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
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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
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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
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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
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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
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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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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
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This note was uploaded on 02/20/2008 for the course CSE 260 taught by Professor Baden during the Fall '06 term at UCSD.

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

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