Efficient Run time Short for the Irregular Block Structured | CMSC 714, Study notes of Computer Science

Download Efficient Run time Short for the Irregular Block Structured | CMSC 714 and more Study notes Computer Science in PDF only on Docsity! Efficient Run-time Support for Irregular Block-Structured Applications By Stephen J. Fink and Scott B. Baden and Scott R. Kohn Presented for your delectation by Asad B. Sayeed Background ● Main type of application considered: scientific numerical methods. ● These applications often use structured irregular representations to improve accuracy. – Difficult to implement. – Cause unpredictable/irregular communication patterns, impeding performance optimization. ● Goal: assist programmer in arranging parallelism so the data layout and distribution best exploit memory arrangements. Programming Model ● Programs begin with a single (logical) control thread. ● for_all loop iterations: each one executes independently on one SPMD process. ● Storage model: distribute each block of data to its own logical address space, one space per processor. ● Little compiler automation, even for consistency. – Programmer explicitly describes data decomposition and also data motion (via block copy operations). Data Layout Abstractions ● Four core data decompositions abstractions: Point, Region, Grid, XArray, inherited from KeLP's predecessor LPARX. KeLP innovation: FloorPlans. – Point: represents point in n-dim space. – Region: rectangular subset of Points.. – Grid: array of data indexed by Region. – XArray : array of Grids of different (irregular) shape. – FloorPlans: array of Regions representing processor assignments for XArray. Data Layout Abstractions ● Regions are constructed by Region calculus. ● XArrays and FloorPlans: ata Layout and Data Motion Summary of classes: Table 1 A Brief Synopsis of the KeLP Data Types Geometric structural abstractions Name Definition Interpretation PointD {int Jp, int 4, ... , int dp) A point in 2” RegionD {PointD /, PointD A} A rectangular subset of 7° FloorPlanD Array of (RegionD 2, int p) A set of regions, each assigned to a processor p MotionPlanD List of ((int f, RegionD R}, Block-structured communication (int ¢, RegionD R;})} pattern between two FloorPlans Data types that interpret abstractions Name Description GridD A multidimensional array whose index space is a RegionD XArrayD An array of GridDs; structure represented by a FloorPlanD MoverD Object that atomically performs the data motion pattern described by a MotionPlan Simple Data Motion Example ● fillpatch: fills in ghost cells from logically overlapping grids. ● Code and example of irregular XArray. Bigger Ghost Cells Example ● Elliptic PDE solver: ● Region2, etc are 2D arrays. Implementation Issues ● KeLP predecessor: LPARX. – LPARX allowed asynchronous one-sided communication: creates barriers for process state global synchronization. – KeLP: bans copy operations from for_all loops, eliminating this problem; ie, only for_1 loops perform copies. Each process stores relevant portion of movement plan. ● Mover: implemented via nonblocking MPI send – In and out buffers allocated to each process. – Receives data while it waits for sends to finish. Performance ● Comparison to MPI. – Three benchmarks involving heavy matrix computation: NAS-FT, NAS-MG, SUMMA. – Very conservatively translated to KeLP from MPI. Performance ● Adaptive multigrid: lda3d – Eigenvalue solver from “ab initio” materials science. – Highly irregular communication.