Concurrency Control in Asynchronous Computations

Abstract

When independently executing processes share data, some form of concurrency control is needed to enforce the atomicity and sequencing constraints imposed by the program. We believe that concurrency control is hard largely because existing architectural support is inadequate. We define a new class of interconnection networks called isotach networks and explore isotach-based concurrency control by describing techniques that use the isotach network to achieve causal message delivery, atomicity, sequential consistency, and cache coherence.We show processes can pipeline their accesses to shared data in an isotach system without sacrificing sequential consistency. We define the isochron, a multicast with strong ordering properties implemented on an isotach network, and describe techniques by which processes can use isochrons to execute atomic actions without obtaining locks or other exclusive access rights. We describe compatible techniques for enforcing data dependences and show that the isotach network is capable of combining concurrently issued operations on the same shared variable while maintaining atomicity and sequential consistency. We describe a family of new cache coherence protocols called delta-cache protocols that extend isotach-based concurrency control techniques to systems that replicate and migrate shared variables, yielding protocols that are both scalable and more highly concurrent than existing protocols. We report results of a simulation study comparing the performance of isotach and conventional systems. The study shows that isotach systems outperform conventional systems under workloads with atomicity and sequencing constraints.

Delta coherence protocols

“…1 However, we can implement atomic actions with internal data dependences, using isochrons together with a class of operations called split operations. 10 Adapting the protocol to enforce atomicity requires changing only the scheduling algorithm. Each CC schedules requests so that all requests in the same isochron have the same effective execution pulse, and schedules each isochron so that it has an effective execution pulse no less than the previously scheduled isochron.…”

Logical Time Coherence Maintenance

“…Section 5.7 develops correctness criteria for these coherence protocols and presents the static owner update protocol, in which the owner copy is a distinguished copy that services misses and distributes updates. This protocol extends a previously defined isotach-based coherence protocol [Wil93] to non-equidistant networks.…”

“…They allow processes to control the logical times at which the messages they send are received and, in some cases, the logical receive times of response messages as well. Our thesis is that the theory of isotach systems, first given by Williams [Wil93], can be extended to increase the flexibility of isotach systems in ways that allow them to serve a wider range of networks and applications.…”

“…Isochrons are flat atomic actions in fault free systems. Although isochrons are not as powerful as structured atomic actions, isochrons can execute structured atomic actions when used with split operations [Wil93].…”

“…Williams developed two delta update coherence protocols for equidistant networks [Wil93]. In addition to developing several new members of this protocol family, we combine her protocols and extend them to non-equidistant networks.…”