"WINNER MULTIMEDIA MONSTER: Cell's nine processors make it
supercomputer on a chip
GOAL: Make a new microprocessor architecture that beats all others a
handling graphics and broadband multimedia.
WHY IT'S A WINNER: It met that goal and is being designed int
highvolume massmarket items like game consoles and televisions
ORGANIZATIONS: IBM, Sony and Toshiba
CENTER OF ACTIVITY: Austin, Texas.
NUMBER OF PEOPLE ON THE PROJECT: 400 at its peak.
BUDGET: US $400 million
http://img150.imageshack.us/img150/1163/toshibaibmsony8fr.jp
WE'RE FLYING AT ABOUT MARCH 1.5 around Mount Saint Helens, i
Washington state. IBM Corp. senior programmer Barry L. Minor is a
the controls, rocketing us over the crater and then down to the lak
at its base to skim over the tree trunks that have been floatin
there since the volcano exploded over 25 years ago. The flight i
exhilarating, even though it's just a simulation projected on
widescreen monitor in a cluttered testing lab
Then, at the flick of a switch, Minor turns the simulation over fro
his new Cell processor to a dualprocessor Apple Power Mac G5, and th
scenery freezes. The G5 almost audibly groans under the burden, thoug
it's no slouch. In fact, it's currently the top of the line for PCs
But Cell is something different entirely. It's a bet on wha
consumers will do with data and how best to suit microprocessors t
the task and it's really, really fast. Cell, which is shorthand fo
Cell Broadband Engine Architecture, is a US $400 million joint effor
of IBM, Sony, and Toshiba. It was originally conceived as th
microprocessor to power Sony's third-generation game console
PlayStation 3, to be released this spring, but it is expected to fin
a home in lots of other broad-band-connected consumer items and i
servers too
Executives at Sony Corp., in Tokyo, wanted more than just a
incremental improvement over PlayStation 2's processor, the Emotio
Engine. What they got was a 36fold acceleration, to a whopping 19
billion floating-point operations per second (192 gigaflops). Becaus
Cell is a combination of general-purpose and multimedia processors, i
defies an exact comparison with other upcoming chips, but it's though
to be more powerful than the chips driving competing game systems
Cell can calculate at such blazing speed, in part, because it's mad
up of nine processors on a single chip of silicon, optimized for th
kind of realtime calculations needed in today's broadband, media-ric
environment. A specially designed 300gigabit-per-second bus knits th
processors into a single machine, and interface technology fro
Rambus Inc., Los Altos, Calif., gives it fast access to memory an
other offchip systems
So far, microprocessor watchers have been impressed with what they'v
seen of Cell. "To bring huge parallel processing onto a singl
chip in a clean and efficient way is a real accomplishment,
says Ruby B. Lee, a professor of electrical engineering at Princeto
University and an IEEE Fellow
A graphics-heavy item such as PlayStation 3 isn't just a showcase fo
an unusual chip. For IBM it's a philosophical statement. "Gamin
is the next interface driving computing," says James A. Kahle
Cell's chief architect with the IBM Technology Group, in Austin
Texas [see photo, "Multicellular"]. Just as moving fro
punch cards to electronic displays changed what people expected o
computers, the highly collaborative, realtime realism of today'
games will set the standard for what people want from computers i
the future
But even now, the sheer desire for power in the gaming marke
guarantees that Cell will he made in volumes that more than make u
for the loss last year of IBM's highest profile customer, Appl
Computer Inc. Market research firm iSuppli Corp., in El Segundo
Calif., predicts that 37 million game consoles will be sold this yea
alone worldwide. By 2007, when all three game console makers will hav
released their nextgeneration products, the market will have grown t
44 million. And though Cell is exclusive to the PlayStation 3, IB
has a lock on the rest of the console market. Its microprocessor
will power both of Sony's competitors, Microsoft's Xbox and
Nintendo's GameCube.
The Cell powered PlayStation 3 can expect to pick up a little less
than half of what could become a market worth up to $9.5 billion in
2007, according to iSuppli senior analyst Chris Crotty. And, of
course, there are other high-volume plans for Cell.
Toshiba Corp., in Tokyo, for one, plans to build television sets
around it. The company has already shown that a single Cell processor
can decode and display 48 compressed video streams at once,
potentially allowing a television viewer to choose a channel based on
dozens of thumbnail videos displayed simultaneously on the screen. And
in a smaller market, Cell has already found its first outside customer
in medical and militarysystems maker Mercury Computer Systems Inc., in
Chelmsford, Mass., which is developing a two Cell blade server due out
by April.
With two such massive consumer electronics makers as Toshiba and Sony
behind it, Cell is an obvious attempt to control the "digital
living room," as technology executives have dubbed their dream
of a home where all the media players are intelligent and networked
together. "[Sony's] goal is to make a computer fun...to make it
an entertainment platform," says Sony's Cell director Masakazu
Suzuoki. "But even if we make the Cell system an entertainment
platform, there's nothing if there's no content."
Indeed, experts say Cell's success hinges on whether programmers
outside IBM, Sony, and Toshiba will be able to exploit the gigaflops
that Cell has to offer. Tony Massimini, chief of technology at the
consulting firm Semico Research Corp., in Phoenix, puts it bluntly:
"Cell has strong potential, assuming that the game developers
satisfy their customers' needs. But if the games suck, who wants to
buy it?"
That Cell has more than one processor core on a single chip is more a
sign of the times than a revolution. All the microprocessor stalwarts
are moving to multicore design. The principal reason is that the old
way of doing things—increasing the number of calculations per second
by shrinking the processors into a tighter knot of tinier transistors
and then dialing up the clock speed has essentially crashed headlong
into the brick wall of heat generation.
Because transistors using today's technology are so small, even when
they are supposed to be in the "off" state, infinitesimal
currents still leak through them. That leakage warms them constantly,
and with the extra heat generated when transistors switch
"on" or "off," it produces a microfurnace on a
chip. If chip makers had continued on their old path, by the year
2015, microprocessors would be throwing off more watts per square
millimeter than the surface of the sun.
As a result, the industry has shifted from maximizing performance to
maximizing performance per watt, mainly by putting more than one
microprocessor on a single chip and running them all well below their
top speed. Because the transistors are switching less frequently, the
processors generate less heat. And because there are at least two hot
spots on each chip, the heat is spread more evenly over it, so it's
less damaging to the circuitry and easier to get rid of with fans and
heat sinks.
Multicore processors on the market today are generally
symmetrical—that is, they have two copies of essentially the same
core on one chip. Cell, on the other hand, has an asymmetric
architecture that contains two different kinds of cores [see photo,
"Cell City Map"]. One, the Power processing element, is
similar to the CPU in a Mac, it runs the Linux operating system and
divides up work for the other eight processors to do. Those
eight—called Synergistic processing elements—are designed
specifically to juggle multimedia applications: video compression and
decompression, encryption and decryption of copyrighted content, and,
especially, rendering and modifying graphics.
[img:318ec18a5e]
http://img227.imageshack.us/img227/971/cell2hc.jpg[/img:318ec18a5e]
The Synergistic elements were built from the ground up to do what are
called singleprecision floatingpoint calculations—the kind of
operations needed for dazzling threedimensional graphics and a host
of other multimedia tasks. The design traded flexibility-a
Synergistic element is not versatile enough to run the Linu
operating system on its own—for eye-popping speed. When pushed to it
5.6gigahertz limits, a single unit can do 44.8 billion singleprecisio
floatingpoint calculations per second. Not wanting to cut Cell of
from a role in scientific computing, its designers included circuitr
in each Synergistic element that can do the more exactin
calculations, called doubleprecision, that scientists demand, but it
performance is only about onetenth that of the singleprecision unit
In fact, the Synergistic elements are so fast that a single one coul
easily consume the entire bandwidth on the interconnects to th
offchip memory, leaving its siblings starved for data and stalle
out. IBM and its partners had to design a special chunk of circuitr
into Cell just to prevent that problem
Apart from its raw power, Cell has content-protection tricks tha
should make it attractive to multimedia applications makers. Fo
instance, the Synergistic element's architecture prevents an
application or external device from accessing the element's loca
memory, so that, for instance, a program cannot steal a music fil
that is being decrypted by the processor. "Once you bring you
code in and decrypt it, it can execute in a virtually truste
environment," says IBM's Cell architect Charles R. Johns
"All the data it calculates on, sends out, and brings in i
fully protected.
The isolation function can be used in several ways, says Kahle
"We knew we couldn't anticipate all the different security need
in the future, but we wanted to know we had the right hardware t
support a very robust security system.
Barry Minor's Mount Saint Helens simulator is a good example of ho
Cell's different processors work together. His program takes
satellite photo of the volcano, lines it up with an elevation map
and then turns it into a detailed 3D terrain on the fly. The Moun
Saint Helen's data has a resolution of 2.4 meters. The city o
Austin, where the Cell design center is, once gave Minor access t
its 15.4centimeter-resolution satellite map. "You could land i
Michael Dell's backyard and check out his view," Minor says wit
a grin
What's happening inside the processor is a finely choreographed dance
The Power processing element starts by figuring out where the joystic
is pointing the simulator in the stored 2D maps. Then it divides tha
scene into 32 portions, four for each Synergistic element. Thoug
perfectly capable of it, the Power processing element does n
calculations on the actual data. Instead, it plays to its strength a
a controller, figuring out which chunk of work should go to each o
the other cores according to how complex the scene is and which core
have more or less time on their hands
The Synergistic elements then go to work. They pull their portion o
the data into their local memories, which they can access at grea
speed. Then each runs a rendering algorithm on the data and stores i
off the chip in the system memory. When the processors are done, the
signal the Power element, which instructs one of the synergisti
units to run a video compression algorithm. That processor compresse
its sister units' finished products and then pushes them out to b
displayed on the screen or streamed to a PDA or some other device
Because the compression takes less time than rendering the graphics
the compressing processor automatically switches gears when it'
finished and runs the rendering algorithm on a portion of data unti
it's needed for compression again. With each frame, the proces
starts over
This dance works so well for two reasons. The first has to do with th
way Cell handles memory. Rather than waste several clock cycle
waiting for the right data to arrive from memory, a Synergisti
element works only on data stored in its own 256 kilobytes of memory
to which it has a high-bandwidth connection. More important, Cell'
memory-handling engines can be programmed to keep data streamin
through the processor. "We can get over 128 memory transaction
going in flight at once," boasts Michael N. Day, a distinguishe
engineer at IBM.
The memory-access engine takes in new data and sends out the old just
in time for the synergistic unit to perform the necessary
calculations. When Cell runs Minor's volcano simulator, it waits for
data to arrive from memory for only 1 percent of the time, the G5, in
contrast, stands idle for about 40 percent of the time.
Cell's other key to speed has to do with breaking problems into parts
that can be done in parallel. In Minor's simulation, it probably
seems obvious that an image can be divided up into eight strips and
these worked on independently. What wasn't so obvious was that the
3-D rendering could be done four pieces of data at a time within each
synergistic processor. Such four-way parallel computing is called
single instruction multiple data, or SIMD, and it is particularly
well suited to the manipulation of graphics and other multimedia.
In these problems, you typically want to perform the same operation on
each of the elements in a large chunk of data. For example, to
increase the brightness of an image, you'd want to add the same
number to every pixel in it. Since around the mid-1990s,
general-purpose processors such as the Intel x86 architectures have
been doing SIMD computing using a set of multimedia-specific
instructions, explains Princeton's Lee, a multimedia instructions
pioneer.
But SIMD instructions run far faster on Cell's Synergistic processors,
because the Cell processors were designed from the start to handle
them. And don't forget: there are eight such processors on each chip.
Cell programmers spend most of their time turning complex algorithms
into efficient SIMD algorithms, says Minor. "Once you've done
that, you're 80 percent done."
The Chip's commercial success will depend on whether programmers can
learn to exploit its full potential. To that end, the developers have
from the beginning put a high priority on crafting the appropriate
software tools.
One of the key deadlines the Cell development team had to meet was
having its software ready and tested in time for the arrival of the
first chips, in spring 2004. The software team was running programs
on a Cell simulator two full years before it got the first chip—and
when the chip finally arrived, both the operating system and the
applications worked on the first try. "Had we waited to do
software development until the chip came back, it would have been a
disaster," says Theodore R. Maeurer, software manager at IBM.
With such a head start on the software, the group could focus on how
to familiarize new programmers with Cell. "A programmer has to
do a really nice job of laying out the data transfers and so
forth," says Day. But soon that job will be turned over to the
compiler and the programming tools. IBM software engineers are also
developing tools that will make it easier for programmers to divide
tasks between the Power element and the Synergistic cores, and
they're making others to automatically find solutions to problems
that fit well with the Synergistic units' SIMD strengths. The company
has already released more than 700 pages of documents to applications
developers and will begin releasing tools and compilers, as well.
Cell's asymmetric architecture signals the beginning of a big shift in
how computers are programmed, says Craig Steffen, a senior research
scientist at the National Center for Super-computing Applications,
Urbana-Champaign, Ill., who gained some fame lashing together 70
PlayStation 2 consoles to form a $50 000 supercomputer.
"How do you program with eight engines running full speed without
them constantly stopping and waiting for data?" Steffen asks.
Cell will force mainstream programmers to wrestle with that question.
But ultimately, parallel programming will become fairly routine, he
predicts. "Over the next several years, we won't think of an
asymmetric processor as anything different."
Indeed, some think Cell is an indication of what's to come in other
microprocessors. "In the future, we'll see convergence of
general-purpose multiprocessors and game- and media-oriented
processors," says Princeton's Lee. "Media processors will
become more general purpose, and general purpose, more
multimedia." And with any luck, that will make your living room
a more entertaining place.
IEEE Spectrum, January 2006
MICROPROCESSORS By Samuel K. Moore
