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Limited Multi-path Routing on Extended Generalized Fat-trees

Limited Multi-path Routing on Extended Generalized Fat-trees. Santosh Mahapatra, Xin Yuan, and Wickus Nienaber Department of Computer Science, Florida State University. Fat-tree. Fat-tree or folded-Clos topologies have been widely used in HPC environments.

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Limited Multi-path Routing on Extended Generalized Fat-trees

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  1. Limited Multi-path Routing on Extended Generalized Fat-trees • Santosh Mahapatra, Xin Yuan, and Wickus Nienaber • Department of Computer Science, • Florida State University

  2. Fat-tree • Fat-tree or folded-Clos topologies have been widely used in HPC environments. • Such interconnects are often treated as central switches • Large bisection bandwidth • Many paths between two end nodes • Most practical fat-trees can be described by the extended generalized fat-tree (XGFT).

  3. Extended generalized fat-trees

  4. Routing on XGFT • Single-path routing • One path between each pair of end nodes. • D-mod-k, S-mod-k • Multi-path routing • Better load balancing than single path routing • Existing schemes consider all possible paths for multi-path routing (unlimited multipath routing) • The number of paths between two nodes is large • May not be possible due to resource constraints

  5. Limited Multi-path routing on XGFT • Multi-path routing with a limit on the number of paths between each pair of nodes • Single path routing and traditional multipath routing are special cases of limited multi-path routing • Limited multi-path routing may combine the advantages of single-path routing and unlimited multi-path routing

  6. Issues in limited multi-path routing • Path calculation • Single-path routing on fat-tree is fairly well understood • How to select K paths out of M potential paths (to optimize route objectives) – we will call such a limited multi-path routing K-path routing. • Performance • How well the routing performs with 2, 3, 4, …, paths between each pair of nodes?

  7. Path calculation for K-path routing: random • In a fat-tree, all shortest paths between any two end nodes can be easily enumerated. • Random K-path routing • Randomly pick K paths from all possible paths • Previous study has shown that when K=1, random routing is not as good as D-mod-k or S-mod-k.

  8. Path calculation for K-path routing: Shift-K • Base on D-mod-K routing • D-mod-k routing offers high performance for single path routing • Shift-K scheme tries to create K copies of D-mod-K routing to achieve good load balancing. • Enumerate paths between any two nodes and number them based on the number of top level switch • For (src, dst), if D-mod-K says path i, shift-K will use paths (i+j) mod M, j = 0, 1, …, K-1.

  9. Shift-K example 6 7 5 4 3 2 1 0 D-mod-k says path 7 for 063 Shift-K routing for 3 paths: path 7, path 0, and path 1

  10. Shift-K problem 6 7 5 4 3 2 1 0 Links at lower level is still not balanced: Shift-K routing for 3 paths: path 7, path 0, and path 1 link 6480 is used by both path 0 and 1.

  11. disjoint-k routing 6 7 5 4 3 2 1 0 • Still based on D-mod-k, shift such that paths are as disjoint • as possible: • disjoint-K routing for 3 paths: • path 7, path 1 and path 3

  12. Results • Flow-level and flit-level simulation • Traffic pattern: permutation traffic and random uniform traffic • Performance matrices: • Flow level: maximum link load • Flit level: packet latency and throughput

  13. Results: average maximum link load for permutation traffic

  14. Average maximum link load for permutation traffic

  15. maximum aggregate throughputXGFT(3;4,4,8;1,4,4), random uniform

  16. Average message delay throughputXGFT(3;4,4,8;1,4,4), random uniform

  17. Conclusion • We investigate limited multi-path routing schemes, propose three methods for path calculation, and evaluate the performance. • Limited multi-path routing significantly reduces the latency and improve throughput in comparison to single path routing • Path calculation methods can make significant impact on the performance • The proposed disjoint path method is shown to have good performance.

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