User Manual

OpenSM – Subnet ManagerRev 2.1-1.0.6
Mellanox Technologies
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In more detail, the algorithm works as follows:
1. LASH determines the shortest-path between all pairs of source / destination switches. Note,
LASH ensures the same SL is used for all SRC/DST - DST/SRC pairs and there is no guar
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antee that the return path for a given DST/SRC will be the reverse of the route SRC/DST.
2. LASH then begins an SL assignment process where a route is assigned to a layer (SL) if the
addition of that route does not cause deadlock within that layer. This is achieved by main
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taining and analysing a channel dependency graph for each layer. Once the potential addition
of a path could lead to deadlock, LASH opens a new layer and continues the process.
3. Once this stage has been completed, it is highly likely that the first layers processed will
contain more paths than the latter ones. To better balance the use of layers, LASH moves
paths from one layer to another so that the number of paths in each layer averages out.
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
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.
LASH analyzes routes and ensures deadlock freedom between switch pairs. The link
from HCA between and switch does not need virtual layers as deadlock will not arise
between switch and HCA.
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.