We have profiled several processor updates and novel architectures this week in the wake of the Hot Chips conference, many of which have focused on clever FPGA implementations, specialized ASICs, or additions to well-known architectures, including Power and ARM.
Among the presentations that provided yet another way to loop around the Moore’s Law wall is a 1000-core processor “KiloCore” from UC Davis researchers, which they noted during Hot Chips (and the press repeated) was the first to wrap 1000 processors on a single die. Actually, a Japanese startup, Exascaler, Inc., beat them to this with the PEZY-SC (a 28nm MIMD processor with 1024 cores and has rankings on the Green 500 and a few machines wins in the country). This hiccup aside, the MIMD-based KiloCore approach is interesting–and has some noteworthy results compared to similar efforts.
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Kilgore has proven successful on both energy consumption and performance fronts. As one of the leads, Dr. Bevan Baas, shared from Hot Chips, the processors-per-die curve has remained relatively static over the last several years—with KiloCore representing a huge leap in the chart below.
The trajectory above is nothing new for Dr. Bevan Baas, who has spent decades immersed in low-power, high-performance processor design before becoming a UC Davis professor. In the late 80s, he was one of the designers of a high-end minicomputer in HP’s Computer Systems Division, before joining Atheros Communications, where he helped develop the first IEEE 802.11a wifi LAN. He now focuses on algorithms, architectures, and circuits as part of the VLSI Computation Lab at UC Davis.
“Kilgore has been designed with the needs of computationally-intensive applications and kernels in mind. It is meant to act as a co-processor within a larger system and isn’t intended to run an operating system itself. There could be some cases in applications or systems where it could act as a sole processor, but they wouldn’t be general-purpose systems,” Baas explains.
Each processor holds up to 128 instructions (and those larger are supported for the processors next to a shared memory block). Those are modified during application programming and stacked during runtime. The idea is to program them at once, stack them together, and let them go, Baas says. Applications can also request that processors be reprogrammed during runtime based on signals from the processors. So a processor might get to a point in execution and send a package to the administrator with a reprogramming request. Alternately, groups of processors can do the same. “Most applications we have tested don’t’ use or need this feature, but it is possible,” Baas notes.
Data is passed via messages between processors (which means they don’t need to hop through a processor’s memory). The messages move from processor A’s software to the other’s software. At its simplest, it is a read/write with synchronization step between the processors, which is part of what makes it possible to scale to thousands, or even tens of thousands of processors, with the programmer needing to worry about synchronization routines and the like—the goal is that “they sort themselves out,” according to Baas.
One final word about programming. The way applications are implemented fits the architecture well. They are broken down into mini-programs that are 128 words or less via a set of steps where the small programs are isolated down to coarse-grained tasks, task code is partitioned into serial code blocks parallelizable code blocks are replicated. Ultimately, this means a KiloCore array can run several different tasks simultaneously, as seen in the example pictured. This sounds easy enough in a brief explanation, but one has to imagine some significant development overhead.