Binu K. Mathew

The Impulse memory controller

Impulse is a memory system architecture that adds an optional level of address indirection at the memory controller. Applications can use this level of indirection to remap their data structures in memory. As a result, they can control how their data is accessed and cached, which can improve cache and bus utilization. The Impulse design does not require any modification to processor, cache, or bus designs since all the functionality resides at the memory controller. As a result, Impulse can be adopted in conventional systems without major system changes. We describe the design of the Impulse architecture and how an Impulse memory system can be used in a variety of ways to improve the performance of memory-bound applications. Impulse can be used to dynamically create superpages cheaply, to dynamically recolor physical pages, to perform strided fetches, and to perform gathers and scatters through indirection vectors. Our performance results demonstrate the effectiveness of these optimizations in a variety of scenarios. Using Impulse can speed up a range of applications from 20 percent to over a factor of 5. Alternatively, Impulse can be used by the OS for dynamic superpage creation; the best policy for creating superpages using Impulse outperforms previously known superpage creation policies.

A low-power accelerator for the SPHINX 3 speech recognition system

Accurate real-time speech recognition is not currently possible in the mobile embedded space where the need for natural voice interfaces is clearly important. The continuous nature of speech recognition coupled with an inherently large working set creates significant cache interference with other processes. Hence real-time recognition is problematic even on high-performance general-purpose platforms. This paper provides a detailed analysis of CMUs latest speech recognizer (Sphinx 3.2), identifies three distinct processing phases, and quantifies the architectural requirements for each phase. Several optimizations are then described which expose parallelism and drastically reduce the bandwidth and power requirements for real-time recognition. A special-purpose accelerator for the dominant Gaussiann probability phase is developed for a 0.25μ CMOS process which is then analyzed and compared with Sphinxs measured energy and performance on a 0.13μ 2.4 GHz Pentium 4 system. The results show an improvement in power consumption by a factor of 29 at equivalent processing throughput. However after normalizing for process, the special-purpose approach has twice the throughput, and consumes 104 times less energy than the general-purpose processor. The energy-delay product is a better comparison metric due to the inherent design trade-offs between energy consumption and performance. The energy-delay product of the special-purpose approach is 196 times better than the Pentium 4. These results provide strong evidence that real-time large vocabulary speech recognition can be done within a power budget commensurate with embedded processing using todays technology.

A loop accelerator for low power embedded VLIW processors

The high transistor density afforded by modern VLSI processes has enabled the design of embedded processors that use clustered execution units to deliver high levels of performance. However, delivering data to the execution resources in a timely manner remains a major problem that limits ILP. It is particularly significant for embedded systems where memory and power budgets are limited. A distributed address generation and loop acceleration architecture for VLIW processors is presented. This decentralized on-chip memory architecture uses multiple SRAMs to provide high intra-processor bandwidth. Each SRAM has an associated stream address generator capable of implementing a variety of addressing modes in conjunction with a shared loop accelerator. The architecture is extremely useful for generating application specific embedded processors, particularly for processing input data which is organized as a stream. The idea is evaluated in the context of a fine grain VLIW architecture executing complex perception algorithms such as speech and visual feature recognition. Transistor level Spice simulations are used to demonstrate a 159x improvement in the energy delay product when compared to conventional architectures executing the same applications.

A low power architecture for embedded perception

Recognizing speech, gestures, and visual features are important interface capabilities for future embedded mobile systems. Unfortunately, the real-time performance requirements of complex perception applications cannot be met by current embedded processors and often even exceed the performance of high performance microprocessors whose energy consumption far exceeds embedded energy budgets. Though custom ASICs provide a solution to this problem, they incur expensive and lengthy design cycles and are inflexible. This paper introduces a VLIW perception processor which uses a combination of clustered function units, compiler controlled dataflow and compiler controlled clock-gating in conjunction with a scratch-pad memory system to achieve high performance for perceptual algorithms at low energy consumption. The architecture is evaluated using ten benchmark applications taken from complex speech and visual feature recognition, security, and signal processing domains. The energy-delay product of a 0.13μ implementation of this architecture is compared against ASICs and general purpose processors. Using a combination of Spice simulations and real processor power measurements, we show that the cluster running at 1 GHz clock frequency outperforms a 2.4 GHz Pentium 4 by a factor of 1.75 while simultaneously achieving 159 times better energy delay product than a low power Intel XScale embedded processor.

Algorithmic foundations for a parallel vector access memory system

This paper presents mathematical foundations for the design of a memory controller subcomponent that helps to bridge the processor/memory performance gap for applications with strided access patterns. The Parallel Vector Access (PVA) unit exploits the regularity of vectors or streams to access them efficiently in parallel on a multi-bank SDRAM memory system. The PVA unit performs scatter/gather operations so that only the elements accessed by the application are transmitted across the system bus. Vector operations are broadcast in parallel to all memory banks, each of which implements an efficient algorithm to determine which vector elements it holds. Earlier performance evaluations have demonstrated that our PVA implementation loads elements up to 32.8 times faster than a conventional memory system and 3.3 times faster than a pipelined vector unit, without hurting the performance of normal cache-line fills. Here we present the underlying PVA algorithms for both word interleaved and cache-line inter-leaved memory systems.