Actas JP2011 - Universidad de La Laguna

Actas XXII Jornadas de Paralelismo (JP2011) , La Laguna, Tenerife, 7-9 septiembre 2011and focus the description of the mechanism and themapping tool. Then, we describe the LBDRx mechanismto cover practical topologies from SoC designs.In Section III we describe the mapping tool. Finally,in Section IV we provide evaluation results and concludethe paper in Section V.II. LBDRx descriptionThe description of the proposed LBDRx mechanismwill be presented as an evolution from the basicLBDR mechanism [8] previously proposed (withlow coverage for complex irregular topologies) to themost enhanced version (with full coverage to all thecomplex topologies analyzed).A. Preliminary: Basic IdeaIn regular networks, e.g. a 2D-mesh network, theregular connectivity pattern is useful when designingthe routing algorithm. Indeed, with the Dimension-Order routing (DOR), the implementation is quitestraightforward as messages are forwarded with minimalpaths first in the X direction and then in theY direction. Thus, there is no need for a routing table,only a set of gates is enough. This renders to anefficient implementation of the routing algorithm interms of area, power, and delay.If we consider small irregularities on 2D-mesh networks,for instance due to manufacturing defects,then the inherent irregularity complicates the routingimplementation. For instance, DOR is no longervalid as some paths are not possible now. However,other routing algorithms are still suitable for suchtopologies, for instance, topology-agnostic routingalgorithms like up*/down* [12]. Their implementationis usually performed with routing tables. Effortsto provide efficient implementations of such algorithmsin those irregular topologies have been performedin the recent years.One important method is LBDR, which collapsesall the routing information required on every switchon a small set of bits, thus reducing significantly implementationcosts. LBDR still relies on the factthat the topology is a 2D-mesh network but withsome missing links. Adding some bits enables LBDRto successfully deal with the irregularity induced bymissing links. However, LBDR still relies on the factthat every switch has at most four links connectingneighboring switches (at North, East, West, or Southdirections). LBDR uses, as DOR does, the coordinatesof the destination switch in the message andthe coordinates of the current switch, to computethe appropriate set of output ports. Thus, LBDRstill benefits from the original 2D-mesh layout.In this paper, what we propose is the extensionand applicability of the LBDR concept to trulyapplication-specific and irregular networks (as an examplesee Figure 1). The approach we follow is tomap the topology into a 2D grid (notice, however, wedo not change the initial topology). Once the topologyis mapped, we need to provide coordinates to everyswitch in the network. Based on the coordinatesof the destination switch and the current switch, thederived LBDR logic will decide the output port thatneeds to be used to forward the packet towards itsdestination. In order to correctly map the topologyinto a 2D grid, we have developed a mapping algorithmthat will search the space of combinations andwill deliver the most suitable ones, always guaranteeingdeadlock freedom and connectivity.Due to the mapping performed, and because of thehigh irregularity we will find, some switches will requirea varying number of ports to connect to otherswitches, and in that situation some links will connectswitches not placed closely in the 2D grid. Thiskind of connectivity has not been provided by theoriginal LBDR mechanism, and thus, requires modifications.In this paper, we further extend LBDR forits support in this kind of mappings.B. LBDR Extension: LBDRxWe start the description with the mechanism requiredat every switch to deal with the irregulartopologies. In order to be concise, we take as a referencethe mapping of the initial topology (Figure 1)that appears in Figure 2. This mapping is obtainedwith the mapping tool that will be described in thenext section. The mapping is representative of allthe connectivity patterns between switches that weneed to address in this section.Fig. 2.Mapping example for the initial topology.As we can see in the figure, there are switches withvarying connectivity patterns with other switches.For instance, switch 1, mapped at row 2 and column2, is connected to switches 4, 2, and 21 with differentlink mapped lengths. In particular, mapped length oflinks are 2 hops for links connecting to switches 2 and4, and 3 hops for the link connecting to switch 21.In addition, links with the same number of mappedhops have different orientations/directions, thus, beingdifferent. This is the case for link connecting toswitch 4 which is located one hop north and one hopwest from switch 1, and link connecting to switch 2that is two hops north.As previously described, LBDR relies on switcheswith up to four ports, and each one connectingswitches in one direction in the 2D mesh plane (N,JP2011-676