Download Parallel Computing Using FPGA

159.735 – Studies in parallel and distributed systems
Parallel Computing Using FPGA
Submitted to:
Dr. Martin Johnson
Submitted by: Sohaib Ahmed (07217579)
Parallel Computing Using FPGA
Introduction
Field programmable gate arrays (FPGAs) are emerging in many areas of high
performance computing, either as tailor made signal processor, embedded algorithm
implementation, systolic array, software accelerator or application specific
architecture. FPGAs are so flexible and reconfigurable that they are capable of
massively parallel operations, explicitly tailored to the problem at hand. There are lot
of paradigms to put FPGAs at work in a high performance computing environment
There are number of limitations which restrict FPGAs to reach the performance of
Application Specific Integrated Circuits (ASICs) but they provide the possibility of
changing the hardware design easily while outpacing software implementations on
general purpose processors.
FPGA
FPGA was first invented in the mid 1980s by one of the founder of Xilinx
(www.xilinx.com ), Ross Freeman. It is a semiconductor device which is comprised
of different number of logic elements, interconnects, IOBs (Input/Output blocks). All
these components are user-configurable. It can help in implementing complex digital
circuits. The IOBs form a ring around the outer-edge of the microchip while
rectangular array of logic blocks lies inside the IOB ring. Each IOB provides a facility
to access a selectable I/O to one of the pins found in exterior of the FPGA package as
mentioned in figure 1 (Buell et al, 2007).
Figure 1: FPGA internal structure based on the Xilinx architecture style
(Buell et al., 2007)
A typical FPGA logic block consists of a four-input lookup table (LUT) and flip-flop.
Currently, FPGA consists of DSP blocks which have high-level functionality
embedded into the silicon, high-speed IOBs, embedded memories and processors. It
also contains Configurable logic blocks (CLBs) which comprised of multiple slices. A
slice is a small set of building blocks. Moreover, modern FPGA consist of tens of
thousands of CLBs and a programmable interconnected network in a rectangular grid
(Buell et al, 2007).
ASICs (Application specific integrated circuits) is typically performed a single
function through out the lifetime of a chip while in FPGA, it can be reprogrammed in
such a way that it can perform function in micro-seconds. Source code written in a
hardware description language (HDL) such as Verilog and VHDL provides the
functionality to perform tasks at run time. Synthesis process generates technologymapped netlist (figure 2). A map, place and route process then fits the netlist to the
actual FPGA in architecture. The process produces a bitstream which is used to
reconfigure FPGA. Timing, postsynthesis, functional simulations and verifications
methodologies can validate map, place and route results (Buell et al., 2007)
Figure 2: Typical design flow of FPGA (Buell et al, 2007)
FPGA supports the notion of reconfigurable computing and provides a facility of onchip parallelism which can be mapped directly from the dataflow characteristics of an
application’s parallel algorithm. A recent emergence in high performance can be
achieved by a hybrid approach to make a complex-system on a programmable chip.
Examples are Virtex II Pro, Virtex-4 and Xilinx Devices. The most recent success of
FPGAs in high-performance computing came under Tsubame cluster in Tokyo when
FPGAs increased performance by additional 25% (Wu-Feng & Manocha, 2007).
Reconfigurable Computing
High performance reconfigurable computing (HPRC) are parallel computing system
that allows multiple microprocessors and multiple FPGAs embedded into it. FPGAs
are inherently co-processors which are deployed to execute the small portion of the
application that takes most of the time – under 10-90 rule, the 10 percent of the code
that takes 90 percent of the execution time (Buell et al, 2007).
Reconfigurable computing is also known as configurable computing or custom
computing. It often has impressive performance and it is described in one of the
example mentioned. For a key size of 270 bits, a point multiplication can be computed
in 0.36 ms with a reconfigurable computing design implemented in an XC2V6000
FPGA at 66MHz while an optimized software implementation took 196.71 ms on a
dual-xeon computer at 2.6 GHz. It shows that reconfigurable computing design is
more that 540 times faster while its clock speeds is almost 40 times slower than the
Xeon processors (Todman et al, 2005).
Progress in hardware system and programming software
With the emergence of technologies, many hardware systems have begun to resemble
parallel computers. These systems are not designed for scalability because they
consisted of a single board of one or more microprocessors connected to one or more
FPGA devices. Recently, SRC-6 and SRC-7 have a parallel architecture in which used
cross bar switch that can be piled for further scalability. Traditionally, highperformance computing vendors – specifically, Silicon Graphics Inc. (SGI), Cray and
Linux Networx have incorporated FPGAs in their parallel architectures (Buell et al.,
2007).
From software perspective, developers can create the hardware kernel by using
hardware description languages such as VHDL and Verilog. SRC Computers allow
other hardware description languages including Carte C, Carte Fortran, Impulse
Accelerated Technologies’ Impulse C, Mitrion C from Mitrionics, and Celoxica’s
Handel-C. Annapolis Micro Systems’ CoreFire, Starbridge Systems’ Viva, Xilinx
System Generator and DSPlogic’s reconfigurable computing toolbox are the highlevel graphical programming development tools (El-Ghazawi et al, 2008, Buell et al,
2007).
Reconfigurable logic and traditional processing
Reconfigurable logic consists of programmable computational matrix with a
programmable interconnected network which is used within that computational
matrix. There are the basic differences between reconfigurable logic and traditional
processing which are described in figure 3 (Bondalapati & Prasanna, 2002).

Spatial computation: the data is processed by spatially distributed the
computations rather than temporally sequencing.

Configurable data path: the functionality of the computational units and the
interconnection network can be adapted at run-time.

Distributed control: the computational units process data based on local
configuration rather than an instruction broadcast to all the functional units.

Distributed resources: the required resources for computation such as
computational units and memory.
Figure 3 Performance trends of FPGAs and microprocessors (Smith et al, 2007)
Advantages of FPGAs
There are number of advantages using FPGAs including speed, reduced energy
power consumption. As in reconfigurable computing, hardware circuit is optimised
with the application so that the power consumption will tend to be much lower than
that for a general-purpose processor. FPGAs have other advantages which comprised
of reduction in size, component count (and hence cost), improved time-to-market and
improved flexibility and extendibility. These advantages are especially important for
embedded applications (Todman et al, 2005).
Limitations of FPGAs
There are number of challenges in implementing reconfigurable computing. Todman
et al. (2005) described three such challenges.
1. Structure of reconfigurable fabric
2. Interfaces between the fabric, processor(s)
3. Memory must be efficient
There is another challenge regarding the development of computer-aided design and
compilation tools that map an application to a reconfigurable computing. The problem
is related to know about which part of the application is mapped to the fabric and
which should be mapped to the processor (Todman et al., 2005).
Few limitations of FPGAs in high performance computing should be addressed. These
issues include the need of programming tools that address the overall architecture,
profiling and debugging tools for parallel and reconfigurable performance.
Furthermore, application-portability issues should be required to explore (Buell et al.,
2007).
FPGAs power applications performance
FPGAs offer tremendous performance potential. They can support in number of
different parallel computation applications and implemented in single clock execution
time. If FPGAs are reprogrammable then they can provide on chip facility for a
number of applications. Due to the presence of on-chip memory facilitate coprocessor logic’s memory access bandwidth is not restricted to the number of I/O pins
present in the devices. Moreover, memory is also closely coupled to the algorithm
logic so therefore, no external high-speed memory cache is needed. And due to that
power-consuming cache access and coherency problems can be avoided. The use of
internal memory also means that no additional I/O pins are required to increase its
accessible memory size, simplifying design scaling (Altera, 2007).
With the use of defined structures and the availability of resources in today’s high
performance FPGAs ( i.e the Altera Stratix III family of FPGAs), they can serve as
hardware for wide range of applications. As shown in Table 1 (Altera, 2007) , some of
the practical examples of applications show that at least ten times improvement in
execution time of algorithms as compare to single processor.
Table 1 FPGA Algorithm Acceleration (Altera, 2007)
FPGA application design techniques
Herbordt (2007) described application design techniques to enable substantial FPGA
acceleration. These methods are as follows:
1. Use an algorithm optimal for FPGAs
2. Use a computing mode appropriate for FPGAs
3. Use appropriate FPGA structures
4. Living with Amdahl’s law
5. Hide latency of independent functions
6. Use rate techniques to remove bottlenecks
7. Take advantage of FPGA-specific hardware
8. Use appropriate arithmetic precision
9. Use appropriate arithmetic mode
10. Minimize use of high-cost arithmetic operations
11. Create families of applications, not point solutions
12. Scale application for maximal use of FPGA hardware
Need of highly parallel FPGAs
There is a common approach in increasing performance of single unit is to build a
cluster of low-cost off-the shelf machines. But there are few disadvantages like low
communication bandwidth and costly. Most clusters are loosely coupled based on
external peripherals and due to that it is much costly. Moreover, they are not able to
solve specific problem in parallel. Another approach is related to use parallel
computers and they share their resources which allow for very fast data throughput
but require system design constraints to remain scalable. COPACOBANA (CostOptimized Parallel Code Breaker) utilizes up to 120 Xilinx Spartan-3 FPGAs
connected through a parallel backplane and interfaces the outside world through a
dedicated controller FPGA with an Ethernet interface and a Micro Blaze softprocessor core running uClinux. It uses for cryptanalysis and further work has been
needed to use its architecture and framework for solving other scientific problems
(Guneysu et al., 2007).
COPACOBANA Architecture
It consists of three basic blocks including one controller module, up to 20 FPGA
modules and backbone which is used for providing interconnection between controller
and FPGA modules (figure 4).
Figure 4 Architecture of COPACOBANA (Guneysu et al., 2007)
The FPGAs are directly connected to a common 64-bit data bus on board of the
FPGA module which is interfaced to the backplane data bus via transceivers with 3state outputs. While disconnected from the bus, the FPGAs can communicate locally
via the internal 64-bit bus on the DIMM module. The DIMM format allows for a very
compact component layout, which is important to closely connect the modules by a
bus. Every FPGA module is assigned a unique hardware address, which is
accomplished by Generic Array Logic (GAL) attached to every DIMM socket. Hence,
all FPGA cores can have the same configuration and all FPGA modules can have the
same layout. They can easily be replaced in case of a defect. (Guneysu, 2007).
Conclusion
FPGAs offer a number of paradigms to speed up calculations in a hardware software
co-design environment. They are relatively cost-effective as compare to ASICs and
due to flexible in nature, hardware resources are utilized in an effective way.
However, much work remains to be done for achieving high-performance parallel
computing by using FPGAs.
References
Altera Cooperation White Paper (2007). Accerating high performance computing
with FPGAs. October 2007
Bondalapti,K., & Prasanna,V. (2002). Reconfigurable computing systems.
Proceedings of the IEEE, Vol. 90, No. 7, July, 2002.
Buell, D., El-Ghazawi, T., Gaj,K.,& Kindratenko,V. (2007). High-Performance
reconfigurable computing. IEEE Computer Society, March, 2007.
El-Ghazawi, T., El-Araby,E., Miaoqing Huang, Gaj,K., Kindratenko, V.,& Buell, D.
(2008).The promise of high-performance reconfigurable computing. IEEE
computer society, February, 2008 pp. 69 -76.
Guneysu,T., Paar,C., Pelzl,J., Pfieffer,G.,Schimmler,M., & Schlieffer,C. (2007).
Parallel computing with low cost FPGAs A framework for COPACOBANA.
Herbordt, M.C., VanCourt, T., Yongfeng, G., Shukhwani, B., Conti,A., Model,J. &
Disabello,D. (2007). Achieving high performance with FPGA-Based computing
Smith, M.C., Vetter,J.S., & Alam,S.R. (2005).Scientific computing beyond CPUs:
FPGA implementations of common scientific kernels. MAPLD/187.
Todman,T.J., Constantinides, G.A., Wilton,S.J.E, Luk,W. & Cheung, P.Y.K. (2005).
Reconfigurable computing: architectures and design methods. IEEE Proceedings
of Computer Digital Technologies, Vol. 152, No. 2, March, 2005.
Wu-Feng, Manocha,D. (2007). High performance computing using accelerators,
Parallel Computing, 33 (2007) pp. 645-647.