Computational sprinting
is a new approach to smartphone power and cooling that could give users
dramatic, brief bursts of computing capability to improve current applications
and make new ones possible.
Its developers at the University of Pennsylvania
and the University
of Michigan are pushing
mobile chips beyond their sustainable operating limits, much like a sprinter
who runs extremely fast for a relatively short distance. The researchers will
present a paper on their concept at the International Symposium on High
Performance Computer Architecture in New
Orleans.
“We asked: what if we
designed a chip to run at 16 times the sustainable rate, but only for half a
second? Can we do it without burning out the chip? We did the calculations and
simulations, and we find that it is indeed possible to engineer such a system,”
said one of the study’s authors, Milo M.K. Martin, associate professor in the Department
of Computer and Information Science in Penn’s School of Engineering
and Applied Science.
“Normally, these devices
are designed for sustained performance, so that they can run full bore forever.
We’re proposing a computer system that can perform a giant surge of computation
but then gets tired and has time to rest,” said Thomas Wenisch, study coauthor
and an assistant professor at the U-M Department of Electrical Engineering and
Computer Science.
Smartphones no longer
benefit from increased transistor density and sophistication of computer chips,
the researchers say. They are hamstrung by the heat transistors produce, which
must be vented before it damages the chip. Small mobile devices don’t have room
for the large fans that keep a laptop’s temperature cool enough for it to
function, so only a fraction of a smartphone chip’s transistors can safely
operate at once.
This phenomenon, named “dark silicon” after the increasingly large portions of a silicon chip that
must remain off at a given time, is a major concern to many engineers, who fear
it represents a hard physical limit for making mobile devices faster and more
powerful. One estimate suggests that by 2019, just 9% of the transistors on a
smartphone chip will be able to be active at any time.
Computational sprinting
could circumvent the dark silicon problem by operating in a way that better
takes into consideration the ways mobile devices differ from laptops and
desktops.
A smartphone only rarely
needs to be operating at its maximum processing power, as most of the time it
is waiting in a pocket or purse for a user’s input. But once a user tells a
smartphone to do something computationally intense—such as performing image
recognition tasks, building panoramas out of individual photos, finding
navigation routes, or doing speech recognition or translation—the results need
to come as fast as possible. These so-called “bursty” activities have short
periods of intense computation, followed by long idle times when the device can
cool down and recover.
“In this research, we
performed a broad feasibility study and identified solutions to overcome
engineering challenges of sprinting,” said Martin. “Engineers have expressed
concern about the technology trends surrounding the idea of dark silicon. What
our research indicates is that it’s okay for the silicon to be mostly dark, if
you can use it all for a short burst of intense computation.”
Under the computational
sprinting scheme, up to 15 additional cores would fire up to work in parallel
alongside the chip’s main core for up to one second. This could speed up the
device’s response time 10-fold.
To handle sprinting’s
higher temperatures, the researchers propose a heat-spreading structure that
includes an encapsulated phase change material—something like candle wax—which
would absorb heat by melting during the sprint, then slowly dissipate it by
hardening while the device is at rest.
“This paradigm of
design for responsiveness will let us do things that are not possible today,”
said Wenisch. “Humans only have so much patience, so these interactive apps are
limited by what you can do in the fraction of a second that we’re willing to
wait. If app designers can now get 10 times as much computing done in one
burst, that frees their hands to pursue ideas they would have just discarded
today.”
