Multicore processor technology advantages and challenges

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 04 Issue: 09 | September-2015, Available @ http://www.ijret.org 87
MULTICORE PROCESSOR TECHNOLOGY- ADVANTAGES AND
CHALLENGES
Anil Sethi1
, Himanshu Kushwah2
1
(Department of Electronics, Keshav Mahavidyalaya, University of Delhi, Delhi, India)
anil_sethi14@yahoo.co.in,
2
(Department of Electronics, Keshav Mahavidyalaya, University of Delhi, Delhi, India)
kushwah.du@gmail.com
Abstract
Until recent times, we have worked with processors having a single computing/processing unit (CPU), also called a core. The
clock frequency of the processor, which determines the speed of it, cannot be exceeded beyond a certain limit as with the
increasing frequency, the power dissipation increases and therefore the amount of heating. So manufacturers came up with a new
design of processors, called Multicore processors. A multicore processor has two or more independent computing/processing
units (cores) on the same chip. Multiple cores have advantage that they run on lower frequency as compared to the single
processing unit, which reduces the power dissipation or temperature. These multiple cores work together to increase the
multitasking capability or performance of the system by operating on multiple instructions simultaneously in an efficient manner.
This also means that with multithreaded applications, the amount of parallel computing or parallelism is increased. The
applications or algorithms must be designed in such a way that their subroutines take full advantage of the multicore technology.
Each core or computing unit has its own independent interface with the system bus.. But along with all these advantages, there
are certain issues or challenges that must be addressed carefully when we add more cores. In this paper, we discuss about
multicore processor technology. In addition to this, we also discuss various challenges faced such as power and temperature
(thermal issue), interconnect issue etc. when more cores are added.
Key Words: CMP (Chip Multiprocessor), Clock, Core, ILP (Instructions Level parallelism), TLP (Thread level
Parallelism).
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1. INTRODUCTION
The multicore processor comprises of two or more cores or
computational/processing units that operate in parallel to
read and execute instructions. These multiple processing
units or cores are fabricated on a single die. So, its also
called a Chip Multiprocessor (CMP). The key factor about
multicore processor is that it gives the same performance of
a single faster processor at lower power dissipation and at a
lower clock frequency by handling more tasks or
instructions in parallel. [1] This enhances the ILP
(Instructions Level Parallelism). The performance of a
processor is a function of three major factors, which
includes IPC (instructions per cycle), CPI (clock cycles per
instruction) and clock cycle (or clock frequency). The IPC
can be increased by increasing ILP and TLP (thread level
parallelism). The CPI can be improved by the techniques of
pipelining. But there is a limiting factor towards increasing
the clock frequency. [2]
The dynamic power dissipation is given by-
P = QCV2
f 1.1
 Q- Number of Transistors
 C- Load Capacitance
 V- Supply Voltage
 f- Clock Frequency
So, if we increase the clock frequency, the power dissipation
increases which in turn causes overheating. So, the idea of
multicore technology is to use multiple cores instead of one
(like single processor) at a comparatively lower frequency,
but an overall improvement in the performance is delivered
through multiple cores operating simultaneously on multiple
instructions.[1] Multicore processors work on multiple
instructions and multiple data. Multiple cores execute
multiple threads (multiple processes/instructions) while
using different parts of memory (multiple data). This
enhances TLP. The main memory is shared by all cores.
Each core is associated with its own cache and they all share
the system bus.[3]
Fig.1. Quad cores sharing memory and bus interface

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Intel came up with its first dual core processor in 2005. A
dual core processor contains two cores- (Intel Core Duo,
AMD Phenom II X2), a quad core processor has four cores
(Intel core i5 and i7 processors, AMD Phenom II X4), a
hexa core processor has six cores(AMD Phenom II X6, Intel
Core i7 Extreme Edition 980X), a Octa core processor has
eight cores (AMD FX-8350, Intel Xeon E7-2820), a deca
core has ten cores ( Intel Xeon E7 2850) or more. There are
various topologies to interconnect cores such as Ring
topology, Bus topology, Two-dimensional mesh and
crossbar. The choice of a particular topology is a crucial
factor as it affects performance parameters like speed,
latency etc. The type of topology also affects area consumed
on a chip and power dissipation.
In addition to this, multicore technology uses homogeneous
and heterogeneous cores. In homogeneous configuration, all
the cores are identical and each core has same hardware.
These cores use divide and rule approach for improving the
performance by dividing a more complex application into
less complex applications and execute them simultaneously.
There are many other benefits of this approach such as
reusability, simpler design etc.[1]
In heterogeneous cores, there are dedicated application
specific cores that work on specialized applications. For
example a system comprising of a DSP core that handles a
multimedia application requiring intensive mathematical
computations, while other cores handle some other
applications simultaneously. Heterogeneous core is more
complex, but has its own benefits also. Multicore Processors
sometimes take advantage of both homogeneous and
heterogeneous configurations to improve performance. The
IBM multicore processor, CELL uses this approach. [1]
Various applications that benefit from multicore technology
are multimedia applications, DSP , servers, graphics,
compilers etc. and also applications which exhibit thread
level parallelism. The efficient use of multicore technology
requires high level of parallelism.[3] Some multicore
processors allow one or more cores which are not required at
times to turn off to save power.
Fig.2. Multiple threads in different Cores[3]
The performance of a multicore processor strongly depends
on the design of algorithm or application which is governed
by Amdahl’s Law. This law was given by Geve Amdahl, a
computer architect. This law is used to compute the
theoretical maximum speed with multiple processors in
parallel computing. This law states that the speed of a
process in parallel computing is limited by the time needed
for the sequential part of the process. For example , if a
process requires X hours to complete on a single core, and a
portion of the process that takes Y hours to execute which
can not be parallelized, while remaining (X-Y) hours can be
parallelized, then regardless of the number of processors,
the minimum time required for the execution can not be less
then (X-Y) hours.[4]
2. MULTICORE ADVANTAGES
The good processing speed of the multicore processors is
due to the multiple cores which operate simultaneously on
instructions, at lower frequency than the single core. At the
same clock frequency, the multicore processor will process
more data than the single core processor. In addition to this,
multicore processors deliver high performance and handle
complex tasks at a comparatively lower energy or lower
power as compared with a single core, which is crucial
factor in appliances such as mobile phones, laptop etc.
which operate on batteries. Also, since the cores are
fabricated close to each other on the same chip, the signals
travel shorter distance between them due to which there is
less attenuation of signals. Since the signals don’t attenuate
much, more data is transferred in a given time and there is
no need of repeating the signals
3. CHALLENGES OF MULTICORE
PROCESSOR
So far, we have seen the benefits of multicore technology
but there are some problems that arise when more cores are
added. The various challenges that are faced with the
addition of new cores are power and temperature issues, the
level of parallelism in the application or algorithms,
interconnect issues etc. Now we discuss the challenges in
detail:-
3.1 Thermal Issues (Power and temperature):
 To reduce the unnecessary power consumption, the
multicore design also has to make use of a separate
power management unit that can manage or control
unnecessary wastage of power. For this to happen, the
power management unit has to turn off or shut down
the cores which don’t operate at times or the cores that
are not required at times. Also, the cores run at a
comparatively lower frequency than a single processor
to reduce the power dissipation.
 The architecture of the core must be such that the
amount of heat generated in the chip is well distributed
across the chip. The power consumption is also a
function of number of transistors on a chip. When
more cores are added, the transistor density also
increases which contributes to the power consumption.

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The core which overheats in a multicore configuration
is called a hot-spot.
 There are various ways to tackle the issue of power
dissipation and temperature such as thread migration,
DVFS (Dynamic Voltage and Frequency Scaling) etc.
In the thread migration technique, a high power
consuming process or thread is moved to a different
core and a low power consuming process or thread is
moved to an overheated core. Also, in order to cool an
overheated core, the work which is being handled by
the overheated core is divided among other cores.
 In DVFS (Dynamic Voltage and Frequency Scaling)
technique, voltage and frequency of the hot core is
reduced since the power dissipation or heating is a
function of both voltage and frequency. This slightly
affects the overall performance also.[2]
3.2 Level of Parallelism:
One of the biggest factors affecting the performance of a
multicore processor is the level of parallelism of the process/
application.
 The lesser the time required to complete a process,
better will be the performance. Performance is directly
related to the amount of parallelism because more the
number of processes that can be executed
simultaneously more will be the parallelism.
Parallelism can be considered at two levels ILP and
TLP.
 TLP increases overall parallelism by breaking a
program into many threads (Small Processes) and
execute them simultaneously.
 To achieve a high level of parallelism and an overall
high performance, software developers must write such
algorithms that can take full advantage of multicore
design. In other words, all the cores should be used in
the most efficient manner. If the algorithms written are
not compatible with the multicore design, then it may
happen that one or more cores starve for data. In such a
case, the process will run on one of the cores, while
other cores will sit idle. So, in a nutshell, the success of
multicore technology strongly depends on the way the
algorithms are written. Also, companies like Microsoft
and Apple have designed their operating systems
which can run efficiently on multicore configuration.
3.3 Interconnect Issues:
Since there are so many components on chip in a multicore
processor like cores, caches, network controllers etc., the
interaction between them can affect the performance if the
interconnection issues are not resolved properly. In the
initial processors, bus was used for communication between
the components. In order to reduce the latency, crossbar and
mesh topologies are used for interconnection of components.
Also, as the parallelism increases at the thread level,
communication also increases off-chip for memory access,
I/O etc. For issues like this, packet based interconnection is
actively used. This packet based interconnection has been
used by Intel (Quick path interconnect) and is also being
used by AMD.[1][2]
4. CONCLUSIONS
In this paper, we reviewed the basic concepts of the
multicore technology. We studied that it is possible to
increase the performance of a processor without increasing
the clock frequency, by simply adding more cores. The
limitation of a single core processor at a high clock
frequency has opened the gates for multicore technology and
has become the trend of the industries today. However,
adding more cores also gives rise to certain issues or
challenges that must be carefully addressed in order to get
the most out of multicore technology. We also studied
various challenges associated with increasing the number of
cores like power and temperature (thermal issues), level of
parallelism, interconnect issues etc. The applications/
algorithms which run on multicore environment must be
compatible with it. Research is constantly going on in the
areas like developing more efficient applications/ algorithms
for multicore environment and also in other areas in order to
get the maximum performance throughput from multicore
processors. Industries are constantly working towards
achieving better and better performance from multicore
processors.
REFERENCES
[1]. Venu, Balaji. "Multicore processors- An overview."
[2]. Jr., Ransford Hyman. "Performance issues on
Multicore Processors”.
[3]. Barbic, Jernej. "Multicore architectures." 2007.
[4]. Martin, Christian. "Multicore Processors: Challenges,
Opportunities, emerging Trends." Embedded World
2014 Exhibition and Conference . 2014.

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