Consistency Fences for Partial Order Delivery to Reduce Latency

For appropriate workloads, partially ordered message delivery can greatly reduce message latency. For example, updates to screens (e.g., remote desktops, VNC) may not have to be totally ordered with respect to different regions of the screen, but ordered with respect to updates to the same region. Similarly, updates to disjoint regions of a file (e.g., bulk-data transfer of sensor data) can be applied in any order, as long as updates (or reads) to the same region of the file are ordered in a consistent way, per data consistency models. Therefore, we introduce the concept of a consistency fence (CF), inspired by a memory fence from data consistency models, as a mechanism to control, specify, and reason about partial orders. If messages are lost on a network, partial ordering via CFs provides a framework to tolerate the latency associated with retransmission, for key workloads. In a set of simple experiments, based on screen update workloads, we show the latency benefits of partial ordering with CFs. We also show how forward error-correction (FEC) can be combined with CFs and partial ordering to reduce cumulative latency (represented as a cumulative distribution function), as compared to total ordering of messages.