Spatial/Temporal Locality-Based Load Sharing in Speculative Discrete Event Simulation on Multi-core Machines

Shared-memory multi-processor/multi-core machines have become a reference for many application contexts. In particular, the recent literature on speculative parallel discrete event simulation has reshuffled the architectural organization of simulation systems in order to deeply exploit the main features of this type of machine. A core aspect dealt with has been the full sharing of the workload at the level of individual simulation events, which enables keeping the rollback incidence minimal. However, making each worker thread continuously switch its execution between events destined to different simulation objects does not favor locality. This problem appears even more evident in the case of Non-Uniform-Memory-Access (NUMA) machines, in which memory accesses generating a cache miss to be served by a far NUMA node give rise to both higher latency and higher traffic at the level of the NUMA interconnection. In this article, we propose a workload-sharing algorithm in which the worker threads can have short-term binding with specific simulation objects to favor spatial locality. The new bindings-carried out when a thread decides to switch its execution to other simulation objects-are based on both (a) the timeline according to which the object states have passed through the caching hierarchy and (b) the (dynamic) placement of objects within the NUMA architecture. At the same time, our solution still enables the worker threads to focus their activities on the events to be processed whose timestamps are closer to the simulation commit horizon-hence, we exploit temporal locality along virtual time and keep the rollback incidence minimal. In our design, we exploit lock-free constructs to support scalable thread synchronization while accessing the shared event pool. Furthermore, we exploit a multi-view approach of the event pool content, which additionally favors local accesses to the parts of the event pool that are currently relevant for the thread activity. Our solution has been released as an integration within the USE (Ultimate-Share-Everything) open-source speculative simulation platform available to the community. Furthermore, in this article we report the results of an experimental study that shows the effectiveness of our proposal.