Improving Performance of Highly-Programmable Concurrent Applications by Leveraging Parallel Nesting and Weaker Isolation Levels

The recent development of multi-core computer architectures has largely affected the creation of everyday applications, requiring the adoption of concurrent programming to significantly utilize the divided processing power of computers. Applications must be split into sections able to execute in parallel, without any of these sections conflicting with one another, thereby necessitating some form of synchronization to be declared. The most commonly used methodology is lock-based synchronization; although, to improve performance the most, developers must typically form complex, low-level implementations for large applications, which can easily create potential errors or hindrances.

An abstraction from database systems, known as transactions, is a rising concurrency control design aimed to circumvent the challenges with programmability, composability, and scalability in lock-based synchronization. Transactions execute their operations speculatively and are capable of being restarted (or rolled back) when there exist conflicts between concurrent actions. As such issues can occur later in the lifespans of transactions, entire rollbacks are not that effective for performance. One particular method, known as nesting, was created to counter that drawback. Nesting is the act of enclosing transactions within other transactions, essentially dividing the work into pieces called sub-transactions. These sub-transactions can roll back without affecting the entire main transaction, although general nesting models only allow one sub-transaction to perform work at a time.

The first main contribution in this thesis is SPCN, an algorithm that parallelizes nested transactions while automatically processing any potential conflicts that may arise, eliminating the burden of additional processing from the application developers. Two versions of SPCN exist: Strict, which enforces the sub-transactions' work to be made visible in a serialized order; and Relaxed, which allows sub-transactions to distribute their information immediately as they finish (therefore invalidation may occur after-the-fact and must be handled). Despite the additional logic required by SPCN, it outperforms traditional closed nesting by 1.78x at the lowest and 3.78x at the highest in the experiments run.

Another method to alter transactional execution and boost performance is to relax the rules of visibility for parallel operations (known as their isolation). Depending on the application, correctness is not broken even if some transactions see external work that may later be undone due to a rollback, or if an object is written while another transaction is using an older instance of its data. With lock-based synchronization, developers would have to explicitly design their application with varying amounts of locks, and different lock organizations or hierarchies, to change the strictness of the execution. With transactional systems, the processing performed by the system itself can be set to utilize different rulings, which can change the performance of an application without requiring it to be largely redesigned.

This notion leads to the second contribution in this thesis: AsR, or As-Serializable transactions. Serializability is the general form of isolation or strictness for transactions in many applications. In terms of execution, its definition is equivalent to only one transaction running at a time in a given system. Many transactional systems use their own internal form of locking to create Serializable executions, but it is typically too strict for many applications. AsR transactions allow the internal processing to be relaxed while additional meta-data is used external to the system, without requiring any interaction from the developer or any changes to the given application. AsR transactions offer multiple orders of magnitude more in throughput in highly-contentious scenarios, due to their capability to outlast traditional levels of isolation.