LNet Router Config Guide2

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This document provides procedures to configure and tune an LNet router. It will also cover detailed instructions set on setting up connectivity of an Infiniband network to Intel OPA nodes using LNet router.


LNet supports different network types like Ethernet, InfiniBand, Intel Omni-Path and other proprietary network technologies such as the Cray’s Gemini. It routes LNet messages between different LNet networks using LNet routing. LNet’s routing capabilities provide an efficient protocol to enable bridging between different types of networks. LNet is part of the Linux kernel space and allows for full RDMA throughput and zero copy communications when supported by underlying network. Lustre can initiate a multi-OST read or write using a single Remote Procedure Call (RPC), which allows the client to access data using RDMA at near peak bandwidth rates. With Multi-Rail (MR) feature implemented in Lustre 2.10.X, it allows for multiple interfaces of same type on a node to be grouped together under the same LNet (ex tcp0, o2ib0, etc.). These interfaces can then be used simultaneously to carry LNet traffic. MR also has the ability to utilize multiple interfaces configured on different networks. For example, OPA and MLX interfaces can be grouped under their respective LNet and then can be utilized with MR feature to carry LNet traffic simultaneously.

LNet Configuration Example

An LNet router is a specialized Lustre client where Lustre file system is not mounted and only the LNet is running. A single LNet router can serve different file systems.

For the above example:

  • Servers are on LAN1, a Mellanox based InfiniBand network –
  • Clients are LAN2, an Intel OPA network –
  • Routers on LAN1 and LAN2 at, and, respectively

The network configuration on the nodes can be done either by adding the module parameters in lustre.conf /etc/modprobe.d/lustre.conf or dynamically by using the lnetctl command utility. Also, current configuration can be exported to a YAML format file and then the configuration can be set by importing that YAML file anytime needed.

Network Configuration by adding module parameters in lustre.conf

options lnet networks="o2ib1(ib0)" routes="o2ib2 [email protected]"
options lnet networks="o2ib1(ib0),o2ib2(ib1)" "forwarding=enabled"
options lnet networks="o2ib2(ib0)" routes="o2ib1 [email protected]" 

NOTE: Restarting LNet is necessary to apply the new configuration. To do this, it is needed to unconfigure the LNet network and reconfigure again. Make sure that the Lustre network and Lustre file system are stopped prior to unloading the modules.

// To unload and load LNet module
modprobe -r lnet
modprobe lnet
// To unconfigure and reconfigure LNet
lnetctl lnet unconfigure
lnetctl lnet configure 

Dynamic Network Configuration using lnetctl command

lnetctl net add --net o2ib1 --if ib0
lnetctl route add --net o2ib2 --gateway [email protected]
lnetctl peer add --nid [email protected]
lnetctl net add --net o2ib1 --if ib0
lnetctl net add --net o2ib2 --if ib1
lnetctl peer add --nid [email protected]
lnetctl peer add --nid [email protected]
lnetctl set routing 1
lnetctl net add --net o2ib2 --if ib0
lnetctl route add --net o2ib1 --gateway [email protected]
lnetctl peer add --nid [email protected] 

Importing/Exporting configuration using a YAML file format

// To export the current configuration to a YAML file
lnetctl export FILE.yaml
lnetctl export > FILE.yaml
// To import the configuration from a YAML file
lnetctl import FILE.yaml
lnetctl import < FILE.yaml 

There is a default lnet.conf file installed at /etc/lnet.conf which has an example configuration in YAML format. Another example of a configuration in a YAML file is:

    - net type: o2ib1
      local NI(s):
        - nid: [email protected]
          status: up
              0: ib0
              peer_timeout: 180
              peer_credits: 8
              peer_buffer_credits: 0
              credits: 256
          lnd tunables:
              peercredits_hiw: 64
              map_on_demand: 32
              concurrent_sends: 256
              fmr_pool_size: 2048
              fmr_flush_trigger: 512
              fmr_cache: 1
          tcp bonding: 0
          dev cpt: -1
          CPT: "[0]"
    - net: o2ib2
      gateway: [email protected]
      hop: 1
      priority: 0
      state: up
    - primary nid: [email protected]
      Multi-Rail: False
      peer ni:
        - nid: [email protected]
          state: up
          max_ni_tx_credits: 8
          available_tx_credits: 8
          min_tx_credits: 7
          tx_q_num_of_buf: 0
          available_rtr_credits: 8
          min_rtr_credits: 8
          refcount: 4
    numa_range: 0
    max_intf: 200
    discovery: 1 

LNet provides a mechanism to monitor each route entry. LNet pings each gateway identified in the route entry on regular, configurable interval (live_router_check_interval) to ensure that it is alive. If sending over a specific route fails or if the router pinger determines that the gateway is down, then the route is marked as down and is not used. It is subsequently pinged on regular, configurable intervals (dead_router_check_interval) to determine when it becomes alive again.

Multi-Rail LNet Configuration Example

If the routers are MR enabled, we can add the routers as peers with multiple interfaces to the clients and the servers, the MR algorithm will ensure that both interfaces of the routers are used while sending traffic to the router. However, single interface failure will still cause the entire router to go down. With the network topology example in Figure 1 above, LNet MR can be configured like below:

lnetctl net add --net o2ib1 --if ib0,ib1
lnetctl route add --net o2ib2 --gateway [email protected]
lnetctl peer add --nid [email protected],[email protected]
lnetctl net add --net o2ib1 --if ib0,ib1
lnetctl net add --net o2ib2 --if ib2,ib3
lnetctl peer add --nid [email protected],[email protected]
lnetctl peer add --nid [email protected],[email protected]
lnetctl set routing 1
lnetctl net add --net o2ib2 --if ib0,ib1
lnetctl route add --net o2ib1 --gateway [email protected]
lnetctl peer add --nid [email protected],[email protected] 

Fine-Grained Routing

The routes parameter, by identifying LNet routers in a Lustre configuration, is used to tell a node which route to use when forwarding traffic. It specifies a semi-colon-separated list of router definitions.

routes=dest_lnet [hop] [priority] [email protected]_lnet; \ dest_lnet [hop] [priority] [email protected]_lnet 

An alternative syntax consists of a colon-separated list of router definitions:

routes=dest_lnet: [hop] [priority] [email protected]_lnet \ [hop] [priority] [email protected]_lnet 

When there are two or more LNet routers, it is possible to give weighted priorities to each router using the priority parameter. Here are some possible reasons for using this parameter:

  • One of the routers is more capable than the other.
  • One router is a primary router and the other is a back-up.
  • One router is for one section of clients and the other is for another section.

Each router is moving traffic to a different physical location. The priority parameter is optional and need not be specified if no priority exists. The hop parameter specifies the number of hops to the destination. When a node forwards traffic, the route with the least number of hops is used. If multiple routes to the same destination network have the same number of hops, the traffic is distributed between these routes in a round-robin fashion. To reach/transmit to the LNet dest_lnet, the next hop for a given node is the LNet router with the NID router_NID in the LNet src_lnet. Given a sufficiently well-architected system, it is possible to map the flow to and from every client or server. This type of routing has also been called fine-grained routing.

Advanced Routing Parameters

In a Lustre configuration where different types of LNet networks are connected by routers, several kernel module parameters can be set to monitor and improve routing performance. These parameters are set in /etc/modprobe.d/lustre.conf file. The routing related parameters are:

  • auto_down - Enable/disable (1/0) the automatic marking of router state as up or down. The default value is 1. To disable router marking, set:
options lnet auto_down=0 
  • avoid_asym_router_failure - Specifies that if even one interface of a router is down for some reason, the entire router is marked as down. This is important because if nodes are not aware that the interface on one side is down, they will still keep pushing data to the other side presuming that the router is healthy, when it really is not. To turn it on:
options lnet avoid_asym_router_failure=1 
  • live_router_check_interval - Specifies a time interval in seconds after which the router checker will ping the live routers. The default value is 60. To set the value to 50, use:
options lnet live_router_check_interval=50 
  • dead_router_check_interval - Specifies a time interval in seconds after which the router checker will check the dead routers. The default value is 60. To set the value to 50:
options lnet dead_router_check_interval=50 
  • router_ping_timeout - Specifies a timeout for the router checker when it checks live or dead routers. The router checker sends a ping message to each dead or live router once every dead_router_check_interval or live_router_check_interval respectively. The default value is 50. To set the value to 60:
options lnet router_ping_timeout=60 
  • check_routers_before_use - Specifies that routers are to be checked before use. Set to off by default. If this parameter is set to on, the dead_router_check_interval parameter must be given a positive integer value.
options lnet check_routers_before_use=1 

The router_checker obtains the following information from each router:

  • time the router was disabled
  • elapsed disable time

If the router_checker does not get a reply message from the router within router_ping_timeout seconds, it considers the router to be down. If a router that is marked “up” responds to a ping, the timeout is reset. If 100 packets have been sent successfully through a router, the sent-packets counter for that router will have a value of 100. The statistics data of an LNet router can be found from /proc/sys/lnet/stats. If no interval is specified, then statistics are sampled and printed only one time. Otherwise, statistics are sampled and printed at the specified interval (in seconds). These statistics can be displayed using lnetctl utility as well like below:

# lnetctl stats show
    msgs_alloc: 0
    msgs_max: 2
    errors: 0
    send_count: 887
    recv_count: 887
    route_count: 0
    drop_count: 0
    send_length: 656
    recv_length: 70048
    route_length: 0
    drop_length: 0 

The ping response also provides the status of the NIDs of the node being pinged. In this way, the pinging node knows whether to keep using this node as a next-hop or not. If one of the NIDs of the router is down and the avoid_asym_router_failure is set, then that router is no longer used.

LNet Dynamic Configuration

LNet can be configured dynamically using the lnetctl utility. The lnetctl utility can be used to initialize LNet without bringing up any network interfaces. This gives flexibility to the user to add interfaces after LNet has been loaded. In general the lnetctl format is as follows: lnetctl cmd subcmd [options] The following configuration items are managed by the tool:

  • Configuring/Unconfiguring LNet
  • Adding/Removing/Showing Networks
  • Adding/Removing/Showing Peers
  • Adding/Removing/Showing Routes
  • Enabling/Disabling routing
  • Configuring Router Buffer Pools

Configuring/Unconfiguring LNet

After LNet has been loaded via modprobe (modprobe lnet), the lnetctl utility can be used to configure LNet without bringing up networks that are specified in the module parameters. It can also be used to configure network interfaces specified in the module parameters by providing the --all option.

// To configure LNet
lnetctl lnet configure [--all]
// To unconfigure LNet
lnectl lnet unconfigure 

Adding/Removing/Showing Networks

Now LNet is ready to be configured with networks to be added. To add an o2ib1 LNet network on ib0 and ib1 interfaces:

lnetctl net add --net o2ib1 --if ib0,ib1