Distributed Breakpoint Detection in Message-Passing Programs

Global conditions in debugging distributed programs

In a message-passing program, there are at least as many threads as processors, and the programmer must deal with large numbers of them on a massively parallel machine. On our target machine, the CM-5, we had previously developed Prism, a programming environment that supports debugging, data visualization, and performance analysis of data-parallel programs. We discuss how our new version, Node Prism, extends Prism's capabilities for message-passing programs. It looks and feels like the data-parallel version, but it uses new methods for user-debugger interaction that promote greater understanding of parallel programs. It offers scalable expression, execution, and interpretation of all debugging operations, making it easier to debug and understand message-passing programs. >

Enumerating consistent global states of a computation is a fundamental problem in parallel computing with applications to debugging, testing and runtime verification of parallel programs. Breadth-first search (BFS) enumeration is especially useful for these applications as it finds an erroneous consistent global state with the least number of events possible. The total number of executed events in a global state is called its rank. BFS also allows enumeration of all global states of a given rank or within a range of ranks. If a computation on n processes has m events per process on average, then the traditional BFS (Cooper-Marzullo and its variants) requires \(\mathcal {O}(\frac{m^{n-1}}{n})\) space in the worst case, whereas our algorithm performs the BFS requires \(\mathcal {O}(m^2n^2)\) space. Thus, we reduce the space complexity for BFS enumeration of consistent global states exponentially, and give the first polynomial space algorithm for this task. In our experimental evaluation of seven benchmarks, traditional BFS fails in many cases by exhausting the 2 GB heap space allowed to the JVM. In contrast, our implementation uses less than 60 MB memory and is also faster in many cases.

Parallel debugging focuses on solving four problems: nondeterminism, global states, communication related errors and performance debugging. PVM is a widely used programming environment for network of workstations, but it provides little support on debugging. The authors have developed an integrated parallel debugger (called BUSTER) for PVM programs running on workstation clusters. They introduce the ideas and implementation techniques of the debugger.

Within the standard weak-coupling limit, the reduced dynamics of open quantum spin chains with their two end spins coupled to two distinct heat baths at different temperatures are mainly derived using the so-called \textit{global} and \textit{local} approaches, in which, respectively, the spin self-interaction is and is not taken into account. In order to compare the differences between the two regimes, we concentrate on an open three-site $XX$ spin-chain, provide systematic techniques to addressing the global and local asymptotic states and then compare the asymptotic spin-transport features by studying the spin flux through the middle site. %we examine the transport properties of an open three-site $XX$ spin-chain. %We analytically derive the exact steady state of the open chain in the global approach and its first order expansion with respect to the self-interaction in the local one. %The steady state transport properties through the middle spin is then studied %in order to compare the physical scenarios associated with the two regimes. %By analytical and numerical means, Basing on the analytical expressions of the stationary states in the two regimes, we find that the local approach misses important global effects emerging as spin sink and source terms that can only be due to non negligible inter-spin interactions. Moreover, we show that the local asympotic transport features cannot be recovered from the global ones by letting the inter-spin coupling vanish thus pointing to the existence of different coupling ranges where only one approach is physically tenable and possibly a region where the two descriptions may coexist.