opensm is an InfiniBand compliant Subnet Manager and Administration, and runs on top of OpenIB.
opensm provides an implementation of an InfiniBand Subnet Manager and Administration. Such a software entity is required to run for in order to initialize the InfiniBand hardware (at least one per each InfiniBand subnet).
opensm also now contains an experimental version of a performance manager as well.
opensm defaults were designed to meet the common case usage on clusters with up to a few hundred nodes. Thus, in this default mode, opensm will scan the IB fabric, initialize it, and sweep occasionally for changes.
opensm attaches to a specific IB port on the local machine and configures only the fabric connected to it. (If the local machine has other IB ports, opensm will ignore the fabrics connected to those other ports). If no port is specified, it will select the first "best" available port.
opensm can present the available ports and prompt for a port number to attach to.
By default, the run is logged to two files: /var/log/messages and /var/log/opensm.log. The first file will register only general major events, whereas the second will include details of reported errors. All errors reported in this second file should be treated as indicators of IB fabric health issues. (Note that when a fatal and non-recoverable error occurs, opensm will exit.) Both log files should include the message "SUBNET UP" if opensm was able to setup the subnet correctly.
BIT LOG LEVEL ENABLED
0x01 - ERROR (error messages)
0x02 - INFO (basic messages, low volume)
0x04 - VERBOSE (interesting stuff, moderate volume)
0x08 - DEBUG (diagnostic, high volume)
0x10 - FUNCS (function entry/exit, very high volume)
0x20 - FRAMES (dumps all SMP and GMP frames)
0x40 - ROUTING (dump FDB routing information)
0x80 - currently unused.
Without -D, OpenSM defaults to ERROR + INFO (0x3). Specifying -D 0 disables all messages. Specifying -D 0xFF enables all messages (see -V). High verbosity levels may require increasing the transaction timeout with the -t option.
-d0 - Ignore other SM nodes
-d1 - Force single threaded dispatching
-d2 - Force log flushing after each log message
-d3 - Disable multicast support
The following environment variables control opensm behavior:
OSM_TMP_DIR - controls the directory in which the temporary files generated by opensm are created. These files are: opensm-subnet.lst, opensm.fdbs, and opensm.mcfdbs. By default, this directory is /var/log.
OSM_CACHE_DIR - opensm stores certain data to the disk such that subsequent runs are consistent. The default directory used is /var/cache/opensm. The following file is included in it:
guid2lid - stores the LID range assigned to each GUID
When opensm receives a HUP signal, it starts a new heavy sweep as if a trap was received or a topology change was found.
Also, SIGUSR1 can be used to trigger a reopen of /var/log/opensm.log for logrotate purposes.
The default name of OpenSM partitions configuration file is /etc/opensm/partitions.conf. The default may be changed by using --Pconfig (-P) option with OpenSM.
The default partition will be created by OpenSM unconditionally even when partition configuration file does not exist or cannot be accessed.
The default partition has P_Key value 0x7fff. OpenSM's port will have full membership in default partition. All other end ports will have partial membership.
Line content followed after '#' character is comment and ignored by parser.
General file format:
<Partition Definition>:<PortGUIDs list> ;
PartitionName - string, will be used with logging. When omitted
empty string will be used.
PKey - P_Key value for this partition. Only low 15 bits will
be used. When omitted will be autogenerated.
flag - used to indicate IPoIB capability of this partition.
defmember=full|limited - specifies default membership for port guid
list. Default is limited.
Currently recognized flags are:
ipoib - indicates that this partition may be used for IPoIB, as
result IPoIB capable MC group will be created.
rate=<val> - specifies rate for this IPoIB MC group
(default is 3 (10GBps))
mtu=<val> - specifies MTU for this IPoIB MC group
(default is 4 (2048))
sl=<val> - specifies SL for this IPoIB MC group
(default is 0)
scope=<val> - specifies scope for this IPoIB MC group
(default is 2 (link local)). Multiple scope settings
are permitted for a partition.
Note that values for rate, mtu, and scope should be specified as defined in the IBTA specification (for example, mtu=4 for 2048).
PortGUID - GUID of partition member EndPort. Hexadecimal
numbers should start from 0x, decimal numbers
are accepted too.
full or limited - indicates full or limited membership for this
port. When omitted (or unrecognized) limited
membership is assumed.
There are two useful keywords for PortGUID definition:
- 'ALL' means all end ports in this subnet.
- 'SELF' means subnet manager's port.
Empty list means no ports in this partition.
White space is permitted between delimiters ('=', ',',':',';').
The line can be wrapped after ':' followed after Partition Definition and between.
PartitionName does not need to be unique, PKey does need to be unique. If PKey is repeated then those partition configurations will be merged and first PartitionName will be used (see also next note).
It is possible to split partition configuration in more than one definition, but then PKey should be explicitly specified (otherwise different PKey values will be generated for those definitions).
Default=0x7fff : ALL, SELF=full ;
NewPartition , ipoib : 0x123456=full, 0x3456789034=limi, 0x2134af2306 ;
YetAnotherOne = 0x300 : SELF=full ;
YetAnotherOne = 0x300 : ALL=limited ;
ShareIO = 0x80 , defmember=full : 0x123451, 0x123452;
# 0x123453, 0x123454 will be limited
ShareIO = 0x80 : 0x123453, 0x123454, 0x123455=full;
# 0x123456, 0x123457 will be limited
ShareIO = 0x80 : defmember=limited : 0x123456, 0x123457, 0x123458=full;
ShareIO = 0x80 , defmember=full : 0x123459, 0x12345a;
ShareIO = 0x80 , defmember=full : 0x12345b, 0x12345c=limited, 0x12345d;
The following rule is equivalent to how OpenSM used to run prior to the partition manager:
There are a set of QoS related low-level configuration parameters. All these parameter names are prefixed by "qos_" string. Here is a full list of these parameters:
qos_max_vls - The maximum number of VLs that will be on the subnet
qos_high_limit - The limit of High Priority component of VL
Arbitration table (IBA 7.6.9)
qos_vlarb_low - Low priority VL Arbitration table (IBA 7.6.9)
qos_vlarb_high - High priority VL Arbitration table (IBA 7.6.9)
Both VL arbitration templates are pairs of
VL and weight
qos_sl2vl - SL2VL Mapping table (IBA 7.6.6) template. It is
a list of VLs corresponding to SLs 0-15 (Note
that VL15 used here means drop this SL)
Typical default values (hard-coded in OpenSM initialization) are:
The syntax is compatible with rest of OpenSM configuration options and values may be stored in OpenSM config file (cached options file).
In addition to the above, we may define separate QoS configuration parameters sets for various target types. As targets, we currently support CAs, routers, switch external ports, and switch's enhanced port 0. The names of such specialized parameters are prefixed by "qos_<type>_" string. Here is a full list of the currently supported sets:
qos_ca_ - QoS configuration parameters set for CAs.
qos_rtr_ - parameters set for routers.
qos_sw0_ - parameters set for switches' port 0.
qos_swe_ - parameters set for switches' external ports.
Prefix routes control how the SA responds to path record queries for off-subnet DGIDs. By default, the SA fails such queries. Note that IBA does not specify how the SA should obtain off-subnet path record information. The prefix routes configuration is meant as a stop-gap until the specification is completed.
Each line in the configuration file is a 64-bit prefix followed by a 64-bit GUID, separated by white space. The GUID specifies the router port on the local subnet that will handle the prefix. Blank lines are ignored, as is anything between a # character and the end of the line. The prefix and GUID are both in hex, the leading 0x is optional. Either, or both, can be wild-carded by specifying an asterisk instead of an explicit prefix or GUID.
When responding to a path record query for an off-subnet DGID, opensm searches for the first prefix match in the configuration file. Therefore, the order of the lines in the configuration file is important: a wild-carded prefix at the beginning of the configuration file renders all subsequent lines useless. If there is no match, then opensm fails the query. It is legal to repeat prefixes in the configuration file, opensm will return the path to the first available matching router. A configuration file with a single line where both prefix and GUID are wild-carded means that a path record query specifying any off-subnet DGID should return a path to the first available router. This configuration yields the same behaviour formerly achieved by compiling opensm with -DROUTER_EXP.
OpenSM now offers five routing engines:
1. Min Hop Algorithm - based on the minimum hops to each node where the path length is optimized.
2. UPDN Unicast routing algorithm - also based on the minimum hops to each node, but it is constrained to ranking rules. This algorithm should be chosen if the subnet is not a pure Fat Tree, and deadlock may occur due to a loop in the subnet.
3. Fat Tree Unicast routing algorithm - this algorithm optimizes routing for congestion-free "shift" communication pattern. It should be chosen if a subnet is a symmetrical or almost symmetrical fat-tree of various types, not just K-ary-N-Trees: non-constant K, not fully staffed, any Constant Bisectional Bandwidth (CBB) ratio. Similar to UPDN, Fat Tree routing is constrained to ranking rules.
4. LASH unicast routing algorithm - uses Infiniband virtual layers (SL) to provide deadlock-free shortest-path routing while also distributing the paths between layers. LASH is an alternative deadlock-free topology-agnostic routing algorithm to the non-minimal UPDN algorithm avoiding the use of a potentially congested root node.
5. DOR Unicast routing algorithm - based on the Min Hop algorithm, but avoids port equalization except for redundant links between the same two switches. This provides deadlock free routes for hypercubes when the fabric is cabled as a hypercube and for meshes when cabled as a mesh (see details below).
OpenSM also supports a file method which can load routes from a table. See 'Modular Routing Engine' for more information on this.
The basic routing algorithm is comprised of two stages:
1. MinHop matrix calculation
How many hops are required to get from each port to each LID ?
The algorithm to fill these tables is different if you run standard (min hop) or Up/Down.
For standard routing, a "relaxation" algorithm is used to propagate min hop from every destination LID through neighbor switches
For Up/Down routing, a BFS from every target is used. The BFS tracks link direction (up or down) and avoid steps that will perform up after a down step was used.
2. Once MinHop matrices exist, each switch is visited and for each target LID a
decision is made as to what port should be used to get to that LID.
This step is common to standard and Up/Down routing. Each port has a counter counting the number of target LIDs going through it.
When there are multiple alternative ports with same MinHop to a LID, the one with less previously assigned ports is selected.
If LMC > 0, more checks are added: Within each group of LIDs assigned to same target port,
a. use only ports which have same MinHop
b. first prefer the ones that go to different systemImageGuid (then the previous LID of the same LMC group)
c. if none - prefer those which go through another NodeGuid
d. fall back to the number of paths method (if all go to same node).
Effect of Topology Changes
OpenSM will preserve existing routing in any case where there is no change in the fabric switches unless the -r (--reassign_lids) option is specified.
This option causes OpenSM to reassign LIDs to all
end nodes. Specifying -r on a running subnet
may disrupt subnet traffic.
Without -r, OpenSM attempts to preserve existing
LID assignments resolving multiple use of same LID.
If a link is added or removed, OpenSM does not recalculate the routes that do not have to change. A route has to change if the port is no longer UP or no longer the MinHop. When routing changes are performed, the same algorithm for balancing the routes is invoked.
In the case of using the file based routing, any topology changes are currently ignored The 'file' routing engine just loads the LFTs from the file specified, with no reaction to real topology. Obviously, this will not be able to recheck LIDs (by GUID) for disconnected nodes, and LFTs for non-existent switches will be skipped. Multicast is not affected by 'file' routing engine (this uses min hop tables).
Min Hop Algorithm
The Min Hop algorithm is invoked by default if no routing algorithm is specified. It can also be invoked by specifying '-R minhop'.
The Min Hop algorithm is divided into two stages: computation of min-hop tables on every switch and LFT output port assignment. Link subscription is also equalized with the ability to override based on port GUID. The latter is supplied by:
This option provides the means to define a set of ports
(by guid) that will be ignored by the link load
equalization algorithm. Note that only endports (CA,
switch port 0, and router ports) and not switch external
ports are supported.
LMC awareness routes based on (remote) system or switch basis.
Purpose of UPDN Algorithm
The UPDN algorithm is designed to prevent deadlocks from occurring in loops of the subnet. A loop-deadlock is a situation in which it is no longer possible to send data between any two hosts connected through the loop. As such, the UPDN routing algorithm should be used if the subnet is not a pure Fat Tree, and one of its loops may experience a deadlock (due, for example, to high pressure).
The UPDN algorithm is based on the following main stages:
1. Auto-detect root nodes - based on the CA hop length from any switch in the subnet, a statistical histogram is built for each switch (hop num vs number of occurrences). If the histogram reflects a specific column (higher than others) for a certain node, then it is marked as a root node. Since the algorithm is statistical, it may not find any root nodes. The list of the root nodes found by this auto-detect stage is used by the ranking process stage.
Note 1: The user can override the node list manually.
Note 2: If this stage cannot find any root nodes, and the user did
not specify a guid list file, OpenSM defaults back to the
Min Hop routing algorithm.
2. Ranking process - All root switch nodes (found in stage 1) are assigned a rank of 0. Using the BFS algorithm, the rest of the switch nodes in the subnet are ranked incrementally. This ranking aids in the process of enforcing rules that ensure loop-free paths.
3. Min Hop Table setting - after ranking is done, a BFS algorithm is run from each (CA or switch) node in the subnet. During the BFS process, the FDB table of each switch node traversed by BFS is updated, in reference to the starting node, based on the ranking rules and guid values.
At the end of the process, the updated FDB tables ensure loop-free paths through the subnet.
Note: Up/Down routing does not allow LID routing communication between switches that are located inside spine "switch systems". The reason is that there is no way to allow a LID route between them that does not break the Up/Down rule. One ramification of this is that you cannot run SM on switches other than the leaf switches of the fabric.
UPDN Algorithm Usage
Activation through OpenSM
Use '-R updn' option (instead of old '-u') to activate the UPDN algorithm. Use '-a <root_guid_file>' for adding an UPDN guid file that contains the root nodes for ranking. If the `-a' option is not used, OpenSM uses its auto-detect root nodes algorithm.
Notes on the guid list file:
1. A valid guid file specifies one guid in each line. Lines with an invalid
format will be discarded.
2. The user should specify the root switch guids. However, it is also possible to specify CA guids; OpenSM will use the guid of the switch (if it exists) that connects the CA to the subnet as a root node.
Fat-tree Routing Algorithm
The fat-tree algorithm optimizes routing for "shift" communication pattern. It should be chosen if a subnet is a symmetrical or almost symmetrical fat-tree of various types. It supports not just K-ary-N-Trees, by handling for non-constant K, cases where not all leafs (CAs) are present, any CBB ratio. As in UPDN, fat-tree also prevents credit-loop-deadlocks.
If the root guid file is not provided ('-a' or '--root_guid_file' options),
the topology has to be pure fat-tree that complies with the following rules:
- Tree rank should be between two and eight (inclusively)
- Switches of the same rank should have the same number
of UP-going port groups*, unless they are root switches,
in which case the shouldn't have UP-going ports at all.
- Switches of the same rank should have the same number
of DOWN-going port groups, unless they are leaf switches.
- Switches of the same rank should have the same number
of ports in each UP-going port group.
- Switches of the same rank should have the same number
of ports in each DOWN-going port group.
- All the CAs have to be at the same tree level (rank).
If the root guid file is provided, the topology doesn't have to be pure
fat-tree, and it should only comply with the following rules:
- Tree rank should be between two and eight (inclusively)
- All the Compute Nodes** have to be at the same tree level (rank).
Note that non-compute node CAs are allowed here to be at different
* ports that are connected to the same remote switch are referenced as 'port group'.
** list of compute nodes (CNs) can be specified by '-u' or '--cn_guid_file' OpenSM options.
Topologies that do not comply cause a fallback to min hop routing. Note that this can also occur on link failures which cause the topology to no longer be "pure" fat-tree.
Note that although fat-tree algorithm supports trees with non-integer CBB ratio, the routing will not be as balanced as in case of integer CBB ratio. In addition to this, although the algorithm allows leaf switches to have any number of CAs, the closer the tree is to be fully populated, the more effective the "shift" communication pattern will be. In general, even if the root list is provided, the closer the topology to a pure and symmetrical fat-tree, the more optimal the routing will be.
The algorithm also dumps compute node ordering file (opensm-ftree-ca-order.dump) in the same directory where the OpenSM log resides. This ordering file provides the CN order that may be used to create efficient communication pattern, that will match the routing tables.
Activation through OpenSM
Use '-R ftree' option to activate the fat-tree algorithm. Use '-a <root_guid_file>' to provide root nodes for ranking. If the `-a' option is not used, routing algorithm will detect roots automatically. Use '-u <root_cn_file>' to provide the list of compute nodes. If the `-u' option is not used, all the CAs are considered as compute nodes.
Note: LMC > 0 is not supported by fat-tree routing. If this is specified, the default routing algorithm is invoked instead.
LASH Routing Algorithm
LASH is an acronym for LAyered SHortest Path Routing. It is a deterministic shortest path routing algorithm that enables topology agnostic deadlock-free routing within communication networks.
When computing the routing function, LASH analyzes the network topology for the shortest-path routes between all pairs of sources / destinations and groups these paths into virtual layers in such a way as to avoid deadlock.
Note 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.
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 guarantee 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 maintaining 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, 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, avoiding 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.
Note: QoS support has to be turned on in order that SL/VL mappings are used.
Note: LMC > 0 is not supported by the LASH routing. If this is specified, the default routing algorithm is invoked instead.
DOR Routing Algorithm
The Dimension Order Routing algorithm is based on the Min Hop algorithm and so uses shortest 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.
To learn more about deadlock-free routing, see the article "Deadlock Free Message Routing in Multiprocessor Interconnection Networks" by William J Dally and Charles L Seitz (1985).
To learn more about the up/down algorithm, see the article "Effective Strategy to Compute Forwarding Tables for InfiniBand Networks" by Jose Carlos Sancho, Antonio Robles, and Jose Duato at the Universidad Politecnica de Valencia.
To learn more about LASH and the flexibility behind it, the requirement for layers, performance comparisons to other algorithms, see the following articles:
"Layered Routing in Irregular Networks", Lysne et al, IEEE Transactions on Parallel and Distributed Systems, VOL.16, No12, December 2005.
"Routing for the ASI Fabric Manager", Solheim et al. IEEE Communications Magazine, Vol.44, No.7, July 2006.
"Layered Shortest Path (LASH) Routing in Irregular System Area Networks", Skeie et al. IEEE Computer Society Communication Architecture for Clusters 2002.
Modular Routine Engine
Modular routing engine structure allows for the ease of "plugging" new routing modules.
Currently, only unicast callbacks are supported. Multicast can be added later.
One existing routing module is up-down "updn", which may be activated with '-R updn' option (instead of old '-u').
General usage is: $ opensm -R 'module-name'
There is also a trivial routing module which is able to load LFT tables from a file.
- this will load switch LFTs and/or LID matrices (min hops tables)
- this will load switch LFTs according to the path entries introduced
in the file
- no additional checks will be performed (such as "is port connected",
- in case when fabric LIDs were changed this will try to reconstruct
LFTs correctly if endport GUIDs are represented in the file
(in order to disable this, GUIDs may be removed from the file
The file format is compatible with output of 'ibroute' util and for whole fabric can be generated with dump_lfts.sh script.
To activate file based routing module, use:
opensm -R file -U /path/to/lfts_file
If the lfts_file is not found or is in error, the default routing algorithm is utilized.
The ability to dump switch lid matrices (aka min hops tables) to file and later to load these is also supported.
The usage is similar to unicast forwarding tables loading from a lfts file (introduced by 'file' routing engine), but new lid matrix file name should be specified by -M or --lid_matrix_file option. For example:
opensm -R file -M ./opensm-lid-matrix.dump
The dump file is named 'opensm-lid-matrix.dump' and will be generated in standard opensm dump directory (/var/log by default) when OSM_LOG_ROUTING logging flag is set.
When routing engine 'file' is activated, but the lfts file is not specified or not cannot be open default lid matrix algorithm will be used.
There is also a switch forwarding tables dumper which generates a file compatible with dump_lfts.sh output. This file can be used as input for forwarding tables loading by 'file' routing engine. Both or one of options -U and -M can be specified together with '-R file'.