Efficient techniques to provide scalability for token-based cache coherence protocols

Cache coherence protocols based on tokens can provide low latency without relying on non-scalable interconnects thanks to the use of efficient requests that are unordered. However, when these unordered requests contend for the same memory block, they may cause protocols races. To resolve the races and ensure the completion of all the cache misses, token protocols use a starvation prevention mechanism that is inefficient and non-scalable in terms of required storage structures and generated traffic. Besides, token protocols use non-silent invalidations which increase the latency of write misses proportionally to the system size. All these problems make token protocols non-scalable. To overcome the main problems of token protocols and increase their scalability, we propose a new starvation prevention mechanism named Priority Requests. This mechanism resolves contention by an efficient, elegant, and flexible method based on ordered requests. Furthermore, thanks to Priority Requests, efficient techniques can be applied to limit the storage requirements of the starvation prevention mechanism, to reduce the total traffic generated for managing protocol races, and to reduce the latency of write misses. Thus, the main problems of token protocols can be solved, which, in turn, contributes to wide their efficiency and scalability.Cuesta Sáez, BA. (2009). Efficient techniques to provide scalability for token-based cache coherence protocols [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/6024Palanci

Increasing the effectiveness of directory caches by avoiding the tracking of noncoherent memory blocks

© 2013 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.A key aspect in the design of efficient multiprocessor systems is the cache coherence protocol. Although directory-based protocols constitute the most scalable approach, the limited size of the directory caches together with the growing size of systems may cause frequent evictions and, consequently, the invalidation of cached blocks, which jeopardizes system performance. Directory caches keep track of every memory block stored in processor caches in order to provide coherent access to the shared memory. However, a significant fraction of the cached memory blocks do not require coherence maintenance (even in parallel applications) because they are either accessed by just one processor or they are never modified. In this paper, we propose to deactivate the coherence protocol for those blocks that do not require coherence. This deactivation means directory caches do not have to keep track of noncoherent blocks, which reduces directory cache occupancy and increases its effectiveness. Since the detection of noncoherent blocks is carried out by the operating system, our proposal only requires minor hardware modifications. Simulation results show that, thanks to our proposal, directory caches can avoid the tracking of about 66 percent (on average) of the blocks accessed by a wide range of applications, thereby improving the efficiency of directory caches. This contributes either to shortening the runtime of parallel applications by 15 percent (on average) while keeping directory cache size or to maintaining performance while using directory caches 16 times smaller.This work was supported by the Spanish MICINN, Consolider Programme and Plan E funds, as well as European Commission FEDER funds, under Grants CSD2006-00046 and TIN2009-14475-C04-01. It was also partly supported by (PROMETEO from Generalitat Valenciana (GVA) under Grant ROMETEO/2008/060). B. Cuesta was with Universitat Politecnica de Valencia while working on this paper.Cuesta Sáez, BA.; Ros Bardisa, A.; Gómez Requena, ME.; Robles Martínez, A.; Duato Marín, JF. (2013). Increasing the effectiveness of directory caches by avoiding the tracking of noncoherent memory blocks. IEEE Transactions on Computers. 62(3):482-495. https://doi.org/10.1109/TC.2011.241S48249562

Temporal-Aware Mechanism to Detect Private Data in Chip Multiprocessors

© 2013 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Most of the data referenced by sequential and parallel applications running in current chip multiprocessors are referenced by only one thread and can be considered as private data. A lot of recent proposals leverage this observation to improve many aspects of chip multiprocessors, such as reducing coherence overhead or the access latency to distributed caches. The effectiveness of those proposals depend to a large extent on the amount of detected private data. However, the mechanisms proposed so far do not consider thread migration and the private use of data within different application phases. As a result, a considerable amount of data is not detected as private. In order to make this detection more accurate and reaching more significant improvements, we propose a mechanism that is able to account for both thread migration and private data within application phases. Simulation results for 16-core systems show that, thanks to our mechanism, the average number of pages detected as private significantly increases from 43% in previous proposals up to 74% in ours. Finally, when our detection mechanism is used to deactivate the coherence for private data in a directory protocol, our proposal improves execution time by 13% with respect to previous proposals.This work was supported by the Spanish MINECO, as well as European Commission FEDER funds, under grant TIN2012-38341-C04-01/03 and by the VIRTICAL project (grant agreement no 288574) which is funded by the European Commission within the Research Programme FP7.Ros Bardisa, A.; Cuesta Sáez, BA.; Gómez Requena, ME.; Robles Martínez, A.; Duato Marín, JF. (2013). Temporal-Aware Mechanism to Detect Private Data in Chip Multiprocessors. En Proceedings of the International Conference on Parallel Processing. IEEE. 562-571. https://doi.org/10.1109/ICPP.2013.70S56257

Extending magny-cours cache coherence

One cost-effective way to meet the increasing demand for larger high-performance shared-memory servers is to build clusters with off-the-shelf processors connected with low-latency point-to-point interconnections like HyperTransport. Unfortunately, HyperTransport addressing limitations prevent building systems with more than eight nodes. While the recent High-Node Count HyperTransport specification overcomes this limitation, recently launched twelve-core Magny-Cours processors have already inherited it and provide only 3 bits to encode the pointers used by the directory cache which they include to increase the scalability of their coherence protocol. In this work, we propose and develop an external device to extend the coherence domain of Magny-Cours processors beyond the 8-node limit while maintaining the advantages provided by the directory cache. Evaluation results for systems with up to 32 nodes show that the performance offered by our solution scales with the number of nodes, enhancing the directory cache effectiveness by filtering additional messages. Particularly, we reduce execution time by 47 percent in a 32-die system with respect to the 8-die Magny-Cours configuration.This work was supported by the Spanish MICINN, Consolider Programme and Plan E funds, as well as European Commission FEDER funds, under Grants CSD2006-00046 and TIN2009-14475-C04-01/03. It was also partly supported by (PROMETEO from Generalitat Valenciana (GVA) under Grant PROMETEO/2008/060).Ros Bardisa, A.; Cuesta Sáez, BA.; Fernández-Pascual, R.; Gómez Requena, ME.; Acacio Sánchez, ME.; Robles Martínez, A.; García Carrasco, JM.... (2012). Extending magny-cours cache coherence. IEEE Transactions on Computers. 61(5):593-606. https://doi.org/10.1109/TC.2011.65S59360661

Evolución climática y ambiental del Parque Nacional de Picos de Europa desde el último máximo glaciar

17 páginas, 11 figuras.-- Ramírez, L. & Asensio, B. (eds.).[EN]: A multi-proxy study of the Enol Lake sequence (Cantabrian Mountains, Northern Spain) together with detailed geomorphological mapping provides a first record of glacier evolution and climate change over the last 40,000 years in the Picos de Europa National Park. The Enol glacial retreated from its maximum extent prior to 40 kyr BP thus supporting previous evidences that the maximum extent of southern Europe glaciers occurs earlier than in northern Europe mountains. A slight improvement in climate conditions is detected at 26 kyrs ago with the change from proglacial sediments to glaciolacustrine sedimentation and a final step at 18 kyrs ago with the onset of organic-rich sediments. Last deglaciation occurred in two phases ending with a cold and dry episode (13,500-11,600 cal yrs BP) embedding the Younger Dryas event. The Holocene is divided in three intervals: a humid and warmer period (11,600-8700 cal yrs BP); a tendency towards drier climate during the Middle Holocene (8700- 4650 cal yrs BP) and a final recovery of humidity when the landscape is already modified by human activity (pastoral activities, deforestation) (4650-2200 cal yrs BP). Additionally, the impact of the end of the Little Ice Age (LIA) and 20th century warming and the varying influence of anthropogenic activities is evidenced by the integration of sedimentary, geochemical, pollen and diatom data in a 210Pb chronological framework. Multidisciplinary studies focused on exceptional lacustrine records, such as the Lago Enol sequence presented here, allows discerning between the climatic and the anthropogenic influences on the evolution of both the lake system and the catchment area, including the vegetation cover. Thus, since the current state of the environment is the result of those influences, this type of studies is very useful for implementing new policies of conservation within the National Park.[ES]: El análisis de múltiples indicadores en la secuencia sedimentológica del Lago de Enol (Cordillera Cantábrica, Norte de España), junto con la elaboración de una cartografía geomorfológica de detalle, han proporcionado por primera vez, un registro de la evolución glaciar y del cambio climático durante los últimos 40,000 años en el Parque Nacional de Picos de Europa. El glaciar de Enol comenzó a retirarse de su máxima extensión hace más de 40.000 años, apoyando así evidencias anteriores de que la máxima extensión de los glaciares en el sur de Europa ocurrió antes que en las montañas del norte de Europa. Una ligera mejora de las condiciones climáticas se observa ya hace 26.000 años, cuando Enol deja de tener una sedimentación típica de lago proglaciar y pasa a un ambiente glaciolacustre, aunque no es hasta hace unos 18.000 años cuando empiezan a depositarse sedimentos orgánicos. La última deglaciación ocurrió en dos fases, finalizando con un periodo frío y seco (13.500-11.600 años cal. BP) que incluye el evento global conocido como Dryas Reciente. El Holoceno se ha dividido en tres intervalos, comenzando con un periodo húmedo y templado (11.600-8700 años cal. BP), seguido por una tendencia hacia mayor aridez durante el Holoceno Medio (8700-4650 años cal. BP), y una recuperación final de la humedad cuando el paisaje está ya modificado por la actividad humana debido esencialmente al pastoralismo y la deforestación (4650-2200 años cal BP). Además, el estudio integrado de la sedimentología, geoquímica, polen y asociaciones de diatomeas en dos sondeos cortos del Lago de Enol, ha evidenciado el impacto del final de la Pequeña Edad del Hielo (PEH) y del calentamiento global durante el siglo XX, junto a una influencia variable de las actividades antrópicas. Estudios multidisciplinares focalizados en registros lacustres excepcionales, como la secuencia del Lago de Enol (objeto de estudio en este proyecto), han permitido distinguir entre las influencias climáticas y antrópicas en la evolución pasada tanto del sistema lacustre como de sus alrededores, incluyendo la cobertura vegetal. Así, dado que el estado actual del ecosistema es, en gran parte, el resultado de esas influencias, este tipo de estudios ayudará a implementar nuevas políticas de conservación en el Parque Nacional.Este estudio ha sido financiado por el proyecto “Evolución climática y ambiental del Parque Nacional de Picos de Europa desde el último máximo glaciar - ref: 53/2006” del Organismo Autónomo de Parques Nacionales. Adicionalmente, se ha conseguido financiación a través de los proyectos LIMNOCLIBER (REN2003-09130-C02-02), IBERLIMNO (CGL2005-20236-E/CLI) y LIMNOCAL (CGL2006-13327-C04-01) y del proyecto Consolider GRACCIE (CSD2007-00067) del Ministerio de Ciencia y Tecnología.Peer reviewe