented by models. Tasks can be contributions:1)SASA(Sorting And
periodic, aperiodic or sporadic tasks. Sequential Assignment) algorithm
The release time of the tasks is random 2)EDF Scheduling 3) mapping FT-EDF.
and cannot be predicted. Tasks can be The rest of the paper is
executed based on some priorities organized as follows. Section 2 describes
(deadline).Priorities can be either fixed the task splitting on multiprocessors with
or dynamic. Static scheduling and an algorithm SASA.Section 3 discusses
dynamic scheduling is possible. the task execution on each
multiprocessor along with an algorithm.
In section 4, the mapping of fault
tolerance with real time scheduling is The earlier existing algorithm does not
discussed and section 5 explains the sort the tasks based on the periods before
conclusion of the paper. assigning the tasks.Shinpei Kato [1] has
given an algorithm for sorting the tasks
2. TASK ASSIGNMENT but not to achieve fault tolerance. This
algorithm provides a path to sort the
The SASA algorithm given below can be tasks and assign all the tasks to all
used for assigning the tasks to the multiprocessors using Bin Packing
processors. It assumes that the tasks are algorithm to achieve fault tolerance.
sorted so that Ti<Ti+1.The concept used
in the algorithm is as follows: First all 3. TASK EXECUTION
the tasks are been sorted in ascending
order. Then in sequential manner, all the EDF(Earliest deadline First) algorithm
sorted tasks are assigned to each executes the tasks based on their
processor such that processor utilization deadlines.That is, the task which has got
does not exceed the threshold value. If the earliest deadline will get the highest
any task is not able to be assigned to any priority to execute.This real time
of the processors then the task can be scheduling algorithm can be mapped
split into two and assigned to two with the fault tolerant
different processors as two different scheduling(Primary Backup) by calling
tasks .These tasks can be executed such the subroutine function FT-EDF [2].
a way that, as soon as first part of the
task completes its execution it preempts Algorithm for Task Execution
the next half in the other processor.
1. When any task is released on
processor Pm, call the scheduler
Algorithm for Task Assignment on Pm .
1. Sort the Task Set T such that 2. If portion 2 of task I is assigned
T1<=T2<=……………<=Tn. on Pm then call scheduler on Pm-
2. Initialize the processor utilization 1 to execute portion 1 of task i.
upper bound variable UTH to the 3. Call the subroutine function FT-
threshold value. EDF.
3. Compare the processor Ui 4. Repeat for all the processors.
Utilization (When Ti is assigned)
to the UTH. Each processor will have its own
4. If it is less, then assign the task scheduler and when the split task arrives,
Ti on Pi. then the processor Pn+1 calls the
5. Else repeat on Pi+1 from step 3. processor Pn scheduler to execute the
6. If a task Tm is not able to be portion 1 of that task. The message
assigned on any of the processor, transfer of scheduler or processor is not
then split the task and assign it to expensive as they share a common
different processor such that Ui memory [3].Hence using EDF and
of the processor I should not Primary Backup model, all the tasks can
exceed UTH. be scheduled and executed in such a way
7. Repeat step 6 for all remaining that their deadlines are not missed.
tasks
5. MAPPING FT with EDF 3. Try Overloading
Try to overload the backup copy
Fault Tolerance is the ability to on an existing backup slot on any
continue operating despite the processor other than Pi. The
failure of a limited subset of their backups of 2 primary tasks that
hardware or software. There can are scheduled on the same
be either hardware fault or processor must not overlap. If the
software fault which may hinder processor fails, it will not be
the real time systems to meet possible to schedule the two
their deadlines. backups simultaneously since
Fault detection can be they are on the same time slot
achieved either through online or (overloaded).
offline. One way of achieving is
Primary-Backup model. Here, 4. EDF algorithm
the tasks are assumed to be If there is no existing backup slot
periodic and two instances of that can be overloaded, then
each task(a primary and a schedule the backup on the latest
backup) are scheduled on a uni possible free slot depending upon
processor system. One of the the dead line of the task. The task
restrictions to this approach is with the earliest deadline is
that the period of its preceding scheduled first.
tasks. It also assumes that the
execution time of the backup is 5. De-allocation of backup
shorter than that of the primary. If a schedule has been found for
The following are the both the primary and backup
steps form the procedure used to copy for a task, commit the task,
implement the backup otherwise reject it. If the primary
overloading algorithm copy executes successfully, the
corresponding backup copy is de-
1. EDF Schedulability allocated.
Check if all the tasks can be
scheduled successfully using the 6. Backup Execution.
earliest deadline first algorithm. If there is a permanent or
If the schedulability test fails, transient fault in the processor,
then reject the set of tasks saying the processor crashes and then all
that they are not schedulable. the backups of the tasks that were
running on this system are
2. Searching for Timeslot executed on different processors.
When task Ti arrives, check each
processor to find if the primary 5. CONCLUSION
copy of the task can be scheduled
between release time and The proposed algorithm SASA(Sort And
deadline. Say it is scheduled on Sequential Assignment) , is a real time
processor Pi. task assignment algorithm for multi
processor. It enhances the system
performance by improving the time
efficiency of the system. It further
strives to achieve fault tolerance with
task split. This approach can be
implemented in any of the real time
applications
6.REFERENCES
[1]Shinpei Kato and Nobuyuki Yamasaki,’’Real
Time scheduling with Task Splitting on
Multiprocessors”,13th International Conference
on Embedded and Real-Time Computing
Systems and Applications(RTCSA 2007),IEEE
Society
[2] A.Christy Persya and T.R.G Nair,” Fault
Tolerant Real Time Systems”,1st International
Conference on Managing next Generation
Software Applications(MNGSA 2008),IRCSIT
&CSIR ,Karunya University,Coimbatore
[3]Jane W.S. Liu., Real Time Systems (Pearson
Edition, 2000).
[4] C.M.Krishna Kang G.Shin., Real Time
Systems (McGraw-Hill International Edition,
1997).
[5] Hakem Beitollahi and Geert
Deconinck,”Fault Tolerant Partitioning
Scheduling Algorithms in Real –Time
Multiprocessor Systems”,12th Pacific Rim
International Symposium on Dependable
Computing (PRDC 2006),IEEE Society.
