Liran Liss

A Local Algorithm for Ad Hoc Majority Voting via Charge Fusion

We present a local distributed algorithm for a general Majority Vote problem: different and time-variable voting powers and vote splits, arbitrary and dynamic interconnection topologies and link delays, and any fixed majority threshold. The algorithm combines a novel, efficient anytime spanning forest algorithm, which may also have applications elsewhere, with a “charge fusion” algorithm that roots trees at nodes with excess “charge” (derived from a node’s voting power and vote split), and subsequently transfers charges along tree links to oppositely charged roots for fusion. At any instant, every node has an ad hoc belief regarding the outcome. Once all changes have ceased, the correct majority decision is reached by all nodes, within a time that in many cases is independent of the graph size. The algorithm’s correctness and salient properties have been proved, and experiments with up to a million nodes provide further validation and actual numbers. To our knowledge, this is the first locality-sensitive solution to the Majority Vote problem for arbitrary, dynamically changing communication graphs.

Veracity radius: capturing the locality of distributed computations

This paper focuses on local computations of distributed aggregation problems on fixed graphs. We define a new metric on problem instances, Veracity Radius (VR), which captures the inherent possibility to compute them locally. We prove that VR yields a tight lower bound on output-stabilization time, i.e., the time until all nodes fix their outputs, as well as a lower bound on quiescence time. We present an efficient aggregation algorithm, I-LEAG, which reaches both output stabilization and quiescence within a time that is proportional to the VR of the problem instance, and is also efficient in terms of per-node communication and memory. We empirically show that the VR metric also effectively captures the performance of previously suggested efficient aggregation protocols, and that I-LEAG significantly outperforms these protocols in several respects.

UCX: An Open Source Framework for HPC Network APIs and Beyond

This paper presents Unified Communication X (UCX), a set of network APIs and their implementations for high throughput computing. UCX comes from the combined effort of national laboratories, industry, and academia to design and implement a high-performing and highly-scalable network stack for next generation applications and systems. UCX design provides the ability to tailor its APIs and network functionality to suit a wide variety of application domains and hardware. We envision these APIs to satisfy the networking needs of many programming models such as Message Passing Interface (MPI), OpenSHMEM, Partitioned Global Address Space (PGAS) languages, task-based paradigms and I/O bound applications. To evaluate the design we implement the APIs and protocols, and measure the performance of overhead-critical network primitives fundamental for implementing many parallel programming models and system libraries. Our results show that the latency, bandwidth, and message rate achieved by the portable UCX prototype is very close to that of the underlying driver. With UCX, we achieved a message exchange latency of 0.89 us, a bandwidth of 6138.5 MB/s, and a message rate of 14 million messages per second. As far as we know, this is the highest bandwidth and message rate achieved by any network stack (publicly known) on this hardware.

GWiQ-P: an efficient decentralized grid-wide quota enforcement protocol

Mega grids span several continents and may consist of millions of nodes and billions of tasks executing at any point in time. This setup calls for scalable and highly available resource utilization control that adapts itself to dynamic changes in the grid environment as they occur. In this paper, we address the problem of enforcing upper bounds on the consumption of grid resources. We propose a grid-wide quota enforcement system, called GWiQ-P. GWiQ-P is light-weight, and in practice is infinitely scalable, satisfying concurrently any number of resource demands, all within the limits of a global quota assigned to each user. GWiQ-P adapts to dynamic changes in the grid as they occur, improving future performance by means of improved locality. This improved performance does not impair the systems ability to respond to current requests, tolerate failures, or maintain the allotted quota levels.

Efficient exploitation of kernel access to Infiniband: a software DSM example

The Infiniband (IB) system area network (SAN) enables applications to access hardware directly from the user level, reducing the overhead of user-kernel crossings during data transfer. However, distributed applications that exhibit close coupling between network and OS services may benefit from accessing IB from the kernel through IBs native verbs interface, which permits tight integration of these services. We assess this approach using a sequential-consistency distributed shared memory (DSM) system as an example. We first develop primitives that abstract the low-level communication and kernel details, and efficiently serve the applications communication, memory,and scheduling needs. Next, we combine the primitives to form a kernel DSM protocol. The approach is evaluated using our full-fledged Linux kernel DSM implementation over Infiniband.

In-kernel integration of operating system and infiniband functions for high performance computing clusters: a DSM example

The infiniband (IB) system area network (SAN) enables applications to access hardware directly from user level, reducing the overhead of user-kernel crossings during data transfer. However, distributed applications that exhibit close coupling between network and OS services may benefit from accessing IB from the kernel through IBs native verbs interface, which permits tight integration of these services. We assess this approach using a sequential-consistency distributed shared memory (DSM) system as an example. We first develop primitives that abstract the low-level communication and kernel details, and efficiently serve the applications communication, memory, and scheduling needs. Next, we combine the primitives to form a kernel DSM protocol. The approach is evaluated using our full-fledged Linux kernel DSM implementation over infiniband. We show that overheads are reduced substantially, and overall application performance is improved in terms of both absolute execution time and scalability relative to an entirely user level implementation.