IEEE Spectrum May, 2011 - 14

$900 million

amount that Google bid for Nortel Networks' patent portfolio. the telecom-equipment maker
went bankrupt in 2009 and has been selling pieces of itself to satisfy creditors.

China's
Godson
Gamble

8-core ring: In
most microprocessors,
information circles around
a ring-shaped interconnect
to reach processor cores.

mesh structure sets
supercomputer
processor apart
from the competition

T

he Dawning 6000
supercomputer, which
Chinese researchers
expect to unveil in the third
quarter of 2011, will have
something quite different
under its hood. Unlike
its forerunners, which
employed American‑born
chips, this machine will
harness the country's
homegrown high‑end
processor, the Godson‑3B.
With a peak frequency of
1.05 gigahertz, the Godson is
slower than its competitors'
wares, at least one of which
operates at more than 5 GHz,
but the chip still turns heads
with its record‑breaking
energy efficiency. It can
execute 128 billion floating‑
point operations per second
using just 40 watts-double
or more the performance per
watt of competitors.
The Godson has an
eccentric interconnect
structure-for relaying
messages among multiple
processor cores-that also
garners attention. While Intel
and IBM are commercializing
chips that will shuttle com‑
munications between cores
merry‑go‑round style on
a "ring interconnect," the
Godson connects cores using
14

9-core meSh: Some new
high-end processors use a
mesh network. these can
be more complex but also
more energy efficient.

a modified version of the
gridlike interconnect system
called a mesh network. The
processor's designers, led
by Weiwu Hu at the Chinese
Academy of Sciences, in
Beijing, seem to be placing
their bets on a new kind
of layout for future high‑
end computer processors.
A mesh design goes hand
in hand with saving energy,
says Matthew Mattina, chief
architect at the San Jose,
Calif.-based Tilera Corp., a
chipmaker now shipping 36‑
and 64‑core processors using
on‑chip mesh interconnects.
Imagine a ring intercon‑
nect as a traffic roundabout.
Getting to some exits
requires you to drive nearly
around the entire circle.
Traveling away from your
destination before getting
there, says Mattina, requires

Na * IEEE SpEctrum * may 2011

8-core godSon:
the chinese processor
relies on a modified mesh
network that contains
extra direct connections
to move data efficiently.
iLLustratioNs: gaViN poteNza

more transistor switching
and therefore consumes
more energy. A mesh
network is more like a city's
crisscrossed streets. "In a
mesh, you always traverse
the minimum amount of
wire-you're never going
the wrong way," he says.
On the 8‑core Godson
chip, 4 cores form a tightly
bound unit-each core sits
on a corner of a square of
interconnects, as in a usual
mesh. Godson researchers
have also connected each
corner to its opposite, using a
pair of diagonal interconnects
to form an X through the
square's center. A "crossbar"
interconnect then serves as
an overpass, linking this
4‑core neighborhood to a
similar 4‑core setup nearby.
Godson developers
believe that their modified

mesh's scalability will prove
a key advantage, as chip
designers cram more cores
onto future chips. Yunji
Chen, a Godson architect,
says that competitors'
ring interconnects may
have trouble squeezing
in more than 32 cores.
Indeed, one of the ring's
benefits could prove its
future liability. Linking new
cores to a ring is fairly easy,
says K.C. Smith, an emeritus
professor of electrical and
computer engineering at
the University of Toronto.
After all, there's only one
path to send information-
or two in a bidirectional
ring. But sharing a common
communication path also
means that each additional
core adds to the length of
wire that messages must
travel and increases the
demand for that path. With
a large number of cores,
"the timing around this
ring just gets out of hand,"
Smith says. "You can't get
service when you need it."
Of course, adding more
cores in a mesh also stresses
the system. Even if you have
a grid of paths providing
multiple communication
channels, more cores
increase the demand for
the network, and more
demand makes traveling
long distances difficult: Try
driving across New York
City at rush hour. Still, the
bandwidth scaling of a mesh
interconnect is superior
to that of a ring, Tilera's
Mattina says. He notes
that the total bandwidth
available with a mesh
interconnect increases as
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Table of Contents for the Digital Edition of IEEE Spectrum May, 2011
