A simple deterministic algorithm for guaranteeing the forward progress of transactions

This paper describes a remarkably simple deterministic (not probabilistic) contention-management algorithm for guaranteeing the forward progress of transactions - avoiding deadlocks, livelocks, and other anomalies. The transactions must be finite (no infinite loops), but on each restart, a transaction may access different shared-memory locations. The algorithm supports irrevocable transactions as long as the transaction satisfies a simple ordering constraint. In particular, a transaction that accesses only one shared-memory location is never aborted. The algorithm is suitable for both hardware and software transactional-memory systems. It also can be used in some contexts as a locking protocol for implementing transactions "by hand."

Randomized Routing on Fat-Trees

Fat-trees are a class of routing networks for hardware-efficient parallel computation. This paper presents a randomized algorithm for routing messages on a fat-tree. The quality of the algorithm is measured in terms of the load factor of a set of messages to be routed, which is a lower bound on the time required to deliver the messages. We show that if a set of messages has load factor lambda on a fat-tree with n processors, the number of delivery cycles (routing attempts) that the algorithm requires is O(lambda + lg n lg lg n) with probability 1-O(1/n). The best previous bound was O(lambda lg n) for the offline problem in which the set of messages is known in advance. In the context of a VLSI model that equates hardware cost with physical volume, the routing algorithm can be used to demonstrate that fat-trees are universal routing networks. Specifically, we prove that any routing network can be efficiently simulated by a fat-tree of comparable hardware cost

Transactions Everywhere

Arguably, one of the biggest deterrants for software developers who might otherwise choose to write parallel code is that parallelism makes their lives more complicated. Perhaps the most basic problem inherent in the coordination of concurrent tasks is the enforcing of atomicity so that the partial results of one task do not inadvertently corrupt another task. Atomicity is typically enforced through locking protocols, but these protocols can introduce other complications, such as deadlock, unless restrictive methodologies in their use are adopted. We have recently begun a research project focusing on transactional memory [18] as an alternative mechanism for enforcing atomicity, since it allows the user to avoid many of the complications inherent in locking protocols. Rather than viewing transactions as infrequent occurrences in a program, as has generally been done in the past, we have adopted the point of view that all user code should execute in the context of some transaction. To make this viewpoint viable requires the development of two key technologies: effective hardware support for scalable transactional memory, and linguistic and compiler support. This paper describes our preliminary research results on making “transactions everywhere” a practical reality.Singapore-MIT Alliance (SMA

Provably Efficient Adaptive Scheduling for Parallel Jobs

Scheduling competing jobs on multiprocessors has always been an important issue for parallel and distributed systems. The challenge is to ensure global, system-wide efficiency while offering a level of fairness to user jobs. Various degrees of successes have been achieved over the years. However, few existing schemes address both efficiency and fairness over a wide range of work loads. Moreover, in order to obtain analytical results, most of them require prior information about jobs, which may be difficult to obtain in real applications. This paper presents two novel adaptive scheduling algorithms -- GRAD for centralized scheduling, and WRAD for distributed scheduling. Both GRAD and WRAD ensure fair allocation under all levels of workload, and they offer provable efficiency without requiring prior information of job's parallelism. Moreover, they provide effective control over the scheduling overhead and ensure efficient utilization of processors. To the best of our knowledge, they are the first non-clairvoyant scheduling algorithms that offer such guarantees. We also believe that our new approach of resource request-allotment protocol deserves further exploration. Specifically, both GRAD and WRAD are O(1)-competitive with respect to mean response time for batched jobs, and O(1)-competitive with respect to makespan for non-batched jobs with arbitrary release times. The simulation results show that, for non-batched jobs, the makespan produced by GRAD is no more than 1.39 times of the optimal on average and it never exceeds 4.5 times. For batched jobs, the mean response time produced by GRAD is no more than 2.37 times of the optimal on average, and it never exceeds 5.5 times.Singapore-MIT Alliance (SMA

A Media Player for Use in Distance Education

We have developed a media player for use in distance education. The player can incorporate several time-indexed sources, including video, audio, PowerPoint, and text index. We have converted all the SMA 5503 Introduction to Algorithms lecture videos for use with the player. Student response to the player has been excellent. The media player's graphical user interface (see Fig. 1) permits a student viewer to navigate a video by the text index. The text index can be produced either automatically by a PowerPoint plug-in developed by David Mycue of MIT's Academic Media Production Systems, or manually by a teaching assistant using an ordinary text editor. The student can flip easily through PowerPoint slides, invoking the video only when needed, rather than being forced to watch video presentation of material the student already understands. The player also allows playback to be accelerated up to twice real-time speed without changing the speaker's tone, thereby allowing the student to watch a video in much less time that would otherwise be possible. A single-click button allows the student to skip backward in the video a few seconds, easing the replay of critical material. Another single-click button allows the user to skip forward. Although none of the features in this media player are unique or original, their combination provides a platform for lecture viewing, superior to the one currently being used by SMA. Ongoing work on the player includes adding more features to the player, such as allowing students to add multiple streams of other information (e.g., notes, snapshots of the lecture, etc.) which will be synchronized with the video in the same manner as the text indices and the slides in the current version.Singapore-MIT Alliance (SMA

Parallel Computation of the Minimal Elements of a Poset

Computing the minimal elements of a partially ordered finite set (poset) is a fundamental problem in combinatorics with numerous applications such as polynomial expression optimization, transversal hypergraph generation and redundant component removal, to name a few. We propose a divide-and-conquer algorithm which is not only cache-oblivious but also can be parallelized free of determinacy races. We have implemented it in Cilk++ targeting multicores. For our test problems of sufficiently large input size our code demonstrates a linear speedup on 32 cores.National Science Foundation (U.S.). (Grant number CNS-0615215)National Science Foundation (U.S.). (Grant number CCF- 0621511

Programming with Exceptions in JCilk

JCilk extends the Java language to provide call-return semantics for multithreading, much as Cilk does for C. Java's built-in thread model does not support the passing of exceptions or return values from one thread back to the "parent" thread that created it. JCilk imports Cilk's fork-join primitives spawn and sync into Java to provide procedure-call semantics for concurrent subcomputations. This paper shows how JCilk integrates exception handling with multithreading by defining semantics consistent with the existing semantics of Java's try and catch constructs, but which handle concurrency in spawned methods. JCilk's strategy of integrating multithreading with Java's exception semantics yields some surprising semantic synergies. In particular, JCilk extends Java's exception semantics to allow exceptions to be passed from a spawned method to its parent in a natural way that obviates the need for Cilk's inlet and abort constructs. This extension is "faithful" in that it obeys Java's ordinary serial semantics when executed on a single processor. When executed in parallel, however, an exception thrown by a JCilk computation signals its sibling computations to abort, which yields a clean semantics in which only a single exception from the enclosing try block is handled. The decision to implicitly abort side computations opens a Pandora's box of subsidiary linguistic problems to be resolved, however. For instance, aborting might cause a computation to be interrupted asynchronously, causing havoc in programmer understanding of code behavior. To minimize the complexity of reasoning about aborts, JCilk signals them "semisynchronously" so that abort signals do not interrupt ordinary serial code. In addition, JCilk propagates an abort signal throughout a subcomputation naturally with a built-in CilkAbort exception, thereby allowing programmers to handle clean-up by simply catching the CilkAbort exception. The semantics of JCilk allow programs with speculative computations to be programmed easily. Speculation is essential for parallelizing programs such as branch-and-bound or heuristic search. We show how JCilk's linguistic mechanisms can be used to program a solution to the "queens" problem and an implemention of a parallel alpha-beta search.Singapore-MIT Alliance (SMA

Hardware Transactional Memory

This work shows how hardware transactional memory (HTM) can be implemented to support transactions of arbitrarily large size, while ensuring that small transactions run efficiently. Our implementation handles small transactions similar to Herlihy and Moss's scheme in that it holds tentative updates in a cache. Unlike their scheme, which uses a special fully associative cache, ours augments the ordinary processor cache and provides a mechanism to handle cache spills of uncommitted transactional data. Consequently, our scheme runs faster for small transactions while correctly handling transactions of arbitrarily large size. Although transactions are small in the common case, we argue that HTM should not restrict the size of transactions, because it complicates the programmer/compiler model and precludes some important programs from exploiting transactional memory. We show that the Linux 2.4.19 kernel can be automatically and efficiently “transactified” if boundless transactions can be supported. Our experimental results show that the largest transaction touches over 7000 64-byte cache lines, whereas 99.94\% of the transactions touch fewer than 64 cache lines. We further show that synchronized methods in Java can be easily compiled to our HTM scheme, thereby providing the advantages of nonblocking atomicity (including absence of deadlock) in a straightforward fashion. Our HTM scheme for boundless transactions uses an efficiently implementable hardware snapshot and the ordinary set-associative L2 cache extended with less than two bits per cache line. One of the bits tells whether the cached item is part of a transaction (as in the Herlihy-Moss scheme), and all the lines in an associative set share another bit telling whether a line has overflowed from the cache and is now stored in a special overflow area of main memory. We provide empirical results to show that our scheme does not adversely affect the processor pipeline or hinder speculative execution.Singapore-MIT Alliance (SMA