Is your memory hierarchy stopping your microprocessor from performing at the high level it should be? Memory Systems: Cache, DRAM, Disk shows you how to resolve this problem. The book tells you everything you need to know about the logical design and operation, physical design and operation, performance characteristics and resulting design trade-offs, and the energy consumption of modern memory hierarchies. You learn how to to tackle the challenging optimization problems that result from the side-effects that can appear at any point in the entire hierarchy.As a result you will be able to design and emulate the entire memory hierarchy. . Understand all levels of the system hierarchy -Xcache, DRAM, and disk. . Evaluate the system-level effects of all design choices. . Model performance and energy consumption for each component in the memory hierarchy.

Memory Systems: Cache, DRAM, Disk

Power is a design constraint not only for portable computers and mobile communication devices but also for high-end systems, and the design process should not subordinate it to performance.

Power: a first-class architectural design constraint

In deep sub-micron ICs, growing amounts of on-die memory and scaling effects make embedded memories increasingly vulnerable to reliability and yield problems. As scaling progresses, soft and hard errors in the memory system will increase and single error events are more likely to cause large-scale multi- bit errors. However, conventional memory protection techniques can neither detect nor correct large-scale multi-bit errors without incurring large performance, area, and power overheads. We propose two-dimensional (2D) error coding in embedded memories, a scalable multi-bit error protection technique to improve memory reliability and yield. The key innovation is the use of vertical error coding across words that is used only for error correction in combination with conventional per-word horizontal error coding. We evaluate this scheme in the cache hierarchies of two representative chip multiprocessor designs and show that 2D error coding can correct clustered errors up to 32times32 bits with significantly smaller performance, area, and power overheads than conventional techniques.

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Multi-bit Error Tolerant Caches Using Two-Dimensional Error Coding

Very small instruction caches have been shown to greatly reduce fetch energy. However, for many appli- cations the use of a small filter cache can lead to an unacceptable increase in execution time. In this paper, we propose the Tagless Hit Instruction Cache (TH-IC), a technique for completely eliminating the performance penalty associated with filter caches, as well as a fur- ther reduction in energy consumption due to not having to access the tag array on cache hits. Using a few meta- data bits per line, we are able to more efficiently track the cache contents and guarantee when hits will occur in our small TH-IC. When a hit is not guaranteed, we can instead fetch directly from the L1 instruction cache, eliminating any additional cycles due to a TH-IC miss. Experimental results show that the overall processor en- ergy consumption can be significantly reduced due to the faster application running time and the elimination of tag comparisons for most of the accesses.

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Guaranteeing Hits to Improve the Efficiency of a Small Instruction Cache

In this paper, we address the problem of the efficient exploration of the architectural design space for parameterized embedded systems. The exploration problem is multi-objective (e.g., energy and delay), so the main goal of this work is to find a good approximation of the Pareto-optimal configurations representing the best energy/delay trade-offs by varying the architectural parameters of the target system. In particular, the paper presents a Design Space Exploration (DSE) framework to simulate the target system and to dynamically profile the target applications. In the proposed DSE framework, a set of heuristic algorithms have been analyzed to reduce the overall exploration time by computing an approximated Pareto set of configurations with respect to the selected figures of merit. Once the approximated Pareto set has been built, the designer can quickly select the best system configuration satisfying the constraints. Experimental results, derived from the application of the proposed DSE framework to a superscalar architecture, show that the exploration time can be reduced by three orders of magnitude with respect to the full search approach, while maintaining a good level of accuracy.

Multi-objective design space exploration of embedded systems

Most modern microprocessors employ one or two levels of on-chip caches in order to improve performance. Caches typically are implemented with static RAM cells and often occupy a large portion of the chip area. Not surprisingly, these caches can consume a significant amount of power. In many applications, such as portable devices, energy efficiency is more important than performance. We propose sacrificing some performance in exchange for energy efficiency by filtering cache references through an unusually small first level cache. We refer to this structure as the filter cache. A second level cache, similar in size and structure to a conventional first level cache, is positioned behind the filter cache and serves to mitigate the performance loss. Extensive experiments indicate that a small filter cache still can achieve a high hit rate and good performance. This approach allows the second level cache to be in a low power mode most of the time, thus resulting in power savings. The filter cache is particularly attractive in low power applications, such as the embedded processors used for communication and multimedia applications. For example, experimental results across a wide range of embedded applications show that a direct mapped 255-byte filter cache achieves a 58 percent power reduction while reducing performance by 21 percent. This trade-off results in a 51 percent reduction in the energy-delay product when compared to a conventional design.

M. Gupta

Papers

Filtering memory references to increase energy efficiency