The mrnesbits package (Multi-Resolution Network Emulation and Simulation for BITS) is used to model applications that execute on a mrnes network model. At the most basic level, an application is compromised of elements (called ‘functions’) that receive messages, execute some computation based on the message and its source, and optionally send messages in response to other functions. mrnesbits model syntax defines the topological structure of these applications (called “Computational Patterns”), describes how messages flow between functions, the function’s individual states, and references methods embedded in the simulator that are called to modify those states as messages are processed.
A foundational principle for mrnesbits models is that the simulation time associated with the execution of a function come from a look-up table that holds multiple measurements of that execution time on different CPUs, and under other model dependent parameters (e.g. number of bytes in a message being processed by the function). Another foundational principle is that mrnesbits allow for a large degree of flexibility and specialization in modeling the behavior of modeled software that calls these measured functions. It does so by organizing the flow of control of a simulation at a level of abstraction and through an API that allows for different ‘class’ dependent behavior of functions. Control of function behavior with associated virtual time delays explicitly supports representation of the impact of queuing and contention for processor and communication network resources, so that evaluation of mrnesbits models can capture the impact of congestion at different levels of the system.
mrnes and mrnesbits models are built using the discrete-event simulation paradigm, and use functionality given by the vrtime,evtq, and evtm packages. The models are explicitly event-oriented (as opposed to “process” oriented models that use discrete-event formulations under the covers, not explicitly viewable by the modeler). The simulation clock uses 64 bit integers for time ’ticks’, and 64 bit integers for tie- breaking events whose number of clock ticks are precisely the same. The vrtime package allows one to define the time duration of a tick to be whatever time-scale is of interest to the modeler; for our models of computer and communications activities we typically define a tick to represent 1/10 of a nanosecond.