Lock (computer science)

Abstract: The author examines the questions of whether there are efficient algorithms for software spin-waiting given hardware support for atomic instructions, or whether more complex kinds of hardware support are needed for performance. He considers the performance of a number of software spin-waiting algorithms. Arbitration for control of a lock is in many ways similar to arbitration for control of a network connecting a distributed system. He applies several of the static and dynamic arbitration methods originally developed for networks to spin locks. A novel method is proposed for explicitly queueing spinning processors in software by assigning each a unique number when it arrives at the lock. Control of the lock can then be passed to the next processor in line with minimal effect on other processors. >

Abstract: Programmers have traditionally used locks to synchronize concurrent access to shared data. Lock-based synchronization, however, has well-known pitfalls: using locks for fine-grain synchronization and composing code that already uses locks are both difficult and prone to deadlock. Transactional memory provides an alternate concurrency control mechanism that avoids these pitfalls and significantly eases concurrent programming. Transactional memory language constructs have recently been proposed as extensions to existing languages or included in new concurrent language specifications, opening the door for new compiler optimizations that target the overheads of transactional memory.This paper presents compiler and runtime optimizations for transactional memory language constructs. We present a high-performance software transactional memory system (STM) integrated into a managed runtime environment. Our system efficiently implements nested transactions that support both composition of transactions and partial roll back. Our JIT compiler is the first to optimize the overheads of STM, and we show novel techniques for enabling JIT optimizations on STM operations. We measure the performance of our optimizations on a 16-way SMP running multi-threaded transactional workloads. Our results show that these techniques enable transactional memory's performance to compete with that of well-tuned synchronization.

Abstract: This paper provides a comprehensive treatment of index management in transaction systems. We present a method, called ARIESIIM (Algorithm for Recovery and Isolation Exploiting Semantics for Index Management), for concurrency control and recovery of B+-trees. ARIES/IM guarantees serializability and uses write-ahead logging for recovery. It supports very high concurrency and good performance by (1) treating as the lock of a key the same lock as the one on the corresponding record data in a data page (e.g., at the record level), (2) not acquiring, in the interest of permitting very high concurrency, commit duration locks on index pages even during index structure modification operations (SMOs) like page splits and page deletions, and (3) allowing retrievals, inserts, and deletes to go on concurrently with SMOs. During restart recovery, any necessary redos of index changes are always performed in a page-oriented fashion (i.e., without traversing the index tree) and, during normal processing and restart recovery, whenever possible undos are performed in a page-oriented fashion. ARIES/IM permits different granularities of locking to be supported in a flexible manner. A subset of ARIES/IM has been implemented in the OS/2 Extended Edition Database Manager. Since the locking ideas of ARIES/IM have general applicability, some of them have also been implemented in SQL/DS and the VM Shared File System, even though those systems use the shadow-page technique for recovery.

Abstract: A performance evaluation of the Symmetry multiprocessor system revealed that the synchronization mechanism did not perform well for highly contested locks, like those found in certain parallel applications. Several software synchronization mechanisms were developed and evaluated, using a hardware monitor, on the Symmetry multiprocessor system; the mechanisms were to reduce contention for the lock. The mechanisms remain valuable even when changes are made to the hardware synchronization mechanism to improve support for highly contested locks. The Symmetry architecture is described, and a number of lock algorithms and their use of hardware resources are examined. The performance of each lock is observed from the perspective of both the program itself and the total system performance. >