Srivatsan Ravi

University of Southern California | USC · Information Sciences Institute

Designing an efficient concurrent data structure is a challenge that is not easy to meet. Intuitively, efficiency of an implementation is defined, in the first place, by its ability to process applied operations in parallel, without using unnecessary synchronization. As we show in this paper, even for a data structure as simple as a linked list use...

In many Software-Defined Networking (SDN) deployments the control plane ends up being actually centralized, yielding a single point of failure and attack. This paper models the interaction between the data plane and a distributed control plane consisting of a set of failure-prone and potentially malicious (compromised) control devices, and im...

Designing low-latency cloud-based applications that are adaptable to unpredictable workloads and efficiently utilize modern cloud computing platforms is hard. The actor model is a popular paradigm that can be used to develop distributed applications: actors encapsulate state and communicate with each other by sending events. Consistency is guarante...

Permissionless blockchains protocols such as Bitcoin are inherently limited in transaction throughput and latency. Current efforts to address this key issue focus on off-chain payment channels that can be combined in a Payment-Channel Network (PCN) to enable an unlimited number of payments without requiring to access the blockchain other than to re...

One of the most recent members of the Paxos family of protocols is Generalized Paxos. This variant of Paxos has the characteristic that it departs from the original specification of consensus, allowing for a weaker safety condition where different processes can have a different views on a sequence being agreed upon. However, much like the original...

Several hybrid transactional memory (HyTM) schemes have recently been proposed to complement the fast, but best-effort nature of hardware transactional memory with a slow, reliable software backup. However, the costs of providing concurrency between hardware and software transactions in HyTM are still not well understood. In this paper, we propose...

Permissionless blockchains protocols such as Bitcoin are inherently limited in transaction throughput and latency. Current efforts to address this key issue focus on off-chain payment channels that can be combined in a Payment-Channel Network (PCN) to enable an unlimited number of payments without requiring to access the blockchain other than to re...

The actor model is a popular paradigm for programming scalable cloud applications. Building elastic and scalable cloud applications requires application developers to carefully adjust the application scale (the required resources) and the placement of actors at the runtime. Unfortunately, there is no efficient solution which could manage applicatio...

One of the most recent members of the Paxos family of protocols is Generalized Paxos. This variant of Paxos has the characteristic that it departs from the original specification of consensus, allowing for a weaker safety condition where different processes can have different views on a sequence being agreed upon. However, much like the original Pa...

One of the most recent members of the Paxos family of protocols is Generalized Paxos. This variant of Paxos has the characteristic that it departs from the original specification of consensus, allowing for a weaker safety condition where different processes can have a different views on a sequence being agreed upon. However, much like the original...

The paper presents the first \emph{concurrency-optimal} implementation of a binary search tree (BST). The implementation, based on a standard sequential implementation of an internal tree, ensures that every \emph{schedule} is accepted, i.e., interleaving of steps of the sequential code, unless linearizability is violated. To ensure this property,...

Designing low-latency cloud-based applications that are adaptable to unpredictable workloads and efficiently utilize modern cloud computing platforms is hard. The actor model is a popular paradigm that can be used to develop distributed applications: actors encapsulate state and communicate with each other by sending events. Consistency is guarante...

Transactional memory (TM) is an inherently optimistic abstraction: it allows concurrent processes to execute sequences of shared-data accesses (transactions) speculatively, with an option of aborting them in the future. Early TM designs avoided using locks and relied on non-blocking synchronization to ensure obstruction-freedom: a transaction that...

Big-data processing frameworks like Hadoop and Spark, often used in multi-user environments, have struggled to achieve a balance between the full utilization of cluster resources and fairness between users. In particular, data locality becomes a concern, as enforcing fairness policies may cause poor placement of tasks in relation to the data on whi...

It is common practice to use the epithet “highly concurrent” referring to data structures that are supposed to perform well in concurrent environments. But how do we measure the concurrency of a data structure in the first place? In this paper, we propose a way to do this, which allowed us to formalize the notion of a concurrency-optimal implementa...

Implementing a concurrent data structure typically begins with defining its sequential specification. However, when used \emph{as is}, a nontrivial sequential data structure, such as a linked list, a search tree, or a hash table, may expose incorrect behavior: lost updates, inconsistent responses, etc. To ensure correctness, portions of the sequent...

Transactional memory (TM) is an inherently optimistic synchronization abstraction: it allows concurrent processes to execute sequences of shared-data accesses (transactions) speculatively, with an option of aborting them in the future. Early TM designs avoided using locks and relied on non-blocking synchronization to ensure obstruction-freedom: a t...

Transactional memory (TM) allows concurrent processes to organize sequences of operations on shared data items into atomic transactions. A transaction may commit, in which case it appears to have executed sequentially or it may abort, in which case no data item is updated. The TM programming paradigm emerged as an alternative to conventional fine-g...

Transactional memory (TM) is an inherently optimistic abstraction: it allows concurrent processes to execute sequences of shared-data accesses (transactions) speculatively, with an option of aborting them in the future. Early TM designs avoided using locks and relied on non-blocking synchronization to ensure obstruction-freedom: a transaction that...

Designing a highly concurrent data structure is an important challenge that is not easy to meet. As we show in this paper, even for a data structure as simple as a linked list used to implement the set type, the most efficient algorithms known so far may reject correct concurrent schedules. We propose a new algorithm based on a versioned try-lock t...

Aktuelle Allzweck-CPUs haben mehrere Rechenkerne innerhalb eines einzelnen Chipsatzes. Allerdings erhöht sich die Leistung der Programme auf diesen Architekturen nicht notwendigerweise proportional in der Anzahl der Kerne. Das Entwerfen nebenläufiger Programme um diese Multicores zu nutzen, erfordert die Überwindung einiger nicht-trivialer Herausfo...

Transactional memory (TM) is a convenient synchronization tool that allows concurrent threads to declare sequences of instructions on shared data as speculative \emph{transactions} with "all-or-nothing" semantics. It is known that dynamic transactional memory cannot provide \emph{wait-free} progress in the sense that every transaction commits in a...

Several Hybrid Transactional Memory (HyTM) schemes have recently been proposed to complement the fast, but best-effort, nature of Hardware Transactional Memory (HTM) with a slow, reliable software backup. However, the fundamental limitations of building a HyTM with nontrivial concurrency between hardware and software transactions are still not well...

Several Hybrid Transactional Memory (HyTM) schemes have recently been proposed to complement the fast, but best-effort nature of Hardware Transactional Memory (HTM) with a slow, reliable software backup. However, the costs of providing concurrency between hardware and software transactions in HyTM are still not well understood. In this paper, we pr...

Transactional memory allows the user to declare sequences of instructions as speculative \emph{transactions} that can either \emph{commit} or \emph{abort}. If a transaction commits, it appears to be executed sequentially, so that the committed transactions constitute a correct sequential execution. If a transaction aborts, none of its instructions...

Two new algorithms are given for randomized consensus in a shared-memory model with an oblivious adversary. Each is based on a new construction of a conciliator, an object that guarantees termination and validity, but that only guarantees agreement with ...

In this issue, we have a short column. It features a review of WTTM 2011, the third Workshop on the Theory of Transactional Memory, by Petr Kuznetsov and Srivatsan Ravi. This annual workshop focuses on developing theory for understanding transactional memory systems. It discusses recent achievements as well as remaining challenges. Petr and Srivats...

To leverage modern multi-cores, hardware manufacturers have recently introduced transactional memory that adopts an optimistic computing strategy, complementing classical (pessimistic) locking techniques. Yet, it is unclear which synchronization technique can lead to higher concurrency. In this paper, we compare the level of concurrency one can obt...

Modern concurrent programming benefits from a large variety of synchronization techniques. These include conventional pessimistic locking, as well as optimistic techniques based on conditional synchronization primitives or transactional memory. Yet, it is unclear which of these approaches better leverage the concurrency inherent to multi-cores. In...

The promise of software transactional memory (STM) is to combine an easy-to-use programming interface with an efficient utilization of the concurrent-computing abilities provided by modern machines. But does this combination come with an inherent cost? We evaluate the cost of concurrency by measuring the amount of expensive synchronization that mus...

Transactional memory appears promising for democratizing concurrent programming. Its potential lies in producing code that is extensible as atomicity is preserved under composition of transactions. This new abstraction raises several challenges such as the compliance of transactions with alternative synchronization techniques of legacy code and mem...