Estimated Simulation Resource Requirements¶
Estimating problem sizes for most Enzo calculations is at best an inexact science, given the nature of Adaptive Mesh Refinement (AMR) simulations. The fundamental issue with an AMR calculation in cosmology or in many astrophysical situations where gravitational collapse is important has to do with memory. The amount of memory used at the beginning of the simulation (when you have a single grid or a handful of grids) is far, far less than the memory consumption at the end of the simulation, when there can be hundreds of grids per processor. The amount of memory required can easily grow by an order of magnitude over the course of a cosmological simulation, so it is very important to make sure to take this into account to ensure that enough memory is available in later stages of your simulation. It is also important to realize that in general one should try to keep the largest amount of data per processing core that you can so that individual cores are never data-starved. Data-starved processing units cause poor scaling, as your CPUs will then be sitting idle while waiting for data from other computing nodes. Computational fluid dynamics simulations are notoriously communication-heavy, making this a challenging corner of parameter space to operate in.
This page contains some rules of thumb that will help you along your way, based on data collected up to the release of Enzo v1.5 (so up to Fall 2008), when supercomputers typically have 1GB-2GB of memory per processing unit (a dual-processor node with two cores per processor would have 4-8 GB of memory, for example).
Cosmology or non-cosmology unigrid (non-AMR) simulations¶
These are actually quite straightforward to predict, given that in a unigrid simulation the grid is partitioned up in an approximately equal fashion and then left alone. Experimentation shows that, for machines with 1-2 GB of memory per core, one gets near-ideal scaling with 1283 cells per core (so a 5123cell calculations should be run on 64 processors, and a 10243 cell run should be done on 512 processors). This is comfortably within memory limits for non-cosmology runs, and there is no danger of running up against a node’s memory ceiling (which causes tremendous slowdown, if not outright program failure). Unigrid cosmology runs have a further complication due to the dark matter particles - these move around in space, and thus move from processor to processor. Areas where halos and other cosmological structures form will correspond to regions with greater than average memory consumption. Keeping 1283cells and particles per core seems to scale extremely efficiently up to thousands of processors, though if one is using a machine like an IBM Blue Gene, which typically has far less memory per core than other computers, one might have to go to 643 cells/particles per core so that nodes corresponding to dense regions of the universe don’t run out of memory.
Cosmology adaptive mesh simulations¶
Scaling and problem size is much more difficult to predict for an AMR cosmology run than for its unigrid equivalent. As discussed above, the amount of memory consumed can grow strongly over time. For example, a 5123 root grid simulation with seven levels of adaptive mesh refinement started out with 512 root grid tiles, and ended up with over 400,000 grids! This calculation was run on 512 processors, though memory consumption grew to the point that it had to be run on a system where half of the cores per node were kept this particle mass field over these processors. For each grid, only processors with particles contribute to this sum to reduce the amount of computation and communication. In short, this routine performs a non-blocking MPI_SUM over a select number of processors.
CommunicationCollectParticles(SUBGRIDS_LOCAL) – This routine replaces grid::MoveSubgridParticlesFast(). It keeps the particles on the same processor, but this doesn’t matter here because the children grids are always created on the same processor as its parent and then moved to another processor during load balancing. CommunicationCollectParticles(SIBLINGS_ONLY) – After load balancing is complete on level Lsub, we can safely move the particles to their host processor without the worry of running out of memory.