Lec17-Irregular problems continued

# Lec17-Irregular problems continued - Lecture 17 Irregular...

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Lecture 17 Irregular problems (II)

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11/17/06 Scott B. Baden / CSE 260 / Fall 2006 2 Announcements Come see my talk at the “Last Lecture Series” Wed 11/22 at 12Noon Gallery A, Price Center Free Pizza and Drinks Office hours on Weds 11/22 delayed to 2pm to 3pm Tuesday’s Lecture Be prepared to discuss the papers in class Slides will be available first thing Tuesday morning
11/17/06 Scott B. Baden / CSE 260 / Fall 2006 3 CSE 260 Symposium Scheduling changes Meet over 2 days Thursday 11/30, 1:30 to 3:20PM Friday 12/1, ~1:30 to 3:30 PM Format 15 minute presentation

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11/17/06 Scott B. Baden / CSE 260 / Fall 2006 4 Partitioning How do we divide up the computation and assign to processors? The process is called decomposition or partitioning Related issue: processor mapping
11/17/06 Scott B. Baden / CSE 260 / Fall 2006 5 Unstructured meshes “Unstructured” meshes are another type of non- uniformly spaced mesh Useful when the boundary or surface of the object is complicated Randy Bank, UCSD

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11/17/06 Scott B. Baden / CSE 260 / Fall 2006 6 A typical irregular mesh sweep loop float x[n_node], y[n_node] int E0[n_edge], E1[n_edge] for i = 1 : n_edge // Loop over all edges int n0 = E0[i] int n1 = E1[i] y[n0] += f(x[n0], x[n1]); y[n1] += g(x[n0], x[n1]); end for i=1 n0 n1
11/17/06 Scott B. Baden / CSE 260 / Fall 2006 7 Run time support for unstructured meshes Data dependences across processor boundaries Irregular communication patterns

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11/17/06 Scott B. Baden / CSE 260 / Fall 2006 9 Implementation issues Because the mesh does not have a regular structure, we must keep processor mapping information for each point in the mesh Compare with a uniform mesh, in which the mapping information is much coarser grained: mapping is handled at the level of a subdomain comprising many points
11/17/06 Scott B. Baden / CSE 260 / Fall 2006 10 Requirements We don’t know the assignment of nodes to processors until run time Assume that the mesh doesn’t change once initialized We need to include ghost cells, and want to keep the same loop structure We execute loops using local indices

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11/17/06 Scott B. Baden / CSE 260 / Fall 2006 11 Processor Mapping Invoke a partitioner to map global indices to (processor, local index) pairs
11/17/06 Scott B. Baden / CSE 260 / Fall 2006 12 Global translation Each global index is mapped to a processor and a local index • A global translation table expresses this mapping Global Local

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11/17/06 Scott B. Baden / CSE 260 / Fall 2006 13 Global translation Because we require mapping information for each individual mesh node, we cannot fit it in the memory of a single processor We must distribute it But, where is the mapping information located for a given global index?
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