Comparing Topology based Collective Communication Algorithms Vishal Sharda Ashima Gupta

Work progress No up-to-date open source solution available for fully mapping the network to build upon Nomad, Cheops, Nagios, ENV, Argus etc. outdated Use of a third-party service to determine the topology Java applet to map SERC network

Status of current study Nearly all the research on this problem till now considers tightly-coupled systems. Thus, most of the algorithms assume point- to-point connectivity. So, these have to be modified to adapt to the network of heterogeneous workstations.

All to all broadcast Also known as multinode broadcast Generalization of one-to-all broadcast in which all the processors simultaneously initiate a broadcast. Different processors may send out different message.

Existing Algorithms Direct exchange Circular all-to-all E1 algorithm Liquid Schedule Algorithms for specific topologies like start and mesh.

Direct Exchange Simplest approach Assumes point to point connectivity Each node simultaneously sends data to each other node Involves lot of congestion

Circular all-to-all Let p be the no. of nodes For each node i for step k in {1..p}, node i sends to (i+k)mod p receives from(i-k+P)mod p

E1 algorithm One node receives messages from all other nodes(becomes an expert) Experts are formed by recursively doubling existing experts.

Liquid schedule algorithm Traffic is the set of all collective exchanges. A simultaneous sub traffic is part of the traffic involing non-congesting transfers. Identify the bottleneck links in the network. A liquid schedule is such that all the bottleneck links are utilized in every sub- traffic.

Need for Simulation Deciding the packet to follow a particular route needs to bypass router decisions –Needs DLL operations. Focus on comparing the algorithms –Incorporate simulation-based study like standard experiments.

Basis of Simulation Execute and compare the algorithms for the network specifed at the interface. Some algorithms will take bandwidth of the links into account. Depending on the input, an algorithm may or may not show good results.

Implementation Approach Network represented as weighted undirected graph with weights inversely proportional to bandwidth. Simulating n processes on different nodes in a network with Java threads. First of all, each node will compute the shortest path to all other nodes using single source shortest path algorithm.

Handling Collision Lot of collision involved in all-to-all broadcast. Several approaches possible like partitioning into subnets, choosing alternate link. Our approach will be to stick to the shortest link and if collision detected then wait for random time and sense again.

References Papers : " ECO: Efficient Collective Operations for Communication on Heterogeneous Networks",Bruce B. Lowekamp and Adam Begueliny. "Network Topology Aware Scheduling of Collective Communications",Emin Gabrielyan, Roger D. Hersch. "On General Results for all-to-all broadcast", Ming- Syan Chen et. al “Efficient all-to-all broadcast in star graph interconnecion networks",Yu-Chee Tseng et.al Websites : freemap.qualys.com