The data-routing techniques that undergird the internet could boost the efficiency ofmulticore chips while reducing their power needs, a new study has revealed.
Computer chips have stopped getting faster. In order to keep increasing chips' computational power at the rate to which we've grown accustomed, chipmakers are instead giving them additional "cores," or processing units.
Today, a typical chip might have six or eight cores, all communicating with each other over a single bundle of wires, called a bus.
With a bus, however, only one pair of cores can talk at a time, which would be a serious limitation in chips with hundreds or even thousands of cores, which many electrical engineers envision as the future of computing.
Li-Shiuan Peh, an associate professor of electrical engineering and computer science at MIT, wants cores to communicate the same way computers hooked to the Internet do: by bundling the information they transmit into "packets." Each core would have its own router, which could send a packet down any of several paths, depending on the condition of the network as a whole.
At the Design Automation Conference in June, Peh and her colleagues will present a paper she describes as "summarizing 10 years of research" on such "networks on chip." Not only do the researchers establish theoretical limits on the efficiency of packet-switched on-chip communication networks, but they also present measurements performed on a test chip in which they came very close to reaching several of those limits.
In principle, multicore chips are faster than single-core chips because they can split up computational tasks and run them on several cores at once.
Cores working on the same task will occasionally need to share data, but until recently, the core count on commercial chips has been low enough that a single bus has been able to handle the extra communication load.
That's already changing, however: "Buses have hit a limit," Peh said.
"They typically scale to about eight cores." The 10-core chips found in high-end servers frequently add a second bus, but that approach won't work for chips with hundreds of cores.
Peh and her colleagues have developed two techniques to address these concerns. One is something they call "virtual bypassing."
In the Internet, when a packet arrives at a router, the router inspects its addressing information before deciding which path to send it down. With virtual bypassing, however, each router sends an advance signal to the next, so that it can preset its switch, speeding the packet on with no additional computation.
In her group's test chips, Peh said, virtual bypassing allowed a very close approach to the maximum data-transmission rates predicted by theoretical analysis.
The other technique is something called low-swing signalling.
Digital data consists of ones and zeroes, which are transmitted over communications channels as high and low voltages. Sunghyun Park, a PhD student advised by both Peh and Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor of Electrical Engineering, developed a circuit that reduces the swing between the high and low voltages from one volt to 300 millivolts.
With its combination of virtual bypassing and low-swing signaling, the researchers' test chip consumed 38 percent less energy than previous packet-switched test chips.
The researchers have more work to do, Peh said, before their test chip's power consumption gets as close to the theoretical limit as its data transmission rate does.
But, she added, "if we compare it against a bus, we get orders-of-magnitude savings."
Computer chips have stopped getting faster. In order to keep increasing chips' computational power at the rate to which we've grown accustomed, chipmakers are instead giving them additional "cores," or processing units.
Today, a typical chip might have six or eight cores, all communicating with each other over a single bundle of wires, called a bus.
With a bus, however, only one pair of cores can talk at a time, which would be a serious limitation in chips with hundreds or even thousands of cores, which many electrical engineers envision as the future of computing.
Li-Shiuan Peh, an associate professor of electrical engineering and computer science at MIT, wants cores to communicate the same way computers hooked to the Internet do: by bundling the information they transmit into "packets." Each core would have its own router, which could send a packet down any of several paths, depending on the condition of the network as a whole.
At the Design Automation Conference in June, Peh and her colleagues will present a paper she describes as "summarizing 10 years of research" on such "networks on chip." Not only do the researchers establish theoretical limits on the efficiency of packet-switched on-chip communication networks, but they also present measurements performed on a test chip in which they came very close to reaching several of those limits.
In principle, multicore chips are faster than single-core chips because they can split up computational tasks and run them on several cores at once.
Cores working on the same task will occasionally need to share data, but until recently, the core count on commercial chips has been low enough that a single bus has been able to handle the extra communication load.
That's already changing, however: "Buses have hit a limit," Peh said.
"They typically scale to about eight cores." The 10-core chips found in high-end servers frequently add a second bus, but that approach won't work for chips with hundreds of cores.
Peh and her colleagues have developed two techniques to address these concerns. One is something they call "virtual bypassing."
In the Internet, when a packet arrives at a router, the router inspects its addressing information before deciding which path to send it down. With virtual bypassing, however, each router sends an advance signal to the next, so that it can preset its switch, speeding the packet on with no additional computation.
In her group's test chips, Peh said, virtual bypassing allowed a very close approach to the maximum data-transmission rates predicted by theoretical analysis.
The other technique is something called low-swing signalling.
Digital data consists of ones and zeroes, which are transmitted over communications channels as high and low voltages. Sunghyun Park, a PhD student advised by both Peh and Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor of Electrical Engineering, developed a circuit that reduces the swing between the high and low voltages from one volt to 300 millivolts.
With its combination of virtual bypassing and low-swing signaling, the researchers' test chip consumed 38 percent less energy than previous packet-switched test chips.
The researchers have more work to do, Peh said, before their test chip's power consumption gets as close to the theoretical limit as its data transmission rate does.
But, she added, "if we compare it against a bus, we get orders-of-magnitude savings."
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