4 Interpolation / Extrapolation Tool

It is often desirable to predict performance of non-existent machines, or across architectures. This section describes a tool that rewrites the log files produced by BigSim (also known as bgTrace trace logs) to provide new durations for portions of the application consisting of sequential execution blocks. These new durations can be based upon multiple types of models. The tool can be easily modified to add new types of models if the user requires. The models can be generated from full or partial executions of an application on an existing processor or on a cycle-accurate simulator.

When predicting the runtime of a parallel application on a not-yet-existent parallel platform, there are two important concerns. The first is correctly modeling the interconnection network, which is handled by BigSimulator (also called BigNetSim). The second is determining the durations of the relevant sequential portions of code, which we call Sequential Execution Blocks (SEB), on a new type of processor. The interpolation tool of this section handles only the prediction of SEB durations, using currently three types of implemented models:

1. Scaling of SEB durations observed on an available (existing) processor, via multiplcation of the original durations by a constant factor.
2. Parameterizations of SEBs: each SEB is augmented with user-defined parameters that influence the duration of the SEB. An extrapolation model based on those parameters can predict the durations of SEBs not instrumented in the initial emulation run.
3. Parameterizations with cycle-accurate simulations for non-existent architectures: processor designers use cycle-accurate simulators to simulate the performance of a piece of code on a future processor that is currently unavailable. Timings for each SEB can be estimated in such a cycle-accurate simulator. The cycle-accurate timings can be extrapolated to predict the durations of SEBs not instrumented in the cycle-accurate simulator.

This tool will soon include a new model with support for performance counters. The currently available tool rewrites the log files produced by a run in the BigSim Emulator. The rewritten log files can then be consumed by BigSimulator. This usage flow can be seen in Figure 1, showing that multiple types of models are supported in the tool.

Figure 1: Flow diagram for use of the interpolation tool