05-12-2012, 01:31 PM
Towards PetaScale simulations of turbulence in precipitating clouds
Towards PetaScale.ppt (Size: 4.8 MB / Downloads: 27)
Data decomposition in EULAG
2D horizontal domain grid decomposition
No decomposition in vertical Z-direction
Hallo/ghost cells for collecting information from neighbors
Predefined halo size for array memory allocation
Selective halo size for update to decrease overhead
Typical processors configuration
Computational 2D grid is mapped onto an 1D grid of processors
Neighboring processors exchange messages via MPI
Each processor know its position in physical space (column, row, boundaries) and location of neighbor processors
EULAG – Cartesian grid configuration
Parallel subdomians ALWAYS assume that grid has cyclic BC in both X and Y !!!
In Cartesian mode, the grid indexes are in range: 1…N, only N-1 are independent !!!
F(N)=F(1) –> periodicity enforcement
N may be even or odd number but it must be divided by number of processors in X
The same apply in Y direction.
BOTTLENECK – LOAD BALLANCING
Balanced work loads:
small imbalances result in many wasted processors! (e.g. 100,000 processors with one processor 5% over average workload equivalent to ~5000 idle processors)
No noticed balancing problems in Cartesian model
Unbalancing in spherical code during communication over the poles
Problem with grid partitioning in unstructured mesh model: proper criterion of efficient load balancing (e.g. geometric methods) vs workload of numerical algorithms used