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Fast and accurate visual odometry from a monocular camera
Xin YANG, Tangli XUE, Hongcheng LUO, Jiabin GUO
Front. Comput. Sci.
Abstract   PDF (800KB)

This paper aims at a semi-dense visual odometry system that is accurate, robust, and able to run realtime on mobile devices, such as smartphones, AR glasses and small drones. The key contributions of our system include: 1) the modified pyramidal Lucas-Kanade algorithm which incorporates spatial and depth constraints for fast and accurate camera pose estimation; 2) adaptive image resizing based on inertial sensors for greatly accelerating tracking speed with little accuracy degradation; and 3) an ultrafast binary feature description based directly on intensities of a resized and smoothed image patch around each pixel that is sufficiently effective for relocalization. A quantitative evaluation on public datasets demonstrates that our system achieves better tracking accuracy and up to about 2X faster tracking speed comparing to the state-of-the-art monocular SLAM system: LSD-SLAM. For the relocalization task, our system is 2.0X∼4.6X faster than DBoW2 and achieves a similar accuracy.

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A verification framework for spatio-temporal consistency language with CCSL as a specification language
Yuanrui ZHANG, Frédéric MALLET, Yixiang CHEN
Front. Comput. Sci.
Abstract   PDF (899KB)

The Spatio-Temporal Consistency Language (STeC) is a high-level modeling language that deals natively with spatio-temporal behaviour, i.e., behaviour relating to certain locations and time. Such restriction by both locations and time is of first importance for some types of real-time systems. CCSL is a formal specification language based on logical clocks. It is used to describe some crucial safety properties for real-time systems, due to its powerful expressiveness of logical and chronometric time constraints. We consider a novel verification framework combining STeC and CCSL, with the advantages of addressing spatio-temporal consistency of system behaviour and easily expressing some crucial time constraints. We propose a theory combining these two languages and a method verifying CCSL properties in STeC models. We adopt UPPAAL as the model checking tool and give a simple example to illustrate how to carry out verification in our framework.

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A divide & conquer approach to liveness model checking under fairness & anti-fairness assumptions
Kazuhiro OGATA
Front. Comput. Sci.
Abstract   PDF (626KB)

This paper proposes an approach to making liveness model checking problems under fairness feasible. The proposed method divides such a problem into smaller ones that can be conquered. It is not superior to existing tools dedicated to model checking liveness properties under fairness assumptions in terms of model checking performance but has the following positive aspects: 1) the approach can be used to model check liveness properties under anti-fairness assumptions as well as fairness assumptions, 2) the approach can help humans better understand the reason why they need to use fairness and/or anti-fairness assumptions, and 3) the approach makes it possible to use existing linear temporal logic model checkers to model check liveness properties under fairness and/or anti-fairness assumptions.

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Timestamp reassignment: taming transaction abort for serializable snapshot isolation
Ningnan ZHOU, Xiao ZHANG, Shan WANG
Front. Comput. Sci.
Abstract   PDF (732KB)

Serializable snapshot isolation (SSI) is a promising technique to exploit parallelism for multi-core databases. However, SSI suffers from excessive transaction aborts. Existing remedies have three drawbacks: 1) tracking prohibitively transitive dependencies; 2) aborting on every writewrite conflict; and 3) requiring manual annotation on transaction programs.

In this paper, we propose to suppress transaction aborts by reassigning timestamps. We combine static analysis with augmented query plan. In this way, we save both aborts caused by read-write and write-write conflicts, without tracking transitive dependency and annotating transaction programs. As such, our approach does not exhibit drawbacks of existing methods. Extensive experiments demonstrate the effectiveness and practicality of our approach.

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A novel index system describing program runtime characteristics for workload consolidation
Lin WANG, Depei QIAN, Rui WANG, Zhongzhi LUAN, Hailong YANG, Huaxiang ZHANG
Front. Comput. Sci.
Abstract   PDF (778KB)

Workload consolidation is a common method to improve the resource utilization in clusters or data centers. In order to achieve efficient workload consolidation, the runtime characteristics of a program should be taken into consideration in scheduling. In this paper, we propose a novel index system for efficiently describing the program runtime characteristics. With the help of this index system, programs can be classified by the following runtime characteristics: 1) dependence to multi-dimensional resources including CPU, disk I/O, memory and network I/O; and 2) impact and vulnerability to resource sharing embodied by resource usage and resource sensitivity. In order to verify the effectiveness of this novel index system in workload consolidation, a scheduling strategy, Sche-index, using the new index system for workload consolidation is proposed. Experiment results show that compared with traditional least-loaded scheduling strategy, Sche-index can improve both program performance and system resource utilization significantly.

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Fault-tolerant precise data access on distributed log-structured merge-tree
Tao ZHU, Huiqi HU, Weining QIAN, Huan ZHOU, Aoying ZHOU
Front. Comput. Sci.
Abstract   PDF (857KB)

Log-structured merge tree has been adopted by many distributed storage systems. It decomposes a large database into multiple parts: an in-writing part and several read-only ones. Records are firstly written into a memoryoptimized structure and then compacted into in-disk structures periodically. It achieves high write throughput. However, it brings side effect that read requests have to go through multiple structures to find the required record. In a distributed database system, different parts of the LSM-tree are stored in distributed fashion. To this end, a server in the query layer has to issues multiple network communications to pull data items from the underlying storage layer. Coming to its rescue, this work proposes a precise data access strategy which includes: an efficient structure with low maintaining overhead designed to test whether a record exists in the in-writing part of the LSM-tree; a lease-based synchronization strategy proposed to maintain consistent copies of the structure on remote query servers.We further prove the technique is capable of working robustly when the LSM-Tree is re-organizing multiple structures in the backend. It is also fault-tolerant, which is able to recover the structures used in data access after node failures happen. Experiments using the YCSB benchmark show that the solution has 6x throughput improvement over existing methods.

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The time model for event processing in internet of things
Chunjie ZHOU, Xiaoling WANG, Zhiwang ZHANG, Zhenxing ZHANG, Haiping QU
Front. Comput. Sci.
Abstract   PDF (833KB)

The time management model for event processing in internet of things has a special and important requirement. Many events in real world applications are long-lasting events which have different time granularity with order or out-of-order. The temporal relationships among those events are often complex. An important issue of complex event processing is to extract patterns from event streams to support decision making in real-time. However, current time management model does not consider the unified solution about time granularity, time interval, time disorder, and the difference between workday calendar systems in different organizations. In this work, we analyze the preliminaries of temporal semantics of events. A tree-plan model of out-of-order durable events is proposed. A hybrid solution is correspondingly introduced. A case study is illustrated to explain the time constraints and the time optimization. Extensive experimental studies demonstrate the efficiency of our approach.

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