Problems in Task Scheduling in Multiprocessor System

International Journal of Trend in Scientific Research and Development, Volume 1(4), ISSN: 2456-6470 www.ijtsrd.com
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IJTSRD | May-Jun 2017
Available Online @www.ijtsrd.com
Problems in Task Scheduling in Multiprocessor System
Mukul Varshney
CSE , Sharda
University, India
Jyotsna
CSE , Sharda
University, India
Abha Kiran Rajpoot
CSE , Sharda
University, India
Shivani Garg
CSE , Sharda
University , India
ABSTRACT
This contemporary computer systems are
multiprocessor or multicomputer machines. Their
efficiency depends on good methods of administering
the executed works. Fast processing of a parallel
application is possible only when its parts are
appropriately ordered in time and space. This calls for
efficient scheduling policies in parallel computer
systems. In this work deterministic problems of
scheduling are considered. The classical scheduling
theory assumed that the application in any moment of
time is executed by only one processor. This
assumption has been weakened recently, especially in
the context of parallel and distributed computer
systems. This monograph is devoted to problems of
deterministic scheduling applications (or tasks
according to the scheduling terminology) requiring
more than one processor simultaneously. We name
such applications multiprocessor tasks. In this work
the complexity of open multiprocessor task
scheduling problems has been established. Algorithms
for scheduling multiprocessor tasks on parallel and
dedicated processors are proposed. For a special case
of applications with regular structure which allow for
dividing it into parts of arbitrary size processed
independently in parallel, a method of finding optimal
scattering of work in a distributed computer system is
proposed. The applications with such regular
characteristics are called divisible tasks. The concept
of a divisible task enables creation of tractable
computation models in a wide class of computer
architectures such as chains, stars, meshes,
hypercubes, multistage networks. Divisible task
method gives rise to the evaluation of computer
system performance. Examples of such performance
evaluation are presented. This work summarizes
earlier works of the author as well as contains new
original results.
KEYWORDS: Load Scheduling, SISD, MIMD,
MISD, SIMD, Communication Overhead
1. INTRODUCTION
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533
II. PARALLEL COMPUTER SYSTEMS
A. Hardware
It is common to start a description of parallel systems
with an attempt of classifying types of parallelism and
types of parallel machines. A useful view on
parallelism types is distinguishing between data
parallelism and code parallelism. . Another view of
parallel processing classification considers granularity
of parallelism. Classical computers execute
instructions in the order dictated by the sequence in
the program code. This approach is called control-
driven or von Neumann architecture.
In control-driven computers have been divided into
four classes: SISD (single instruction stream, single
data stream), SIMD (single instruction stream,
multiple data streams), MISD (multiple instruction
streams, single data stream), MIMD (multiple
instruction streams, multiple data streams). A
multicomputer consists of a set of processors with
local memories, interconnected by some kind of
network. We will name by processing element (PE) a
processor with local memory and a network interface.
Tightly-coupled computers can be further dierentiated
by the type of PE interconnection. In this work we
limit considered interconnection types to: bus(es),
point-to-point networks (called also single-stage
networks).
In multistage networks PEs are connected by several
layers of switches while the internal layer switches
have no PEs attached. Multistage networks are
divided here into: trees and multistage cube network
B. Software
In many common applications (programs) great
potential parallelism can be found. Thus, programs
can be executed via many concurrent threads (mutual
relations between the notions of an application, a
thread and a task). Computer systems should provide
support for implementing parallelism of an
application including the issues posed by scheduling.
Parallel operating systems are evolving from
previously existing systems and many ideas have been
"naturally" inherited. Based on acceptable response
time two load types have been distinguished in single-
processor systems: terminal (or interactive) and batch
load. Since batch tasks are submitted to the computer
system far earlier than their actual execution begins,
deterministic scheduling algorithms can be applied.
For the terminal load which requires immediate
response, access to processors is granted on the basis
of FCFS, Round-Robin, multi-level priority queues
etc
III. COMMUNICATION MODE
The next element of the architecture is the
commutation mode. The commutation mode is a
physical protocol for message routing. We describe
commutation modes here because routing functions
are increasingly executed by dedicated hardware. The
methods we refer to in this section are also called
switching or routing techniques. Among various
commutation (or routing) modes we distinguish store-
and-forward, circuit-switched and packet-switched.
In the store-and-forward mode when a PE sends a
message to another PE located at distance d, the
message (either as whole or in packet pieces) is sent
to the closest PE on the path and it is stored there.
In the circuit-switched mode from the transmitter to
the receiver a header of the message is sent which
reserves all the links of a communication path to form
a circuit between both PEs.
In the packet-switched modes the message is split into
packets which consist of its. Flits are also called ow of
control digits. These are words passed over a link in
one control cycle.
Among the packet-switched modes three sub-types
can be identified: wormhole, virtual-cut-through and
buered-Wormholemodes. These modes differ in the
behavior of its and packets when the packet cannot
move forward (e.g. there is no free link). In the
wormhole mode, the progressing of the message in
the pipe is stopped. All the its remain in the
intermediate buffers thus blocking the links.
In the virtual-cut-through mode its continue
progressing on their way until reaching the site where
the rst it is stopped. There a whole packet is waiting
for release of the link. This mode assumes infinite
capacity of buffers
In the buffered wormhole mode, its of some packets
move until they reach the stopped it, then the whole
packet is stored there. Yet, the number of packets that
can be stored is limited by buyer’s capacity.

534
IV. FORMULATION OF TASK
SCHEDULING PROBLEM
A task scheduling problem consists of the application
model, system computing model and performance
evaluation metrics. This section will discuss an
application model, a system computing model
followed by performance evaluation metrics.
A. Application model
Task scheduling of a given application is represented
by directed acyclic graph (DAG) G1= (T, E), where T
is the finite set of m tasks {T1, T2, T3…Tm} and E is
the set of edges {eij }between the tasks Ti and Tj.
Here, each edge represents the precedence constraints
between the tasks Ti and Tj such that Tj can not start
until Ti completes its execution. Each task Ti is
associated with an execution time ET (Ti) and each
edge eij is associated with a communication time CT
(Ti, Tj) for data transfer from Ti to Tj. If there is a
direct path from Ti to Tj then Ti is the predecessor of
Tj and Tj is the successor of Ti. A entry task does not
any predecessor, similarly, an exit task does not any
successors. Layout of a DAG with six tasks is shown
in figure 4 [16]
Where T1, T2, T3, T4, T5 and T6 are different tasks
of the given DAG. The execution time ET (Ti) and
communication time CT (Ti, Tj) [16] of tasks are: ET
(T1) = 2 ET (T2) = 3 ET (T3) = 4 ET (T4) = 4 ET
(T5) = 2 ET (T6) = 3 CT (T1, T2) = 5 CT (T1, T3) =
4 CT (T2, T4) = 3 CT (T2, T5) = 4 CT (T3, T5) = 3
CT (T3, T6) = 5 CT (T4, T6) = 5 CT (T5, T6) = 2
B. Algorithm
Rules Design objectives of the algorithm:
• Able to dynamically regulate task allocation to each
processor according to changes in the system load,
optimize the utilization of processor resources
• Minimize the effect on the working performance of
the processor. The node processor in the algorithm is
defined into the four states as follows:
R I: Suppose A1, A2, B1 and B2 represent the
processor node set in heavy load, busy, light load and
no load states respectively, load balancing is carried
out in following steps:
• Equilibrium operation is conducted between the
heavy loaded node and light loaded node
• Equilibrium operation is conducted between the
busy node and the no load node when there is no
heavy load Node. • Repeat steps 1 and 2 until no
migratable task can be selected or the loads on the
processor Nodes are relatively balanced.
R II: The execution time of the parallel program is
always restricted by the task at the slowest execution

535
speed L denotes the load. So when load L moves from
A1 to B1.
C. Load Evaluation
It’s common to evaluation node load by the array
length of CPU. And this load evaluation mode is
concise and rapid .In order to evaluation load state of
each node; we let character C to represent process
capability of each node in homogeneous
multiprocessor system. Namely, it’s maximum
process number that can be processed by CPU in per
unit time.
V. CONCLUSIONS
In this work we considered selected methods of
scheduling in multiprocessor computer systems. With
the advent of modern computer systems it turned out
that classical scheduling methods in many cases are
not satisfactory Therefore, two new scheduling
models were analyzed here: multiprocessor tasks and
divisible tasks. Multiprocessor tasks require several
processors simultaneously, thus allow for expressing
task parallelism at high level of abstraction. This
model allows for finding simple solutions of problems
which in other setting are again intractable. Moreover,
divisible task concept permits introducing computer
architecture context which in the classical approach is
often highly generalized to make problems
manageable. In this way scheduling problems have
been combined with communication optimization
problems. Experimental simulation has shown that the
algorithm can solve very well the load balancing
problems in a multiprocessor system and significantly
improve the system performance and task processing.
The proposed algorithm is characterized by less load
balancing frequency and less overhead. It can be seen
from the results of the experiment that use of the
algorithm proposed in the paper can get good
performance
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