Garbage collection auto-tuning for Java MapReduce on Multi-Cores

MapReduce has been widely accepted as a simple programming pattern that can form the basis for efficient, large-scale, distributed data processing. The success of the MapReduce pattern has led to a variety of implementations for different computational scenarios. In this paper we present MRJ, a MapReduce Java framework for multi-core architectures. We evaluate its scalability on a four-core, hyperthreaded Intel Core i7 processor, using a set of standard MapReduce benchmarks. We investigate the significant impact that Java runtime garbage collection has on the performance and scalability of MRJ. We propose the use of memory management auto-tuning techniques based on machine learning. With our auto-tuning approach, we are able to achieve MRJ performance within 10% of optimal on 75% of our benchmark tests

Transparent fault tolerance for scalable functional computation

Reliability is set to become a major concern on emergent large-scale architectures. While there are many parallel languages, and indeed many parallel functional languages, very few address reliability. The notable exception is the widely emulated Erlang distributed actor model that provides explicit supervision and recovery of actors with isolated state. We investigate scalable transparent fault tolerant functional computation with automatic supervision and recovery of tasks. We do so by developing HdpH-RS, a variant of the Haskell distributed parallel Haskell (HdpH) DSL with Reliable Scheduling. Extending the distributed work stealing protocol of HdpH for task supervision and recovery is challenging. To eliminate elusive concurrency bugs, we validate the HdpH-RS work stealing protocol using the SPIN model checker. HdpH-RS differs from the actor model in that its principal entities are tasks, i.e. independent stateless computations, rather than isolated stateful actors. Thanks to statelessness, fault recovery can be performed automatically and entirely hidden in the HdpH-RS runtime system. Statelessness is also key for proving a crucial property of the semantics of HdpH-RS: fault recovery does not change the result of the program, akin to deterministic parallelism. HdpH-RS provides a simple distributed fork/join-style programming model, with minimal exposure of fault tolerance at the language level, and a library of higher level abstractions such as algorithmic skeletons. In fact, the HdpH-RS DSL is exactly the same as the HdpH DSL, hence users can opt in or out of fault tolerant execution without any refactoring. Computations in HdpH-RS are always as reliable as the root node, no matter how many nodes and cores are actually used. We benchmark HdpH-RS on conventional clusters and an HPC platform: all benchmarks survive Chaos Monkey random fault injection; the system scales well e.g. up to 1,400 cores on the HPC; reliability and recovery overheads are consistently low even at scale

Garbage collection optimization for non uniform memory access architectures

Cache-coherent non uniform memory access (ccNUMA) architecture is a standard design pattern for contemporary multicore processors, and future generations of architectures are likely to be NUMA. NUMA architectures create new challenges for managed runtime systems. Memory-intensive applications use the system’s distributed memory banks to allocate data, and the automatic memory manager collects garbage left in these memory banks. The garbage collector may need to access remote memory banks, which entails access latency overhead and potential bandwidth saturation for the interconnection between memory banks. This dissertation makes five significant contributions to garbage collection on NUMA systems, with a case study implementation using the Hotspot Java Virtual Machine. It empirically studies data locality for a Stop-The-World garbage collector when tracing connected objects in NUMA heaps. First, it identifies a locality richness which exists naturally in connected objects that contain a root object and its reachable set— ‘rooted sub-graphs’. Second, this dissertation leverages the locality characteristic of rooted sub-graphs to develop a new NUMA-aware garbage collection mechanism. A garbage collector thread processes a local root and its reachable set, which is likely to have a large number of objects in the same NUMA node. Third, a garbage collector thread steals references from sibling threads that run on the same NUMA node to improve data locality. This research evaluates the new NUMA-aware garbage collector using seven benchmarks of an established real-world DaCapo benchmark suite. In addition, evaluation involves a widely used SPECjbb benchmark and Neo4J graph database Java benchmark, as well as an artificial benchmark. The results of the NUMA-aware garbage collector on a multi-hop NUMA architecture show an average of 15% performance improvement. Furthermore, this performance gain is shown to be as a result of an improved NUMA memory access in a ccNUMA system. Fourth, the existing Hotspot JVM adaptive policy for configuring the number of garbage collection threads is shown to be suboptimal for current NUMA machines. The policy uses outdated assumptions and it generates a constant thread count. In fact, the Hotspot JVM still uses this policy in the production version. This research shows that the optimal number of garbage collection threads is application-specific and configuring the optimal number of garbage collection threads yields better collection throughput than the default policy. Fifth, this dissertation designs and implements a runtime technique, which involves heuristics from dynamic collection behavior to calculate an optimal number of garbage collector threads for each collection cycle. The results show an average of 21% improvements to the garbage collection performance for DaCapo benchmarks

Achieving High Performance and High Productivity in Next Generational Parallel Programming Languages

Processor design has turned toward parallelism and heterogeneity cores to achieve performance and energy efficiency. Developers find high-level languages attractive because they use abstraction to offer productivity and portability over hardware complexities. To achieve performance, some modern implementations of high-level languages use work-stealing scheduling for load balancing of dynamically created tasks. Work-stealing is a promising approach for effectively exploiting software parallelism on parallel hardware. A programmer who uses work-stealing explicitly identifies potential parallelism and the runtime then schedules work, keeping otherwise idle hardware busy while relieving overloaded hardware of its burden. However, work-stealing comes with substantial overheads. These overheads arise as a necessary side effect of the implementation and hamper parallel performance. In addition to runtime-imposed overheads, there is a substantial cognitive load associated with ensuring that parallel code is data-race free. This dissertation explores the overheads associated with achieving high performance parallelism in modern high-level languages. My thesis is that, by exploiting existing underlying mechanisms of managed runtimes; and by extending existing language design, high-level languages will be able to deliver productivity and parallel performance at the levels necessary for widespread uptake. The key contributions of my thesis are: 1) a detailed analysis of the key sources of overhead associated with a work-stealing runtime, namely sequential and dynamic overheads; 2) novel techniques to reduce these overheads that use rich features of managed runtimes such as the yieldpoint mechanism, on-stack replacement, dynamic code-patching, exception handling support, and return barriers; 3) comprehensive analysis of the resulting benefits, which demonstrate that work-stealing overheads can be significantly reduced, leading to substantial performance improvements; and 4) a small set of language extensions that achieve both high performance and high productivity with minimal programmer effort. A managed runtime forms the backbone of any modern implementation of a high-level language. Managed runtimes enjoy the benefits of a long history of research and their implementations are highly optimized. My thesis demonstrates that converging these highly optimized features together with the expressiveness of high-level languages, gives further hope for achieving high performance and high productivity on modern parallel hardwar

Adaptive architecture-transparent policy control in a distributed graph reducer

The end of the frequency scaling era occured around 2005 as the clock frequency has stalled for commodity architectures. Thus performance improvements that could in the past be expected with each new hardware generation needed to originate elsewhere. Almost all computer architectures exhibit substantial and growing levels of parallelism, exploiting which became one of the key sources of performance and scalability improvements. Alas, parallel programming proved much more difficult than sequential, due to the need to specify coordination and parallelism management aspects. Whilst low-level languages place the burden on the programmers reducing productivity and portability, semi-implicit approaches delegate the responsibility to sophisticated compilers and run-time systems. This thesis presents a study of adaptive load distribution based on work stealing using history and ancestry information in a distributed graph reducer for a nonstrict functional language. The results contribute to the exploration of more flexible run-time-system-level parallelism control implementing a semi-explicit model of parallelism, which offers productivity and high level of abstraction by delegating the responsibility of coordination to the run-time system. After characterising a set of parallel functional applications, we study the use of historical information to adapt the choice of the victim to steal from in a work stealing scheduler. We observe substantially lower numbers of messages for data-parallel and nested applications. However, this heuristic fails in cases where past application behaviour is not resembling future behaviour, for instance for Divide-&-Conquer applications with a large number of very fine-grained threads and generators of parallelism that move dynamically across processing elements. This mechanism is not specific to the language and the run-time system, and applies to other work stealing schedulers. Next, we focus on the other key work stealing decision of which sparks that represent potential parallelism to donate, investigating the effect of Spark Colocation on the performance of five Divide-&-Conquer programs run on a cluster of up to 256 PEs. When using Spark Colocation, the distributed graph reducer shares related work resulting in a higher degree of both potential and actual parallelism, and more fine-grained and less variable thread size. We validate this behaviour by observing a reduction in average fetch times, but increased amounts of FETCH messages and of inter-PE pointers for colocation, which nevertheless results in improved load balance for three of the five benchmark programs. The results show high speedups and speedup improvements for Spark Colocation for the three more regular and nested applications and performance degradation for two programs: one that is excessively fine-grained and one exhibiting limited scalability. Overall, Spark Colocation appears most beneficial for higher numbers of PEs, where improved load balance and higher degree of parallelism have more opportunities to pay off. In more general terms, we show that a run-time system can beneficially use historical information on past stealing successes that is gathered dynamically and used within the same run and the ancestry information dynamically reconstructed at run time using annotations. Moreover, the results support the view that different heuristics are beneficial for applications using different parallelism patterns, underlining the advantages of a flexible architecture-transparent approach.The Scottish Informatics and Computer Science Alliance (SICSA

Performance analysis methods for understanding scaling bottlenecks in multi-threaded applications

In dit proefschrift stellen we drie nieuwe methodes voor om de prestatie van meerdradige programma's te analyseren. Onze eerste methode, criticality stacks, is bruikbaar voor het analyseren van onevenwicht tussen draden. Om deze stacks te construeren stellen we een nieuwe criticaliteitsmetriek voor, die de uitvoeringstijd van een applicatie opsplitst in een deel voor iedere draad. Hoe groter dit deel is voor een draad, hoe kritischer deze draad is voor de applicatie. De tweede methode, bottle graphs, stelt iedere draad van een meerdradig programma voor als een rechthoek in een grafiek. De hoogte van de rechthoek wordt berekend door middel van onze criticaliteitsmetriek, en de breedte stelt het parallellisme van een draad voor. Rechthoeken die bovenaan in de grafiek zitten, als het ware in de hals van de fles, hebben een beperkt parallellisme, waardoor we ze beschouwen als “bottlenecks” voor de applicatie. Onze derde methode, speedup stacks, toont de bereikte speedup van een applicatie en de verschillende componenten die speedup beperken in een gestapelde grafiek. De intuïtie achter dit concept is dat door het reduceren van de invloed van een bepaalde component, de speedup van een applicatie proportioneel toeneemt met de grootte van die component in de stapel

Effective memory management for mobile environments

Smartphones, tablets, and other mobile devices exhibit vastly different constraints compared to regular or classic computing environments like desktops, laptops, or servers. Mobile devices run dozens of so-called “apps” hosted by independent virtual machines (VM). All these VMs run concurrently and each VM deploys purely local heuristics to organize resources like memory, performance, and power. Such a design causes conflicts across all layers of the software stack, calling for the evaluation of VMs and the optimization techniques specific for mobile frameworks. In this dissertation, we study the design of managed runtime systems for mobile platforms. More specifically, we deepen the understanding of interactions between garbage collection (GC) and system layers. We develop tools to monitor the memory behavior of Android-based apps and to characterize GC performance, leading to the development of new techniques for memory management that address energy constraints, time performance, and responsiveness. We implement a GC-aware frequency scaling governor for Android devices. We also explore the tradeoffs of power and performance in vivo for a range of realistic GC variants, with established benchmarks and real applications running on Android virtual machines. We control for variation due to dynamic voltage and frequency scaling (DVFS), Just-in-time (JIT) compilation, and across established dimensions of heap memory size and concurrency. Finally, we provision GC as a global service that collects statistics from all running VMs and then makes an informed decision that optimizes across all them (and not just locally), and across all layers of the stack. Our evaluation illustrates the power of such a central coordination service and garbage collection mechanism in improving memory utilization, throughput, and adaptability to user activities. In fact, our techniques aim at a sweet spot, where total on-chip energy is reduced (20–30%) with minimal impact on throughput and responsiveness (5–10%). The simplicity and efficacy of our approach reaches well beyond the usual optimization techniques

The Circulation of Ideas in Firms and Markets

Novel early stage ideas face uncertainty on the expertise needed to elaborate them, which creates a need to circulate them widely to find a match. Yet as information is not excludable, shared ideas may be stolen, reducing incentives to innovate. Still, in idea-rich environments inventors may share them without contractual protection. Idea density is enhanced by firms ensuring rewards to inventors, while their legal boundaries limit idea leakage. As firms limit idea circulation, the innovative environment involves a symbiotic interaction: firms incubate ideas and allow employees to leave if they cannot find an internal fit; markets allow for wide circulation of ideas until matched and completed; under certain circumstances ideas may be even developed in both firms and markets.Ideas, Innovation, Entrepreneurship, Firm Organization, Start-Ups