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Incipient plasticity is typically associated with thermally activated events like the nucleation of dislocations in crystalline solids and the activation of shear transformation zones in metallic glasses. A widely employed method of estimating the activation parameters of such mechanisms involves analyzing the statistical distribution of critical loads obtained through a series of repeated measurements. However, the conventional mathematical approach assumes the activation parameters to remain fixed during the sequence of measurements. The present study critically examines this premise and presents a generalized statistical model that allows the statistical variations of activation parameters. Using a simple Monte Carlo scheme, it is demonstrated that even small fluctuations of activation parameters can significantly affect the statistical distribution of measured critical loads. The Monte Carlo calculations, along with atomistic simulations, further show that imposing the assumption of rigidly fixed parameters on an activated process with parametric fluctuations can lead to severe underestimation of the activation parameters. As many experimental studies have consistently reported perplexingly small activation volumes estimated using the conventional statistical approach, we propose that our findings can offer a fresh perspective on this longstanding issue., Comment: 7 figures, 2 tables

Popular blockchains such as Ethereum and several others execute complex transactions in blocks through user-defined scripts known as smart contracts. Serial execution of smart contract transactions/atomic-units (AUs) fails to harness the multiprocessing power offered by the prevalence of multi-core processors. By adding concurrency to the execution of AUs, we can achieve better efficiency and higher throughput. In this paper, we develop a concurrent miner that proposes a block by executing the AUs concurrently using optimistic Software Transactional Memory systems (STMs). It captures the independent AUs in a concurrent bin and dependent AUs in the block graph (BG) efficiently. Later, we propose a concurrent validator that re-executes the same AUs concurrently and deterministically using a concurrent bin followed by a BG given by the miner to verify the proposed block. We rigorously prove the correctness of concurrent execution of AUs and achieve significant performance gain over the state-of-the-art., Comment: 43 pages, 13 figure, 1 Table

We investigate the minimal number of failures that can partition a system where processes communicate both through shared memory and by message passing. We prove that this number precisely captures the resilience that can be achieved by algorithms that implement a variety of shared objects, like registers and atomic snapshots, and solve common tasks, like randomized consensus, approximate agreement and renaming. This has implications for the m&m-model and for the hybrid, cluster-based model., Comment: 22 pages, 3 figures

Deformation twinning, which occurs in fcc metals only under particular conditions of intrinsic material properties, microstructure, and loading conditions, occupies an indispensable place in their deformation mechanism maps. Nonetheless, dedicated studies have seldom been carried out to explore the fundamental properties of twinning dislocations mediating this process. Here we employ a combination of atomistic computations and continuum modeling to investigate the nucleation of twinning dislocation loops in metals with fcc crystal structure. Besides the conventional layer-by-layer model of twin nucleation, the newly proposed alternate-shear mechanism has been investigated, and the energy barrier and shear stress for twinning loop nucleation have been determined. We find the nucleation stress in the latter mechanism to be smaller than that in the former. The study also highlights the non-uniform variations of the critical loop size and fractional Burgers vector with the applied shear load., Comment: 5 figures

We present an algorithm for implementing a store-collect object in an asynchronous crash-prone message-passing dynamic system, where nodes continually enter and leave. The algorithm is very simple and efficient, requiring just one round trip for a store operation and two for a collect. We then show the versatility of the store-collect object for implementing churn-tolerant versions of useful data structures, while shielding the user from the complications of the underlying churn. In particular, we present elegant and efficient implementations of atomic snapshot and generalized lattice agreement objects that use store-collect., Comment: 30 pages

In the modern era of multi-core systems, the main aim is to utilize the cores properly. This utilization can be done by concurrent programming. But developing a flawless and well-organized concurrent program is difficult. Software Transactional Memory Systems (STMs) are a convenient programming interface which assist the programmer to access the shared memory concurrently without worrying about consistency issues such as priority-inversion, deadlock, livelock, etc. Another important feature that STMs facilitate is compositionality of concurrent programs with great ease. It composes different concurrent operations in a single atomic unit by encapsulating them in a transaction. Many STMs available in the literature execute read/write primitive operations on memory buffers. We represent them as Read-Write STMs or RWSTMs. Whereas, there exist some STMs (transactional boosting and its variants) which work on higher level operations such as insert, delete, lookup, etc. on a hash-table. We refer these STMs as Object Based STMs or OSTMs. The literature of databases and RWSTMs say that maintaining multiple versions ensures greater concurrency. This motivates us to maintain multiple version at higher level with object semantics and achieves greater concurrency. So, this paper pro-poses the notion of Optimized Multi-version Object Based STMs or OPT-MVOSTMs which encapsulates the idea of multiple versions in OSTMs to harness the greater concurrency efficiently., Comment: 45 pages

To utilize the multi-core processors properly concurrent programming is needed. Concurrency control is the main challenge while designing a correct and efficient concurrent program. Software Transactional Memory Systems (STMs) provides ease of multithreading to the programmer without worrying about concurrency issues such as deadlock, livelock, priority inversion, etc. Most of the STMs works on read-write operations known as RWSTMs. Some STMs work at high-level operations and ensure greater concurrency than RWSTMs. Such STMs are known as Object-Based STMs (OSTMs). The transactions of OSTMs can return commit or abort. Aborted OSTMs transactions retry. But in the current setting of OSTMs, transactions may starve. So, we proposed a Starvation-Free OSTM (SF-OSTM) which ensures starvation-freedom in object based STM systems while satisfying the correctness criteria as co-opacity. Databases, RWSTMs and OSTMs say that maintaining multiple versions corresponding to each key of transaction reduces the number of aborts and improves the throughput. So, to achieve greater concurrency, we proposed Starvation-Free Multi-Version OSTM (SF-MVOSTM) which ensures starvation-freedom while storing multiple versions corresponding to each key and satisfies the correctness criteria such as local opacity. To show the performance benefits, We implemented three variants of SF-MVOSTM (SF-MVOSTM, SF-MVOSTM-GC and SF-KOSTM) and compared it with state-of-the-art STMs., Comment: 68 pages, 24 figures. arXiv admin note: text overlap with arXiv:1709.01033