Response Time Bounds for DAG Tasks with Arbitrary Intra-Task Priority Assignment

Most parallel real-time applications can be modeled as directed acyclic graph (DAG) tasks. Intra-task priority assignment can reduce the nondeterminism of runtime behavior of DAG tasks, possibly resulting in a smaller worst-case response time. However, intra-task priority assignment incurs dependencies between different parts of the graph, making it a challenging problem to compute the response time bound. Existing work on intra-task task priority assignment for DAG tasks is subject to the constraint that priority assignment must comply with the topological order of the graph, so that the response time bound can be computed in polynomial time. In this paper, we relax this constraint and propose a new method to compute response time bound of DAG tasks with arbitrary priority assignment. With the benefit of our new method, we present a simple but effective priority assignment policy, leading to smaller response time bounds. Comprehensive evaluation with both single-DAG systems and multi-DAG systems demonstrates that our method outperforms the state-of-the-art method with a considerable margin

Utilization-Based Scheduling of Flexible Mixed-Criticality Real-Time Tasks

Mixed-criticality models are an emerging paradigm for the design of real-time systems because of their significantly improved resource efficiency. However, formal mixed-criticality models have traditionally been characterized by two impractical assumptions: once \textit{any} high-criticality task overruns, \textit{all} low-criticality tasks are suspended and \textit{all other} high-criticality tasks are assumed to exhibit high-criticality behaviors at the same time. In this paper, we propose a more realistic mixed-criticality model, called the flexible mixed-criticality (FMC) model, in which these two issues are addressed in a combined manner. In this new model, only the overrun task itself is assumed to exhibit high-criticality behavior, while other high-criticality tasks remain in the same mode as before. The guaranteed service levels of low-criticality tasks are gracefully degraded with the overruns of high-criticality tasks. We derive a utilization-based technique to analyze the schedulability of this new mixed-criticality model under EDF-VD scheduling. During runtime, the proposed test condition serves an important criterion for dynamic service level tuning, by means of which the maximum available execution budget for low-criticality tasks can be directly determined with minimal overhead while guaranteeing mixed-criticality schedulability. Experiments demonstrate the effectiveness of the FMC scheme compared with state-of-the-art techniques.Comment: This paper has been submitted to IEEE Transaction on Computers (TC) on Sept-09th-201

Multi-Path Bound for DAG Tasks

This paper studies the response time bound of a DAG (directed acyclic graph) task. Recently, the idea of using multiple paths to bound the response time of a DAG task, instead of using a single longest path in previous results, was proposed and leads to the so-called multi-path bound. Multi-path bounds can greatly reduce the response time bound and significantly improve the schedulability of DAG tasks. This paper derives a new multi-path bound and proposes an optimal algorithm to compute this bound. We further present a systematic analysis on the dominance and the sustainability of three existing multi-path bounds and the proposed multi-path bound. Our bound theoretically dominates and empirically outperforms all existing multi-path bounds. What's more, the proposed bound is the only multi-path bound that is proved to be self-sustainable

TICK: Tiny Client for Blockchains

In Bitcoin-like systems, when a payee chooses to accept zero-confirmation transactions, it needs to verify the validity of the transaction. In particular, one of the steps is to verify that each referred output of the transaction is not previously spent. The conventional lightweight client design can only support such operation in the complexity of O($N_T$), where $N_T$ is the total number of transactions in the system. This is impractical for lightweight clients. The latest proposals suggest to summarize all the unspent outputs in an ordered Merkle tree. Therefore, a light client can request proof of presence and/or absence of an element in it to prove whether a referred output is previously spent or not, in the complexity of O(log($N_U$)), where $N_U$ is the total number of unspent output in the system. However, updating such ordered Merkle tree is slow, thus making the system impractical --- by our evaluation, when a new block is generated in Bitcoin, it takes more than one minute to update the ordered Merkle tree. We propose a practical client, TICK, to solve this problem. TICK uses the AVL hash tree to store all the unspent outputs. The AVL hash tree can be update in the time of O(M*log($N_U$)), where $M$ is the number of elements that need to be inserted or removed from the AVL hash tree. By evaluation, when a new block is generated, the AVL hash tree can be updated within $1$ second. Similarly, the proof can also be generated in the time of O(log($N_U$)). Therefore, TICK brings negligible run-time overhead, and thus it is practical. Benefited by the AVL hash tree, a storage-limited device can efficiently and cryptographically verify transactions. In addition, rather than requiring new miners to download the entire blockchain before mining, TICK allows new miners to download only a small portion of data to start mining. We implement TICK for Bitcoin and provide an experimental evaluation on its performance by using the current Bitcoin blockchain data. Our result shows that the proof for verifying whether an output of a transaction is spent or not is only several KB. The verification is very fast -- generating a proof generally takes less than $1$ millisecond, and verifying a proof even takes much less time. In addition, to start mining, new miners only need to download several GB data, rather than downloading over 230 GB data

Average austenite grain size evolution simulation during multi-pass shape metal hot rolling process

783-788Constitutive equations describing the flow stress of Q235 steel undergoing hot plastic deformation have been developed by integrating the microstructure models and exponential flow stress models presented in literatures, in which the microstructure models parameters have been re-determined based on the related experiments carried out on cylindrical specimens of the Q235 steel. Therefore, the integrated constitutive equations are capable of calculating the evolution of austenite grain size and investigating the effects of average austenite grain size on the flow stress of Q235 steel during hot deformation. A three-dimensional thermo-mechanically coupled FEM simulation of an 11-pass H-shape metal roughing rolling process with the austenite grain size evolution taken into account have been carried out to verify the rationality of roll pass schedule. The accuracy of the integrated constitutive equations has been preliminary validated by the comparisons between measured and the calculated values of rolling force

Average austenite grain size evolution simulation during multi-pass shape metal hot rolling process

Constitutive equations describing the flow stress of Q235 steel undergoing hot plastic deformation have been developed by integrating the microstructure models and exponential flow stress models presented in literatures, in which the microstructure models parameters have been re-determined based on the related experiments carried out on cylindrical specimens of the Q235 steel. Therefore, the integrated constitutive equations are capable of calculating the evolution of austenite grain size and investigating the effects of average austenite grain size on the flow stress of Q235 steel during hot deformation. A three-dimensional thermo-mechanically coupled FEM simulation of an 11-pass H-shape metal roughing rolling process with the austenite grain size evolution taken into account have been carried out to verify the rationality of roll pass schedule. The accuracy of the integrated constitutive equations has been preliminary validated by the comparisons between measured and the calculated values of rolling force

Timing-anomaly free dynamic scheduling of conditional DAG tasks on multi-core systems

In this paper, we propose a novel approach to schedule conditional DAG parallel tasks, with which we can derive safe response time upper bounds significantly better than the state-of-the-art counterparts. The main idea is to eliminate the notorious timing anomaly in scheduling parallel tasks by enforcing certain order constraints among the vertices, and thus the response time bound can be accurately predicted off-line by somehow “simulating” the runtime scheduling. A key challenge to apply the timing-anomaly free scheduling approach to conditional DAG parallel tasks is that at runtime it may generate exponentially many instances from a conditional DAG structure. To deal with this problem, we develop effective abstractions, based on which a safe response time upper bound is computed in polynomial time. We also develop algorithms to explore the vertex orders to shorten the response time bound. The effectiveness of the proposed approach is evaluated by experiments with randomly generated DAG tasks with different parameter configurations.This work is supported by the Research Grants Council of Hong Kong (GRF 15204917 and 15213818) and National Natrual Science Foundation of China (Grant No. 61672140), and Nanyang Assistant Professorship (NAP) M4082282 and Start-Up Grant (SUG) M4082087 from Nanyang Technological University, Singapore