In terms of speed, energy consumption and size, inexact computer chips like this prototype, are about 15 times more efficient than today’s microchips. Credit: Avinash Lingamneni/Rice University/CSEM

Researchers
have unveiled an “inexact” computer chip that challenges the industry’s
50-year pursuit of accuracy. The design improves power and resource
efficiency by allowing for occasional errors. Prototypes unveiled this
week at the ACM International Conference on Computing Frontiers in
Cagliari, Italy, are at least 15 times more efficient than today’s
technology.

The
research, which earned best-paper honors at the conference, was
conducted by experts from Rice University in Houston, Singapore’s
Nanyang Technological University (NTU), Switzerland’s Center for
Electronics and Microtechnology (CSEM) and the University of California,
Berkeley.

In
terms of speed, energy consumption and size, inexact computer chips
like this prototype, are about 15 times more efficient than today’s
microchips.

“It
is exciting to see this technology in a working chip that we can
measure and validate for the first time,” said project leader Krishna
Palem, who also serves as director of the Rice-NTU Institute for
Sustainable and Applied Infodynamics (ISAID). “Our work since 2003
showed that significant gains were possible, and I am delighted that
these working chips have met and even exceeded our expectations.”

ISAID
is working in partnership with CSEM to create new technology that will
allow next-generation inexact microchips to use a fraction of the
electricity of today’s microprocessors.

“The
paper received the highest peer-review evaluation of all the Computing
Frontiers submissions this year,” said Paolo Faraboschi, the program
co-chair of the ACM Computing Frontiers conference and a distinguished
technologist at Hewlett Packard Laboratories. “Research on approximate
computation matches the forward-looking charter of Computing Frontiers
well, and this work opens the door to interesting energy-efficiency
opportunities of using inexact hardware together with traditional
processing elements.”

The
concept is deceptively simple: Slash power use by allowing processing
components—like hardware for adding and multiplying numbers—to make a
few mistakes. By cleverly managing the probability of errors and
limiting which calculations produce errors, the designers have found
they can simultaneously cut energy demands and dramatically boost
performance.

One
example of the inexact design approach is “pruning,” or trimming away
some of the rarely used portions of digital circuits on a microchip.
Another innovation, “confined voltage scaling,” trades some performance
gains by taking advantage of improvements in processing speed to further
cut power demands.

This comparison shows frames produced with video-processing software on traditional processing elements (left), inexact processing hardware with a relative error of 0.54 percent (middle) and with a relative error of 7.58 percent (right). The inexact chips are smaller, faster and consume less energy. The chip that produced the frame with the most errors (right) is about 15 times more efficient in terms of speed, space and energy than the chip that produced the pristine image (left). Credit: Rice University/CSEM/NTU

In their initial simulated tests in 2011,
the researchers showed that pruning some sections of traditionally
designed microchips could boost performance in three ways: The pruned
chips were twice as fast, used half as much energy and were half the
size. In the new study, the team delved deeper and implemented their
ideas in the processing elements on a prototype silicon chip.

“In
the latest tests, we showed that pruning could cut energy demands 3.5
times with chips that deviated from the correct value by an average of
0.25%,” said study co-author Avinash Lingamneni, a Rice graduate
student. “When we factored in size and speed gains, these chips were 7.5
times more efficient than regular chips. Chips that got wrong answers
with a larger deviation of about 8% were up to 15 times more efficient.”

Project
co-investigator Christian Enz, who leads the CSEM arm of the
collaboration, said, “Particular types of applications can tolerate
quite a bit of error. For example, the human eye has a built-in
mechanism for error correction. We used inexact adders to process images
and found that relative errors up to 0.54% were almost indiscernible,
and relative errors as high as 7.5% still produced discernible images.”

Palem,
the Ken and Audrey Kennedy Professor of Computing at Rice, who holds a
joint appointment at NTU, said likely initial applications for the
pruning technology will be in application-specific processors, such as
special-purpose “embedded” microchips like those used in hearing aids,
cameras and other electronic devices.

The
inexact hardware is also a key component of ISAID’s I-slate educational
tablet. The low-cost I-slate is designed for Indian classrooms with no
electricity and too few teachers. Officials in India’s Mahabubnagar
District announced plans in March to adopt 50,000 I-slates into middle and high school classrooms over the next three years.

The
hardware and graphic content for the I-slate are being developed in
tandem. Pruned chips are expected to cut power requirements in half and
allow the I-slate to run on solar power from small panels similar to
those used on handheld calculators. Palem said the first I-slates and
prototype hearing aids to contain pruned chips are expected by 2013.

Source: Rice University