Zoran Radovic

THROOM: Supporting POSIX multithreaded binaries on a cluster

I propose a set of criteria which distinguish a grand challenge in science or engineering from the many other kinds of short-term or long-term research problems that engage the interest of scientists and engineers. The primary purpose of the formulation and promulgation of a grand challenge is to contribute to the advancement of some branch of science or engineering. A grand challenge represents a commitment by a significant section of the research community to work together towards a common goal, agreed to be valuable and achievable by a team effort within a predicted timescale. The challenge is formulated by the researchers themselves as a focus for the research that they wish to pursue in any case, and which they believe can be pursued more effectively by advance planning and co-ordination. Unlike other common kinds of research initiative, a grand challenge should not be triggered by hope of short-term economic, commercial, medical, military or social benefits; and its initiation should not wait for political promotion or for prior allocation of special funding. The goals of the challenge should be purely scientific goals of the advancement of skill and of knowledge. It should appeal not only to the curiosity of scientists and to the ambition of engineers; ideally it should appeal also to the imagination of the general public; thereby it may enlarge the general understanding and appreciation of science, and attract new entrants to a rewarding career in scientific research. As an example drawn from Computer Science, I revive an old challenge: the construction and application of a verifying compiler that guarantees correctness of a program before running it. A verifying compiler uses automated mathematical and logical reasoning methods to check the correctness of the programs that it compiles. The criterion of correctness is specified by types, assertions, and other redundant annotations that are associated with the code of the program, often inferred automatically, and increasingly often supplied by the original programmer. The compiler will work in combination with other program development and testing tools, to achieve any desired degree of confidence in the structural soundness of the system and the total correctness of its more critical components. The only limit to its use will be set by an evaluation of the cost and benefits of accurate and complete formalization of the criterion of correctness for the software. H. Kosch, L. Böszörményi, H. Hellwagner (Eds.): Euro-Par 2003, LNCS 2790, p. 1, 2003. c

Hierarchical backoff locks for nonuniform communication architectures

This paper identifies node affinity as an important property for scalable general-purpose locks. Nonuniform communication architectures (NUCA), for example CC-NUMA built from a few large nodes or from chip multiprocessors (CMP), have a lower penalty for reading data from a neighbors cache than from a remote cache. Lock implementations that encourages handing over locks to neighbors will improve the lock handover time, as well as the access to the critical data guarded by the lock, but will also be vulnerable to starvation. We propose a set of simple software-based hierarchical backoff locks (HBO) that create node affinity in NUCA. A solution for lowering the risk of starvation is also suggested. The HBO locks are compared with other software-based lock implementations using simple benchmarks, and are shown to be very competitive for uncontested locks while being more than twice as fast for contended locks. An application study also demonstrates superior performance for applications with high lock contention and competitive performance for other programs.

Removing the Overhead from Software-Based Shared Memory

The implementation presented in this paper---DSZOOM-WF---is a sequentially consistent, fine-grained distributed software-based shared memory. It demonstrates a protocol-handling overhead below a microsecond for all the actions involved in a remote load operation, to be compared to the fastest implementation to date of around ten microseconds.The all-software protocol is implemented assuming some basic low-level primitives in the cluster interconnect and an operating system bypass functionality, similar to the emerging InfiniBand standard. All interrupt- and/or poll-based asynchronous protocol processing is completely removed by running the entire coherence protocol in the requesting processor. This not only removes the asynchronous overhead, but also makes use of a processor that otherwise would stall. The technique is applicable to both page-based and fine-grain software-based shared memory.DSZOOM-WF consistently demonstrates performance comparable to hardware-based distributed shared memory implementations.

Efficient Synchronization for Nonuniform Communication Architectures

Scalable parallel computers are often nonuniform communication architectures (NUCAs), where the access time to other processor’s caches vary with their physical location. Still, few attempts of exploring cache-to-cache communication locality have been made. This paper introduces a new kind of synchronization primitives (lock-unlock) that favor neighboring processors when a lock is released. This improves the lock handover time as well as access time to the shared data of the critical region. A critical section guarded by our new RH lock takes less than half the time to execute compared with the same critical section guarded by any other lock on our NUCA hardware. The execution time for Raytrace with 28 processors was improved 2.23 - 4.68 times, while global traffic was dramatically decreased compared with all the other locks. The average execution time was improved 7 - 24% while the global traffic was decreased 8 - 28% for an average over the seven applications studied.

TMA: a trap-based memory architecture

The advances in semiconductor technology have set the shared-memory server trend towards processors with multiple cores per die and multiple threads per core. We believe that this technology shift forces a reevaluation of how to interconnect multiple such chips to form larger systems.This paper argues that by adding support for coherence traps in future chip multiprocessors, large-scale server systems can be formed at a much lower cost. This is due to shorter design time, verification and time to market when compared to its traditional all-hardware counter part. In the proposed trap-based memory architecture (TMA), software trap handlers are responsible for obtaining read/write permission, whereas the coherence trap hardware is responsible for the actual permission check.In this paper we evaluate a TMA implementation (called TMA Lite) with a minimal amount of hardware extensions, all contained within the processor. The proposed mechanisms for coherence trap processing should not affect the critical path and have a negligible cost in terms of area and power for most processor designs.Our evaluation is based on detailed full system simulation using out-of-order processors with one or two dual-threaded cores per die as processing nodes. The results show that a TMA based distributed shared memory system can perform on par with a highly optimized hardware based design.

Exploiting Spatial Store Locality through Permission Caching in Software DSMs

Fine-grained software-based distributed shared memory(SW-DSM) systems typically maintain coherence with in-line checking code at load and store operations to shared memory. The instrumentation overhead of this added checking code can be severe. This paper (1) shows that most of the instrumentation overhead in the fine-grained SW-DSM system DSZOOM is store-related, (2) introduces a new write permission cache (WPC) technique that exploits spatial store locality and batches coherence actions at runtime, (3) evaluates WPC and (4) presents WPC results when implemented in a real SW-DSM system. On average, the WPC reduces the store instrumentation overhead in DSZOOM with 42 (67) percent for benchmarks compiled with maximum (minimum) compiler optimizations.

Exploiting locality: a flexible DSM approach

No single coherence strategy suits all applications well. Many promising adaptive protocols and coherence predictors, capable of dynamically modifying the coherence strategy, have been suggested over the years. While most dynamic detection schemes rely on plentiful of dedicated hardware, the customization technique suggested in this paper requires no extra hardware support for its per-application coherence strategy. Instead, each application is profiled using a low-overhead profiling tool. The appropriate coherence flag setting, suggested by the profiling, is specified when the application is launched. We have compared the performance of a hardware DSM (Sun WildFire) to a software DSM (distributed shared memory) built with identical interconnect hardware and coherence strategy. With no support for flexibility, the software DSM runs on average 45 percent slower than the hardware DSM on the 12 studied applications, while the flexibility can get the software DSM within 11 percent. Our all-software system outperforms the hardware DSM on four applications