Conservative parallel discrete event simulation takes a connection-oriented approach to inter-process communication. Conservative processes modeling physical entities are connected via channels that represent physical links in the target system. By contrast, optimistic strategy and its implementation in time warp machine, take a connectionless approach where a process can communicate freely with any other process without prior connection. This paper presents the case for a connection-oriented optimistic approach and provides a data structure for the event set of time warp processes that significantly reduce the cost of event scheduling and cancellation under connection-oriented operation. Detailed description of the proposed implementation is given and performance of the structure in forward event scheduling and backward event cancellation is tested and the results are discussed.
{"title":"Time warp - connection oriented","authors":"Nasser Kalantery","doi":"10.1145/1013329.1013342","DOIUrl":"https://doi.org/10.1145/1013329.1013342","url":null,"abstract":"Conservative parallel discrete event simulation takes a connection-oriented approach to inter-process communication. Conservative processes modeling physical entities are connected via channels that represent physical links in the target system. By contrast, optimistic strategy and its implementation in time warp machine, take a connectionless approach where a process can communicate freely with any other process without prior connection. This paper presents the case for a connection-oriented optimistic approach and provides a data structure for the event set of time warp processes that significantly reduce the cost of event scheduling and cancellation under connection-oriented operation. Detailed description of the proposed implementation is given and performance of the structure in forward event scheduling and backward event cancellation is tested and the results are discussed.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126165422","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
An important question for network simulation is what level of detail is required to obtain a desired level of accuracy. While in some networks, the level of detail is an open research issue (for example, radio propagation models in wireless networks), it has long been assumed that wired networks could be accurately modeled by fairly simple queues with a bandwidth limit and propagation delay. To our knowledge this assumption has not been widely tested. In this paper we evaluate different levels of detail for an Ethernet simulation. We consider two models for Ethernet simulation; a detailed, CSMA/CD protocol based model and a more abstract model using a DropTail, shared queue. Using Web traffic with two different TCP simulation models, we evaluated the accuracy of these Ethernet models as compared to testbed measurements. We observed the DropTail Ethernet model requires significantly less execution time and can accurately model performance using a bandwidth normalization factor.
{"title":"The effect of detail on Ethernet simulation","authors":"Alefiya Hussain, Aman Kapoor, J. Heidemann","doi":"10.1145/1013329.1013346","DOIUrl":"https://doi.org/10.1145/1013329.1013346","url":null,"abstract":"An important question for network simulation is what level of detail is required to obtain a desired level of accuracy. While in some networks, the level of detail is an open research issue (for example, radio propagation models in wireless networks), it has long been assumed that wired networks could be accurately modeled by fairly simple queues with a bandwidth limit and propagation delay. To our knowledge this assumption has not been widely tested. In this paper we evaluate different levels of detail for an Ethernet simulation. We consider two models for Ethernet simulation; a detailed, CSMA/CD protocol based model and a more abstract model using a DropTail, shared queue. Using Web traffic with two different TCP simulation models, we evaluated the accuracy of these Ethernet models as compared to testbed measurements. We observed the DropTail Ethernet model requires significantly less execution time and can accurately model performance using a bandwidth normalization factor.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"32 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114962060","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We investigated the benefit of exploiting the symmetries of graphs for partitioning. We represent the model to be simulated by a weighted graph. Graph symmetries are studied in the theory of permutation groups and can be calculated in polynomial time with the nauty algorithm by B. McKay (1981). We designed an algorithm to extract useful symmetries from the automorphism group, which can be used to create partitions derived from the graph's structure. Our approach is focused on composite graphs, for which identical subgraphs reoccur in the graph. If these identical subgraphs can be mapped onto each other by symmetries, the subgraphs are replaced by equivalent multivertices, resulting in a 'natural' aggregation of vertices. This approach is applied to parallel simulation of a detailed IP-switch with a conservative synchronous algorithm. The experimental results show that even for good partitions, global and temporal load imbalances are inevitable.
{"title":"Exploiting symmetry for partitioning models in parallel discrete event simulation","authors":"J. Lemeire, B. Smets, P. Cara, E. Dirkx","doi":"10.1145/1013329.1013360","DOIUrl":"https://doi.org/10.1145/1013329.1013360","url":null,"abstract":"We investigated the benefit of exploiting the symmetries of graphs for partitioning. We represent the model to be simulated by a weighted graph. Graph symmetries are studied in the theory of permutation groups and can be calculated in polynomial time with the nauty algorithm by B. McKay (1981). We designed an algorithm to extract useful symmetries from the automorphism group, which can be used to create partitions derived from the graph's structure. Our approach is focused on composite graphs, for which identical subgraphs reoccur in the graph. If these identical subgraphs can be mapped onto each other by symmetries, the subgraphs are replaced by equivalent multivertices, resulting in a 'natural' aggregation of vertices. This approach is applied to parallel simulation of a detailed IP-switch with a conservative synchronous algorithm. The experimental results show that even for good partitions, global and temporal load imbalances are inevitable.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"52 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"122100443","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
An efficient cancellation scheme is essential to the performance of time warp simulations. The pitfalls of rollback echoes, chasing hazards and cascading rollbacks can be identified as being attributable to the inefficiency of the conventional per-event based cancellation scheme. Instead of capturing the happen-before relation between events, which is used by the range based cancellation scheme, the batch based cancellation scheme proposed in this paper utilizes a modified paradigm of vector time, namely, state vector, to capture the dependence of events. We prove that with conformance to specific rules regulating the advancement of LPs (logical processes), the events to be cancelled by a straggler message can be determined using a range of the state vector. Thus, knowledge of the range enables any LP to recover from the receipt of a straggler message at the cost of at most one rollback (i.e., rollback optimal). The results of preliminary experiments conducted using a manufacturing model show that the proposed scheme is successful in reducing the number of antimessages and increasing the ratio of the number of committed events to the number of processed events.
{"title":"Batch based cancellation: a rollback optimal cancellation scheme in time warp simulations","authors":"Yi Zeng, Wentong Cai, S. Turner","doi":"10.1145/1013329.1013343","DOIUrl":"https://doi.org/10.1145/1013329.1013343","url":null,"abstract":"An efficient cancellation scheme is essential to the performance of time warp simulations. The pitfalls of rollback echoes, chasing hazards and cascading rollbacks can be identified as being attributable to the inefficiency of the conventional per-event based cancellation scheme. Instead of capturing the happen-before relation between events, which is used by the range based cancellation scheme, the batch based cancellation scheme proposed in this paper utilizes a modified paradigm of vector time, namely, state vector, to capture the dependence of events. We prove that with conformance to specific rules regulating the advancement of LPs (logical processes), the events to be cancelled by a straggler message can be determined using a range of the state vector. Thus, knowledge of the range enables any LP to recover from the receipt of a straggler message at the cost of at most one rollback (i.e., rollback optimal). The results of preliminary experiments conducted using a manufacturing model show that the proposed scheme is successful in reducing the number of antimessages and increasing the ratio of the number of committed events to the number of processed events.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"66 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129858749","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jason Liu, Yougu Yuan, D. Nicol, R. Gray, Calvin C. Newport, D. Kotz, L. F. Perrone
Computer simulation is the most common approach to studying wireless ad-hoc routing algorithms. The results, however, are only as good as the models the simulation uses. One should not underestimate the importance of validation, as inaccurate models can lead to wrong conclusions. In this paper, we use direct-execution simulation to validate radio models used by ad-hoc routing protocols, against real-world experiments. This paper documents a common testbed that supports direct execution of a set of ad-hoc routing protocol implementations in a wireless network simulator. The testbed reads traces generated from real experiments, and uses them to drive direct-execution implementations of the routing protocols. Doing so we reproduce the same network conditions as in real experiments. By comparing routing behavior measured in real experiments with behavior computed by the simulation, we are able to validate the models of radio behavior upon which protocol behavior depends. We conclude that it is possible to have fairly accurate results using a simple radio model, but the routing behavior is quite sensitive to one of this model's parameters. The implication is that one should: i) use a more complex radio model that explicitly models point-to-point path loss; or ii) use measurements from an environment typical of the one of interest; or iii) study behavior over a range of environments to identify sensitivities.
{"title":"Simulation validation using direct execution of wireless ad-hoc routing protocols","authors":"Jason Liu, Yougu Yuan, D. Nicol, R. Gray, Calvin C. Newport, D. Kotz, L. F. Perrone","doi":"10.1145/1013329.1013333","DOIUrl":"https://doi.org/10.1145/1013329.1013333","url":null,"abstract":"Computer simulation is the most common approach to studying wireless ad-hoc routing algorithms. The results, however, are only as good as the models the simulation uses. One should not underestimate the importance of validation, as inaccurate models can lead to wrong conclusions. In this paper, we use direct-execution simulation to validate radio models used by ad-hoc routing protocols, against real-world experiments. This paper documents a common testbed that supports direct execution of a set of ad-hoc routing protocol implementations in a wireless network simulator. The testbed reads traces generated from real experiments, and uses them to drive direct-execution implementations of the routing protocols. Doing so we reproduce the same network conditions as in real experiments. By comparing routing behavior measured in real experiments with behavior computed by the simulation, we are able to validate the models of radio behavior upon which protocol behavior depends. We conclude that it is possible to have fairly accurate results using a simple radio model, but the routing behavior is quite sensitive to one of this model's parameters. The implication is that one should: i) use a more complex radio model that explicitly models point-to-point path loss; or ii) use measurements from an environment typical of the one of interest; or iii) study behavior over a range of environments to identify sensitivities.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"9 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133689945","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Suiping Zhou, S. Turner, Wentong Cai, Hanfeng Zhao, Xiaolin Pang
In distributed wargame simulations, the states of simulation entities need to be updated and sent to other simulation nodes in a timely manner, which may incur high overhead on the network. Thus, how to efficiently use the limited bandwidth of the network is very important to the performance of distributed wargame simulations. In this paper, a utility model is proposed to evaluate the relative importance of a simulation entity. In our proposed utility model, the utility of an entity is determined by the number of entities on which this entity may have influence and the distances between this entity and the entities within its area of influence. Based on the utility model, we may devise some flexible updating mechanisms which can utilize the bandwidth more efficiently. For example, the state updates of an important entity which may have great influence on the simulation could be sent to other nodes with a high frequency; on the other hand, the state updates of an entity with little importance may be sent to other simulation nodes with a low frequency. In this paper, we focus on the utility computation of a simulation entity rather than the flexible state updating mechanisms for the simulation entities. Our proposed utility computation mechanism is general and can be easily implemented using the HLA services. Experimental results show that the utility computation mechanism incurs little overhead.
{"title":"A utility model for timely state update in distributed wargame simulations","authors":"Suiping Zhou, S. Turner, Wentong Cai, Hanfeng Zhao, Xiaolin Pang","doi":"10.1145/1013329.1013347","DOIUrl":"https://doi.org/10.1145/1013329.1013347","url":null,"abstract":"In distributed wargame simulations, the states of simulation entities need to be updated and sent to other simulation nodes in a timely manner, which may incur high overhead on the network. Thus, how to efficiently use the limited bandwidth of the network is very important to the performance of distributed wargame simulations. In this paper, a utility model is proposed to evaluate the relative importance of a simulation entity. In our proposed utility model, the utility of an entity is determined by the number of entities on which this entity may have influence and the distances between this entity and the entities within its area of influence. Based on the utility model, we may devise some flexible updating mechanisms which can utilize the bandwidth more efficiently. For example, the state updates of an important entity which may have great influence on the simulation could be sent to other nodes with a high frequency; on the other hand, the state updates of an entity with little importance may be sent to other simulation nodes with a low frequency. In this paper, we focus on the utility computation of a simulation entity rather than the flexible state updating mechanisms for the simulation entities. Our proposed utility computation mechanism is general and can be easily implemented using the HLA services. Experimental results show that the utility computation mechanism incurs little overhead.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"143 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124560294","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
If a model shall be executed in a parallel, distributed instead of a sequential manner, typically the entire simulation engine has to be exchanged. To adapt the simulation layer more easily to the requirements of a concrete model to be run in a specific environment a component based simulation layer has been developed for JAMES. A set of different simulator components demonstrates that a component-based design facilitates the exchange of simulators and their combination.
{"title":"A component-based simulation layer for JAMES","authors":"J. Himmelspach, A. Uhrmacher","doi":"10.1145/1013329.1013349","DOIUrl":"https://doi.org/10.1145/1013329.1013349","url":null,"abstract":"If a model shall be executed in a parallel, distributed instead of a sequential manner, typically the entire simulation engine has to be exchanged. To adapt the simulation layer more easily to the requirements of a concrete model to be run in a specific environment a component based simulation layer has been developed for JAMES. A set of different simulator components demonstrates that a component-based design facilitates the exchange of simulators and their combination.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"120 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123758882","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Designing fast parallel discrete event simulation systems for shared-memory parallel computers is simplified by the efficient communication operations enabled by the common memory space. The difficulties involved in designing large shared-memory computers and the resulting high cost of even modest size systems has led to the proliferation of computer systems consisting of small shared-memory computers connected via low-latency message-passing interconnection networks. This paper describes how a network simulation system using a simulation kernel optimized for high performance operation on shared-memory parallel computers has been extended to operate on computers that mix shared-memory and message-passing paradigms. Results are presented showing that the system can achieve over 60 million simulated packet transmissions per second on 32 4-processor nodes. The results demonstrate the advantage of using a mixture of shared-memory and message-passing over using only message-passing in many cases.
{"title":"Performance of a mixed shared/distributed memory parallel network simulator","authors":"C. Kiddle, R. Simmonds, B. Unger","doi":"10.1145/1013329.1013334","DOIUrl":"https://doi.org/10.1145/1013329.1013334","url":null,"abstract":"Designing fast parallel discrete event simulation systems for shared-memory parallel computers is simplified by the efficient communication operations enabled by the common memory space. The difficulties involved in designing large shared-memory computers and the resulting high cost of even modest size systems has led to the proliferation of computer systems consisting of small shared-memory computers connected via low-latency message-passing interconnection networks. This paper describes how a network simulation system using a simulation kernel optimized for high performance operation on shared-memory parallel computers has been extended to operate on computers that mix shared-memory and message-passing paradigms. Results are presented showing that the system can achieve over 60 million simulated packet transmissions per second on 32 4-processor nodes. The results demonstrate the advantage of using a mixture of shared-memory and message-passing over using only message-passing in many cases.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"10 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130758744","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Yeung, M. Takai, R. Bagrodia, A. Mehrnia, B. Daneshrad
In mobile ad hoc network (MANET) studies, it is imperative to use highly detailed device models as they provide high layer protocols with good prediction of underlying wireless communication performance. However, such studies often utilize abstract models for execution speed and simplicity. This paper first shows that physical layer variables including path loss, shadowing, multipath, Doppler have significant effects on the predicted overall networking performance. It then proposes an approach to simulate details of wireless propagation and radio characteristics in networking studies while still maintaining a reasonable simulation execution time. Through our runtime performance studies with detailed OFDM Simulink/MATLAB models and QualNet network simulator, it is shown that the proposed approach can improve the simulation runtime performance by three to four orders of magnitudes without compromising the fidelity of simulation results.
{"title":"Detailed OFDM modeling in network simulation of mobile ad hoc networks","authors":"G. Yeung, M. Takai, R. Bagrodia, A. Mehrnia, B. Daneshrad","doi":"10.1145/1013329.1013335","DOIUrl":"https://doi.org/10.1145/1013329.1013335","url":null,"abstract":"In mobile ad hoc network (MANET) studies, it is imperative to use highly detailed device models as they provide high layer protocols with good prediction of underlying wireless communication performance. However, such studies often utilize abstract models for execution speed and simplicity. This paper first shows that physical layer variables including path loss, shadowing, multipath, Doppler have significant effects on the predicted overall networking performance. It then proposes an approach to simulate details of wireless propagation and radio characteristics in networking studies while still maintaining a reasonable simulation execution time. Through our runtime performance studies with detailed OFDM Simulink/MATLAB models and QualNet network simulator, it is shown that the proposed approach can improve the simulation runtime performance by three to four orders of magnitudes without compromising the fidelity of simulation results.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"65 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114651790","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Resources in large logistic networks are occasionally unavailable or malfunctioning. This implies that performability becomes an issue for quantitative analysis of logistic networks. Different time scales between failures and normal operation often justify the decomposition of a performability model into a single availability model that considers failures and recovery of resources and a family of performance models whose individual instances depend on the state of resources. In this paper, we present an approach that simulates a set of performance models independently and in a distributed manner on a network of workstations. We propose to optimize the achievable quality of results for a given total amount of CPU time by minimizing the confidence intervals for performability measures. This is possible by an adaptive assignment of CPU time to simulate those models whose results have the largest impact on the width of confidence intervals.
{"title":"Hybrid performability analysis of logistic networks","authors":"Falko Bause, P. Buchholz, M. Fischer, P. Kemper","doi":"10.1145/1013329.1013351","DOIUrl":"https://doi.org/10.1145/1013329.1013351","url":null,"abstract":"Resources in large logistic networks are occasionally unavailable or malfunctioning. This implies that performability becomes an issue for quantitative analysis of logistic networks. Different time scales between failures and normal operation often justify the decomposition of a performability model into a single availability model that considers failures and recovery of resources and a family of performance models whose individual instances depend on the state of resources. In this paper, we present an approach that simulates a set of performance models independently and in a distributed manner on a network of workstations. We propose to optimize the achievable quality of results for a given total amount of CPU time by minimizing the confidence intervals for performability measures. This is possible by an adaptive assignment of CPU time to simulate those models whose results have the largest impact on the width of confidence intervals.","PeriodicalId":326595,"journal":{"name":"18th Workshop on Parallel and Distributed Simulation, 2004. PADS 2004.","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2004-05-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128729590","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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