# Running Large Simulations¶

Here we describe how to efficiently run a large simulation on a high number of processors, such as particular parameters to set and suggested number of MPI tasks for a given problem size. For a problem to be scalable, most of the code must be parallel to achieve high performance numbers on large MPI process counts (see Amdahl’s Law). In general, the user wants to pick the number of processors so that computation is still dominant over communication time. If the processor count is too high, communication time will become too large and might even slow down the simulation!

For picking the number of processors for an Enzo run, a good starting point is putting a 643 box on each processor for both AMR and unigrid setups. For example, a 2563 simulation would run well on (256/64)3 = 64 processors. For nested grid simulations, the outer boxes usually require little computation compared to the “zoom-in” region, so the processor count should be based on the inner-most nested grid size. The user can experiment with increasing the processor count from this suggestion, but strong scaling (i.e. linear speedup with processor count) is not to be expected. Little performance gains (as of v2.0) can be expected beyond assigning a 323 cube per processor.

Note

The level-0 grid is only partitioned during the problem initialization. It will never be re-partitioned if the user restarts with a different number of processors. However, some performance gains can be expected even if a processor does not contain a level-0 grid because of the work on finer levels.

## Important Parameters¶

• LoadBalancing: Default is 1, which moves work from overloaded to underutilized processes, regardless of the grid position. New for v2.1: In some cases but not always, speedups can be found in load balancing on a space filling curve (LoadBalancing = 4). Here the grids on each processor will be continuous on the space filling curve. This results in a grouped set of grids, requiring less communication from other processors (and even other compute nodes).
• SubgridSizeAutoAdjust and OptimalSubgridsPerProcessor: New for v2.1 Default is ON and 16, respectively. The maximum subgrid size and edge length will be dynamically adjusted on each AMR level according to the number of cells on the level and number of processors. The basic idea behind increasing the subgrid sizes (i.e. coalescing grids) is to reduce communication between grids.
• MinimumSubgridEdge and MaximumSubgridSize: Unused if SubgridAutoAdjust is ON. Increase both of these parameters to increase the average subgrid size, which might reduce communication and speedup the simulation.
• UnigridTranspose: Default is 0, which is employs blocking MPI communication to transpose the root grid before and after the FFT. In level-0 grids >= 10243, this becomes the most expense part of the calculation. In these types of large runs, Option 2 is recommended, which uses non-blocking MPI calls; however it has some additional memory overhead, which is the reason it is not used by default.

## Compile-time options¶

• max-subgrids: If the number of subgrids in a single AMR level exceeds this value, then the simulation will crash. Increase as necessary. Default: 100,000
• ooc-boundary-yes: Stores the boundary conditions out of core, i.e. on disk. Otherwise, each processor contains a complete copy of the external boundary conditions. This becomes useful in runs with large level-0 grids. For instance in a 10243 simulation with 16 baryon fields, each processor will contain a set of boundary conditions on 6 faces of 10242 with 16 baryon fields. In single precision, this requires 402MB! Default: OFF
• fastsib-yes: Uses a chaining mesh to help locate sibling grids when constructing the boundary conditions. Default: ON
• log2alloc-yes: All arrays created with new are allocated with sizes that are the nearest power of 2. This will result in a roughly 20% increase in overall memory usage, but in some cases, should reduce memory fragmentation. If you are having problems with memory fragmentation, consider enabling this. Default: OFF