The predictability of data values

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Comprehensive characterization of an open source document search engine

This work performs a thorough characterization and analysis of the open source Lucene search library. The article describes in detail the architecture, functionality, and micro-architectural behavior of the search engine, and investigates prominent online document search research issues. In particular, we study how intra-server index partitioning affects the response time and throughput, explore the potential use of low power servers for document search, and examine the sources of performance degradation ands the causes of tail latencies. Some of our main conclusions are the following: (a) intra-server index partitioning can reduce tail latencies but with diminishing benefits as incoming query traffic increases, (b) low power servers given enough partitioning can provide same average and tail response times as conventional high performance servers, (c) index search is a CPU-intensive cache-friendly application, and (d) C-states are the main culprits for performance degradation in document search.Web of Science162art. no. 1

Performance-effective operation below Vcc-min

Continuous circuit miniaturization and increased process variability point to a future with diminishing returns from dynamic voltage scaling. Operation below Vcc-min has been proposed recently as a mean to reverse this trend. The goal of this paper is to minimize the performance loss due to reduced cache capacity when operating below Vcc-min. A simple method is proposed: disable faulty blocks at low voltage. The method is based on observations regarding the distributions of faults in an array according to probability theory. The key lesson, from the probability analysis, is that as the number of uniformly distributed random faulty cells in an array increases the faults increasingly occur in already faulty blocks. The probability analysis is also shown to be useful for obtaining insight about the reliability implications of other cache techniques. For one configuration used in this paper, block disabling is shown to have on the average 6.6% and up to 29% better performance than a previously proposed scheme for low voltage cache operation. Furthermore, block-disabling is simple and less costly to implement and does not degrade performance at or above Vcc-min operation. Finally, it is shown that a victim-cache enables higher and more deterministic performance for a block-disabled cache

Proposition for a sequential accelerator in future general-purpose manycore processors

The number of transistors that can be put on a given silicon area doubles on every technology generation. Consequently, the number of on-chip cores increases quickly, making it possible to build general-purpose processors with hundreds of cores in a near future. However, though having a large number of cores is beneficial for speeding up parallel code sections, it is also important to speed up sequential execution. We argue that it will be possible and desirable to dedicate a large fraction of the chip area and power to high sequential performance. Current processor design styles are restrained by the implicit constraint that a processor core should be able to run continuously; therefore power hungry techniques that would allow very high clock frequencies are not used. The "sequential accelerator" we propose removes the constraint of continuous functioning. The sequential accelerator consists of several cores designed for ultimate instantaneous performance. Those cores are large and power hungry, they cannot run continuously (thermal constraint) and cannot be active simultaneously (power constraint) . A single core is active at any time, inactive cores are power-gated. The execution is migrated periodically to a new core so as to spread the heat generation uniformly over the whole accelerator area, which solves the temperature issue. The "sequential accelerator" will be a viable solution only if the performance penalty due to migrations can be tolerated. Migration-induced cache misses may incur a significant performance loss. We propose some solutions to alleviate this problem. We also propose a migration method, using integrated thermal sensors, such that the migration interval is variable and depends on the ambient temperature. The migration penalty can be kept negligible as long as the ambient temperature is maintained below a threshold

A Real-Time Error Detection (RTD) architecture and its use for reliability and post-silicon validation for F/F based memory arrays

This work proposes in-situ Real-Time Error Detection (RTD): embedding hardware in a memory array for detecting a fault in the array when it occurs, rather than when it is read. RTD breaks the serialization between data access and error-detection and, thus, it can speed-up the access-time of arrays that use in-line error-correction. The approach can also reduce the time needed to root-cause array related bugs during post-silicon validation and product testing. The paper introduces a two-dimensional error-correction scheme based on RTD and, also, presents a proactive error-correction method that combines RTD with demand-scrubbing. The work describes how to build RTD into a memory array with flip-flops to track in real-time the column-parity. A comparison of the proposed two-dimensional ECC scheme, as compared to single-error-correction-double-error-detection, shows that the RTD design has comparable error-detection-and-correction strength and, depending on the array dimensions and configuration, RTD reduces access time by 4% to 26% at an area and power overhead (negative is a reduction) between -7% to 33% and -42% to 86% respectively.Peer ReviewedPostprint (author's final draft

Proposition for a Sequential Accelerator in Future General-Purpose Manycore Processors and the Problem of Migration-Induced Cache Misses

International audienceAs the number of transistors on a chip doubles with every technology generation, the number of on-chip cores also increases rapidly, making possible in a foreseeable future to design processors featuring hundreds of general-purpose cores. However, though a large number of cores speeds up parallel code sections, Amdahl's law requires speeding up sequential sections too. We argue that it will become possible to dedicate a substantial fraction of the chip area and power budget to achieve high sequential performance. Current general-purpose processors contain a handful of cores designed to be continuously active and run in parallel. This leads to power and thermal constraints that limit the core's performance. We propose removing these constraints with a {\it sequential accelerator} ({\bf SACC}). A SACC consists of several cores {\it designed} for ultimate sequential performance. These cores cannot run continuously. A single core is active at any time, the rest of the cores are inactive and power-gated. We migrate the execution periodically to another core to spread heat generation uniformly over the whole SACC area, thus addressing the temperature issue. The SACC will be viable only if it yields significant sequential performance. Migration-induced cache misses may limit performance gains. We propose some solutions to mitigate this problem. We also investigate a migration method using thermal sensors, such that the migration interval depends on the ambient temperature and the migration penalty is negligible under normal thermal conditions

BlackOut: Enabling fine-grained power gating of buffers in Network-on-Chip routers

The Network-on-Chip (NoC) router buffers play an instrumental role in the performance of both the interconnection fabric and the entire multi-/many-core system. Nevertheless, the buffers also constitute the major leakage power consumers in NoC implementations. Traditionally, they are designed to accommodate worst-case traffic scenarios, so they tend to remain idle, or under-utilized, for extended periods of time. The under-utilization of these valuable resources is exemplified when one profiles real application workloads; the generated traffic is bursty in nature, whereby high traffic periods are sporadic and infrequent, in general. The mitigation of the leakage power consumption of NoC buffers via power gating has been explored in the literature, both at coarse (router-level) and fine (buffer-level) granularities. However, power gating at the router granularity is suitable only for low and medium traffic conditions, where the routers have enough opportunities to be powered down. Under high traffic, the sleeping potential rapidly diminishes. Moreover, disabling an entire router greatly affects the NoC functionality and the network connectivity. This article presents BlackOut, a fine-grained power-gating methodology targeting individual router buffers. The goal is to minimize leakage power consumption, without adversely impacting the system performance. The proposed framework is agnostic of the routing algorithm and the network topology, and it is applicable to any router micro-architecture. Evaluation results obtained using both synthetic traffic patterns and real applications in 64-core systems indicate energy savings of up to 70%, as compared to a baseline NoC, with a near-negligible performance overhead of around 2%. BlackOut is also shown to significantly outperformby 35%, on averagetwo current state-of-the-art power-gating solutions, in terms of energy savings. Not tailored to any topology, routing algorithm and NoC router architecture.Router-to-router communication. No need for custom, region-based/global networks.Effective at low, medium and high traffic. Other solutions are more restrictive.+35% energy saving, on average, against two state-of-the-art power-gating solutions.Negligible performance overhead (+2%) compared to the baseline architecture