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Definition
For many signal processing applications programmability and efficiency is
desired. With current technology either programmability or efficiency is achievable,
not both. Conventionally ASIC's are being used where highly efficient systems
are desired. The problem with ASIC is that once programmed it cannot be enhanced
or changed, we have to get a new ASIC for each modification. Other option is microprocessor
based or dsp based applications. These can provide either programmability or efficiency.
Now with stream processors we can achieve both simultaneously. A comparison of
efficiency and programmability of Stream processors and other techniques are done.
We will look into how efficiency and programmability is achieved in a stream processor.
Also we will examine the challenges faced by stream processor architecture.
The
complex modern signal and image processing applications requires hundreds of GOPS
(giga, or billions, of operations per second) with a power budget of a few watts,
an efficiency of about 100 GOPS/W (GOPS per watt), or 10 pJ/op (Pico Joules per
operation). To meet this requirement current media processing applications use
ASICs that are tailor made for a particular application. Such processors require
significant design efforts and are difficult to change when a new media processing
application or algorithm evolve. The other alternative to meet the changing needs
is to go for a dsp or microprocessor, which are highly flexible. But these do
not provide the high efficiency needed by the application. Stream processors provide
a solution to this problem by giving efficiency and programmability simultaneously.
They achieve this by expressing the signal processing problems as signal flow
graphs with streams flowing between computational kernels. Stream processors have
efficiency comparable to ASICs (200 GOPS/W), while being programmable in a high-level
language.
Many
signal processing applications require both efficiency and programmability. The
complexity of modern media processing, including 3D graphics, image compression,
and signal processing, requires tens to hundreds of billions of computations per
second. To achieve these computation rates, current media processors use special-purpose
architectures tailored to one specific application. Such processors require significant
design effort and are thus difficult to change as media-processing applications
and algorithms evolve. Digital television, surveillance video processing, automated
optical inspection, and mobile cameras, camcorders, and 3G cellular handsets have
similar needs. The demand for flexibility in media processing motivates the use
of programmable processors. However, very large-scale integration constraints
limit the performance of traditional programmable architectures. In modern VLSI
technology, computation is relatively cheap - thousands of arithmetic logic units
that operate at multi gigahertz rates can fit on a modestly sized 1 cm 2 die.
The problem is that delivering instructions and data to those ALUs is prohibitively
expensive. For
example, only 6.5 percent of the Itanium 2 die is devoted to the 12 integer and
two floating-point ALUs and their register files; communication, control, and
storage overhead consume the remaining die area. In contrast, the more efficient
communication and control structures of a special purpose graphics chip, such
as the NVIDIA GeForce4, enable the use of many hundreds of floating-point and
integer ALUs to render 3D images. Conventional signal processing solutions can
provide high efficiency or programmability, but are unable to provide both at
the same time. In applications that demand efficiency, a hardwired application-specific
processor-ASIC (application-specific integrated circuit) or ASSP (application-specific
standard part)-has an efficiency of 50 to 500 GOPS/W, but offers little if any
flexibility.
At the other extreme, microprocessors and DSPs (digital signal processors) are
completely programmable but have efficiencies of less than 10 GOPS/W. DSP (digital
signal processor) arrays and FPGAs (field-programmable gate arrays) offer higher
performance than individual DSPs, but have roughly the same efficiency.
Moreover,
these solutions are difficult to program-requiring parallelization, partitioning,
and, for FPGAs, hardware design. Applications today must choose between efficiency
and programmability.
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