User Manual

Rev 2.3-1.0.1
Mellanox Technologies
83
Note that the implementation of LASH in opensm attempts to use as few layers as possible. This
number can be less than the number of actual layers available.
In general LASH is a very flexible algorithm. It can, for example, reduce to Dimension Order
Routing in certain topologies, it is topology agnostic and fares well in the face of faults.
It has been shown that for both regular and irregular topologies, LASH outperforms Up/Down.
The
reason for this is that LASH distributes the traffic more evenly through a network, avoid-
ing the bottleneck issues related to a root node and always routes shortest-path.
The algorithm was developed by Simula Research Laboratory.
Use ‘-R lash -Q’ option to activate the LASH algorithm
QoS support has to be turned on in order that SL/VL mappings are used.
LMC > 0 is not supported by the LASH routing. If this is specified, the default routing
algorithm is invoked instead.
For open regular cartesian meshes the DOR algorithm is the ideal routing algorithm. For toroidal
meshes on the other hand there are routing loops that can cause deadlocks. LASH can be used to
route these cases. The performance of LASH can be improved by preconditioning the mesh in
cases where there are multiple links connecting switches and also in cases where the switches are
not cabled consistently. To invoke this, use '-R lash -Q --do_mesh_analysis'. This will add an
additional phase that analyses the mesh to try to determine the dimension and size of a mesh. If it
determines that the mesh looks like an open or closed cartesian mesh it reorders the ports in
dimension order before the rest of the LASH algorithm runs.
3.2.2.5.5 DOR Routing Algorithm
The Dimension Order Routing algorithm is based on the Min Hop algorithm and so uses short-
est paths. Instead of spreading traffic out across different paths with the same shortest distance, it
chooses
among the available shortest paths based on an ordering of dimensions. Each port must
be consistently cabled to represent a hypercube dimension or a mesh dimension. Paths are
grown from a destination back to a source using the lowest dimension (port) of available paths
at each step. This provides the ordering necessary to avoid deadlock. When there are multiple
links between any two switches, they still represent only one dimension and traffic is balanced
across them unless port equalization is turned off. In the case of hypercubes, the same port must
be used throughout the fabric to represent the hypercube dimension and match on both ends
of the cable. In the case of meshes, the dimension should consistently use the same pair of
ports, one port on one end of the cable, and the other port on the other end, continuing along the
mesh dimension.
Use ‘-R dor’ option to activate the DOR algorithm.
3.2.2.5.6 Torus-2QoS Routing Algorithm
Torus-2QoS is a routing algorithm designed for large-scale 2D/3D torus fabrics. The torus-2QoS
routing engine can provide the following functionality on a 2D/3D torus:
• Free of credit loops routing