Composing Relaxed Transactions

As the classical transactional abstraction is sometimes considered too restrictive in leveraging parallelism, a lot of work has been devoted to devising relaxed transactional models with the goal of improving concurrency. Nevertheless, the quest for improving concurrency has somehow led to neglect one of the most appealing aspects of transactions: software composition, namely, the ability to develop pieces of software independently and compose them into applications that behave correctly in the face of concurrency. Indeed, a closer look at relaxed transactional models reveals that they do jeopardize composition, raising the fundamental question whether it is at all possible to devise such models while preserving composition. This paper shows that the answer is positive. We present outheritance, a necessary and sufficient condition for a (potentially relaxed) transactional memory to support composition. Basically, outheritance requires child transactions to pass their conflict information to their parent transaction, which in turn maintains this information until commit time. Concrete instantiations of this idea have been used before, classical transactions being the most prevalent example, but we believe to be the first to capture this as a general principle as well as to prove that it is, strictly speaking, equivalent to ensuring composition. We illustrate the benefits of outheritance using elastic transactions and show how they can satisfy outheritance and provide composition without hampering concurrency. We leverage this to present a new (transactional) Java package, a composable alternative to the concurrency package of the JDK, and evaluate efficiency through an implementation that speeds up state of the art software transactional memory implementations (TL2, LSA, SwissTM) by almost a factor of 3

Non-blocking Patricia Tries with Replace Operations

This paper presents a non-blocking Patricia trie implementation for an asynchronous shared-memory system using Compare&Swap. The trie implements a linearizable set and supports three update operations: insert adds an element, delete removes an element and replace replaces one element by another. The replace operation is interesting because it changes two different locations of tree atomically. If all update operations modify different parts of the trie, they run completely concurrently. The implementation also supports a wait-free find operation, which only reads shared memory and never changes the data structure. Empirically, we compare our algorithms to some existing set implementations.Comment: To appear in the 33rd IEEE International Conference on Distributed Computing Systems (ICDCS 2013

Efficient means of Achieving Composability using Transactional Memory

The major focus of software transaction memory systems (STMs) has been to facilitate the multiprocessor programming and provide parallel programmers with an abstraction for fast development of the concurrent and parallel applications. Thus, STMs allow the parallel programmers to focus on the logic of parallel programs rather than worrying about synchronization. Heart of such applications is the underlying concurrent data-structure. The design of the underlying concurrent data-structure is the deciding factor whether the software application would be efficient, scalable and composable. However, achieving composition in concurrent data structures such that they are efficient as well as easy to program poses many consistency and design challenges. We say a concurrent data structure compose when multiple operations from same or different object instances of the concurrent data structure can be glued together such that the new operation also behaves atomically. For example, assume we have a linked-list as the concurrent data structure with lookup, insert and delete as the atomic operations. Now, we want to implement the new move operation, which would delete a node from one position of the list and would insert into the another or same list. Such a move operation may not be atomic(transactional) as it may result in an execution where another process may access the inconsistent state of the linked-list where the node is deleted but not yet inserted into the list. Thus, this inability of composition in the concurrent data structures may hinder their practical use. In this context, the property of compositionality provided by the transactions in STMs can be handy. STMs provide easy to program and compose transactional interface which can be used to develop concurrent data structures thus the parallel software applications. However, whether this can be achieved efficiently is a question we would try to answer in this thesis. Most of the STMs proposed in the literature are based on read/write primitive operations(or methods) on memory buffers and hence denoted RWSTMs. These lower level read/write primitive operations do not provide any other useful information except that a write operation always needs to be ordered with any other read or write. Thus limiting the number of possible concurrent executions. In this thesis, we consider Object-based STMs or OSTMs which operate on higher level objects rather than read/write operations on memory locations. The main advantage of considering OSTMs is that with the greater semantic information provided by the methods of the object, the conflicts among the transactions can be reduced and as a result, the number of aborts will also be less. This allows for larger number of permissive concurrent executions leading to more concurrency. Hence, OSTMs could be an efficient means of achieving composability of higher-level operations in the software applications using the concurrent data structures. This would allow parallel programmers to leverage underlying multi-core architecture. To design the OSTM, we have adopted the transactional tree model developed for databases. We extend the traditional notion of conflicts and legality to higher level operations in STMs which allows efficient composability. Using these notions we define the standard STM correctness notion of Conflict-Opacity. The OSTM model can be easily extended to implement concurrent lists, sets, queues or other concurrent data structures. We use the proposed theoretical OSTM model to design HT-OSTM - an OSTM with underlying hash table object. We noticed that major concurrency hot-spot is the chaining data structure within the hash table. So, we have used Lazyskip-list approach which is time efficient compared to normal lists in terms of traversal overhead. At the transactional level, we use timestamp ordering protocol to ensure that the executions are conflict-opaque. We provide a detailed handcrafted proof of correctness starting from operational level to the transactional level. At the operational level we show that HT-OSTM generates legal sequential history. At vi transactional level we show that every such sequential history would be opaque thus co-opaque. The HT-OSTM exports STM insert, STM lookup and STM delete methods to the programmer along-with STM begin and STM trycommit. Using these higher level operations user may easily and efficiently program any parallel software application involving concurrent hash table. To demonstrate the efficiency of composition we build a test application which executes the number of hash-tab methods (generated with a given probability) atomically in a transaction. Finally, we evaluate HT-OSTM against ESTM based hash table of synchrobench and the hash-table designed for RWSTM based on basic time stamp ordering protocol. We observe that HT-OSTM outperforms ESTM by the average magnitude of 106 transactions per second(throughput) for both lookup intensive and update intensive work load. HT-OSTM outperforms RWSTM by 3% & 3.4% update intensive and lookup intensive workload respectivel

Supporting lock-free composition of concurrent data objects

Lock-free data objects offer several advantages over their blocking counterparts, such as being immune to deadlocks and convoying and, more importantly, being highly concurrent. But they share a common disadvantage in that the operations they provide are difficult to compose into larger atomic operations while still guaranteeing lock-freedom. We present a lock-free methodology for composing highly concurrent linearizable objects together by unifying their linearization points. This makes it possible to relatively easily introduce atomic lock-free move operations to a wide range of concurrent objects. Experimental evaluation has shown that the operations originally supported by the data objects keep their performance behavior under our methodology

Supporting Lock-Free Composition of Concurrent Data Objects: Moving Data Between Containers

Lock-free data objects offer several advantages over their blocking counterparts, such as being immune to deadlocks, priority inversion and convoying. They have also been shown to work well in practice. However, composing the operations they provide into larger atomic operations, while still guaranteeing efficiency and lock-freedom, is a challenging algorithmic task. We present a lock-free methodology for composing a wide variety of concurrent linearizable objects together by unifying their linearization points. This makes it possible to relatively easily introduce atomic lock-free move operations to a wide range of concurrent lock-free containers. This move operation allows data to be transferred from one container to another, in a lock-free way, without blocking any of the operations supported by the original container. For a data object to be suitable for composition using our methodology it needs to fulfil a set of requirements. These requirement are however generic enough to be fulfilled by a large set of objects. To show this we have performed case studies on six commonly used lock-free objects (a stack, a queue, a skip-list, a deque, a doubly linked-list and a hash-table) to demonstrate the general applicability of the methodology. We also show that the operations originally supported by the data objects keep their performance behavior under our methodology

Supporting Lock-Free Composition of Concurrent Data Objects: Moving Data Between Containers

Lock-free data objects offer several advantages over their blocking counterparts, such as being immune to deadlocks, priority inversion and convoying. They have also been shown to work well in practice. However, composing the operations they provide into larger atomic operations, while still guaranteeing efficiency and lock-freedom, is a challenging algorithmic task. We present a lock-free methodology for composing a wide variety of concurrent linearizable objects together by unifying their linearization points. This makes it possible to relatively easily introduce atomic lock-free move operations to a wide range of concurrent lock-free containers. This move operation allows data to be transferred from one container to another, in a lock-free way, without blocking any of the operations supported by the original container. For a data object to be suitable for composition using our methodology it needs to fulfil a set of requirements. These requirement are however generic enough to be fulfilled by a large set of objects. To show this we have performed case studies on six commonly used lock-free objects (a stack, a queue, a skip-list, a deque, a doubly linked-list and a hash-table) to demonstrate the general applicability of the methodology. We also show that the operations originally supported by the data objects keep their performance behavior under our methodology