Showing papers by "Miguel Castro published in 1997"

Structure and properties of cobalt clusters up to the tetramer: A density-functional study

Abstract: All-electron calculations have been performed on cobalt clusters up to the tetramer using the local and generalized gradient correction spin density formalisms. These clusters present an abundance of low-lying states. In the case of Co{sub 4}, we have found that two and three-dimensional structures are energetically very close. Symmetric structures exhibit high degeneracy. Jahn-Teller deformations have hence been investigated and have been found to play an important role in such transition-metal clusters. General trends in the magnetic moment per atom and in the attachment energy have been elucidated. {copyright} {ital 1997} {ital The American Physical Society}

HAC: hybrid adaptive caching for distributed storage systems

Abstract: This paper presents HAC, a novel technique for managing the client cache in a distributed, persistent object storage system. HAC is a hybrid between page and object caching that combines the virtues of both while avoiding their disadvantages. It achieves the low miss penalties of a page-caching system, but is able to perform well even when locality is poor, since it can discard pages while retaining their hot objects. It realizes the potentially lower miss rates of object-caching systems, yet avoids their problems of fragmentation and high overheads. Furthermore, HAC is adaptive: when locality is good it behaves like a page-caching system, while if locality is poor it behaves like an object-caching system. It is able to adjust the amount of cache space devoted to pages dynamically so that space in the cache can be used in the way that best matches the needs of the application. The paper also presents results of experiments that indicate that HAC outperforms other object storage systems across a wide range of cache sizes and workloads; it performs substantially better on the expected workloads, which have low to moderate locality. Thus we show that our hybrid, adaptive approach is the cache management technique of choice for distributed, persistent object systems.

Fragment reconstruction: providing global cache coherence in a transactional storage system

Abstract: Cooperative caching is a promising technique to avoid the increasingly formidable disk bottleneck problem in distributed storage systems; it reduces the number of disk accesses by servicing client cache misses from the caches of other clients. However, existing cooperative caching techniques do not provide adequate support for fine grained sharing. We describe a new storage system architecture, split caching, and a new cache coherence protocol, fragment reconstruction, that combine cooperative caching with efficient support for fine grained sharing and transactions. We also present the results of performance studies that show that our scheme introduces little overhead over the basic cooperative caching mechanism and provides better performance when there is fine grained sharing.

Theoretical calculation of carbon clusters

Abstract: Structural parameters and energy have been calculated for C1-3 and C9 clusters using density functional theory through the deMon program. The C1-3 clusters were fully optimized using deMon; different basis sets were tested in order to choose the suitable one to be used in the C9 clusters. In the case of C2 the results were compared with experimental values. DZVP2 basis was selected because it always gave the closest value to the experimental data. The C9 carbon clusters were designed in relation to C(100) diamond surface. The energies of the nonrelaxed and relaxed surfaces with and without hydrogen were calculated. In the same way CH3-relaxed and CH2-relaxed species were calculated. The diamond growth mechanism proposed in the literature was evaluated by an energy analysis. The C(SINGLE BOND)CH3 distance is reported. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 867–875, 1997

Lazy Reference Counting for Transactional Storage Systems

Abstract: Hac is a novel technique for managing the client cache in a distributed, persistent object storage system In a companion paper, we showed that it outperforms other techniques across a wide range of cache sizes and workloads This report describes Hac’s solution to a specifi c problem: how to discard indirection table entries in an indirect pointer swizzling scheme Hac uses lazy reference counting to solve this problem Instead of eagerly updating reference counts when objects are modifi ed and eagerly freeing unreferenced entries, which can be expensive, we perform these operations lazily in the background while waiting for replies to fetch and commit requests Furthermore, we introduce a number of additional optimizations to reduce the space and time overheads of maintaining reference counts The net eff ect is that the overhead of lazy reference counting is low