Soft Machines continues to develop CPUs built using its VISC architecture. If you're not already familiar with VISC, be sure to read our in-depth look at VISC from October of last year. As a quick refresher, VISC works by inserting a middleware layer between the native instruction set architecture of the operating system above and the hardware beneath. This middleware then translates the native ISA into VISC's native instruction set before distributing the workload across the processor's virtual cores.
VISC's trick is that even in single-threaded workloads, it can break that work into chunks that Soft Machines calls "threadlets." In turn, a VISC CPU can distribute the work of a demanding single thread on a virtual core across multiple hardware cores. It can also dynamically provision computing resources in mixed workloads where a demanding thread and a lighter-weight task need simultaneous access to CPU resources. That flexible resource allocation purports to allow VISC to deliver two to three times the instructions per clock of traditional CPUs.
The company says it remains on-track to deliver Shasta, its first commercial core, this year, and it's releasing some more detailed performance targets today for its Shasta, Shasta+, and Tahoe cores to back up its claims of 2-4x increases in performance per watt over competing architectures.
We still don't know many details about the Shasta CPU itself yet. We do know that it presents one to two virtual cores to the operating system on top of two physical cores. Each physical core has 1MB of L2 cache. The CPU uses a 64-bit ISA, and Soft Machines expects it to run at speeds up to 2GHz.
Soft Machines won't make its own chips. Instead, the company will license its VISC cores to hardware partners that want to implement the technology in their own SoCs. The company will also offer a VISC SoC of its own that it'll work with partners to customize for use in devices like smartphones, tablets, and entry-level convertibles.
The company thinks it can deliver up to 2.5 times the performance of competing chips at the same power level, and it believes its chips will be up to four times as efficient when delivering the same performance. As we'll soon see, those figures are averages of the company's test results rather than a hard maximum.
To establish its performance targets for Shasta, Shasta+, and Tahoe, Soft Machines says it's tested a variety of production SoCs and CPUs using the SPEC CPU 2006 benchmarks, including an ARM Cortex-A72, Apple's A9X, and Intel's Skylake Core i5-6200U. The company says it normalized each chip's results to assume a 16-nm process and 1MB L2 or last-level cache per core.
The power and performance results for the Shasta, Shasta+, and Tahoe chips were produced using the company's internal performance simulator using the SimPoint input method. The company says the performance and power usage of its 28-nm prototype silicon correlates well with its simulator results for that chip: within 5% for performance and 10% for power. Given those results, Soft Machines is confident about the results it's sharing today, as well.
The first chart the company is releasing today demonstrates the efficiency of each chip. The Y-axis charts the energy used per unit of SPEC CPU 2006 score, while the X-axis is a geometric mean of the benchmark's integer and floating-point tests. The nearest-term comparison here is the ARM Cortex-A72 versus the Shasta virtual core running on two physical processors.
By this measure, Soft Machines says the Shasta core is two to three times as efficient as a projected 3.23GHz Cortex-A72 when delivering the same performance. If the company's Shasta+ chip arrives on a 10nm process in 2017 as expected, it could be about 4.5 times as efficient as Apple's A9X CPU cores, while 2018's Tahoe chip (running one virtual core on four physical cores) could be as much as seven times as efficient as a Core i5-6200U-class Skylake chip.
The second chart demonstrates how much SPEC CPU 2006 performance each of the tested chips delivers as power and frequency scales. For the same power, the company claims a pair of 2.3GHz Shasta cores running one virtual thread offers 1.8 times as much performance as a theoretical Cortex-A72 at 3.4 GHz, while a Shasta chip providing the same performance as the A72 can run for one-third the power level. Shasta+ and Tahoe could deliver even greater performance.
If these numbers hold true, they offer reason to be optimistic about VISC's performance claims. We'll just have to wait and see whether that's the case later this year when VISC processors are expected to appear in actual hardware.