Implementation of the SNAP Mini-App in Legion
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This is an implementation of the SNAP mini-application in Legion. This work was done by NVIDIA as part of the Fast Forward 2 project and was funded by the U.S. Department of Energy under subcontract B609478 with Lawrence Livermore National Security, LLC. The code is released under the Apache License, Version 2.0. A copy of the license can be found in this repository.

Several notes on the code for this implementation of SNAP.

  • This version of SNAP only implements the mini-KBA sweep algorithm for performing the computation. It also only supports 3D computations. The Legion version issues index space launches for each stage of a sweep for each energy group and direction. This allows Legion to extract task parallelism from the different sweeps. This proves especially useful for the way Legion performs the GPU computation. The GPU implementation launches a single CTA per sweep and energy group and relies on task parallelism to launch multiple kernels onto the GPU to keep all the SMs on the GPU busy. This is unorthodox, but allows for a more efficient implementation that can store per-angle fluxes in the register file as the CTA sweeps through cells.

  • The Legion style of this implementation is designed to illustrate how code should be generated from a higher-level compiler or written for a domain specific library, with good application-specific abstractions and multiple different task variants for each logical task. This allows the application to specialize itself for different target architectures. The downside is that the code can appear to be verbose. This is not an artifact of Legion, but instead of what needs to be done to make a code portable across many different architectures. In general you will notice that there are very few places where Legion shows up in this code. There are fewer than 100 Legion runtime calls which represents less than 2% of all the code in the application. All of the task variants are highly tuned so it is possible to accurately gauge the runtime overhead that is incurred.

  • The Legion version of SNAP also demonstrates the use of custom projection functions when performing index space launches for the different sweeps. The custom projection functions for SNAP end up projecting from three dimensional points for the sweeps down to two dimensional points for handling the per-angle fluxes being passed from previous cells as each sweep progresses.

  • This version of SNAP is the first real Legion application that relies heavily upon using predication to handle the dynamic convergence tests needed in SNAP. This implementation shows how to chain together predicates to perform the convergence tests. It also demonstrates how to use Legion futures and tasks to construct a monad for performing accurate timing of tasks in a deferred execution environment (similar to how monads in Haskell are needed to handle the laziness of the execution model). Users can leverage this as a template for handling other kinds of predicated computations that need to be done in a deferred execution model.

  • Included in this version of SNAP is a custom mapper that demonstrates how a mapper can be specialized for a particular application by overriding specific calls from the default mapper implementation. An interesting observation is that the implementation of the mapper calls for a custom mapper are considerably simpler than the default mapper implementations. The reason for this is that with an application specific mapper the implementation can be tailored directly to the application and know exactly what layouts and locations to use for physical instances.