The format of the <bhost> file is documented in the bhost(5) man page.

lamboot will resolve all names in <bhost> on the node in which lamboot was invoked (the origin node). After that, LAM will only use IP addresses, not names. Specifically, the name resolution configuration on all other nodes is not used. Hence, the the origin node must be able to resolve all the names in <bhost> to addresses that are reachable by all other nodes.

A common mistake is to list localhost (or any name that resolves to the special address 127.0.0.1 -- the loopback TCP/IP device) in a <bhost> file that contains other nodes. In this case, the address 127.0.0.1 would be sent to each of the other nodes as the address of the origin node. If the other nodes try to use 127.0.0.1 to contact the origin node, they will actually be contacting themselves, and would eventually timeout and fail.

The IP addresses obtained from <bhost> are used for LAM's meta messages: startup and shutdown of jobs, out-of-band messages used for coordination, etc. The amount of traffic is fairly low (unless using the "lamd" mode of MPI message passing, in which case all MPI traffic will also utilize LAM's meta messages for transport -- see mpirun(1)). When using the TCP RPI, these IP addresses are also used for MPI message passing via direct sockets between each pair of nodes.

A common case is where a "master" node has multiple network interface cards (NICs) -- one that is connected to a public network, and one that is connected to a private network where parallel jobs are to be run. To include the master node in a <bhost> file, the IP name (or address) of the NIC on the private network should be listed in <bhost>. This ensures that all the other nodes can reach the master node on the private network.

As another example, some configurations have multiple TCP/IP NICs in each node of a parallel job. One NIC is considered "slow" (e.g., 10Mbps), while the other is considered "fast" (e.g., 100Mbps). It is desirable to allow LAM to take advantage of the higher bandwidth on the "fast" network for MPI messages. As such, <bhost> should list the IP names (or addresses) of all the "fast" NICs. However, if the LAM RPI does not use TCP/IP (e.g., the Myrinet/GM RPI), the <bhost> file should probably list the "slow" NICs so that LAM's meta message traffic does not cause overhead and potentially detract from performance on the "fast" network from other high-performance applications.

Normally, name resolution of hostnames is done on the machines where lamboot is invoked. This is done for optimization reasons, so that the list of hostnames only needs to be resolved once (potentially minimizing the amount of DNS or other hostname-lookup network traffic).

However, in some non-uniform networking environments, this is not sufficient because each host may have a different IP address on each of its peers. For example, host A may have address Z on host B, but have address Y on host C.

The -l option to lamboot will cause LAM to distribute hostnames to each node rather than a fully resolved set of IP addresses. Hence, each node where LAM is booted will do its own name resolution on the list of hostnames.

The -ssi switch allows the passing of parameters to various SSI modules. LAM's SSI modules are described in detail in lamssi(7). SSI modules have direct impact on MPI programs because they allow tunable parameters to be set at run time (such as which boot device driver to use, what parameters to pass to that driver, etc.).

The -ssi switch takes two arguments: <key> and <value>. The <key> argument generally specifies which SSI module will receive the value. For example, the <key> "boot" is used to select which RPI to be used for starting processes on remote nodes. The <value> argument is the value that is passed. For example:

lamboot -ssi boot tm

Tells LAM to use the "tm" boot module for native launching in PBSPro / OpenPBS environments (the tm boot module does not require a boot schema).

lamboot -ssi boot rsh -ssi rsh_agent "ssh -x" boot_schema

Tells LAM to use the "rsh" boot module, and tells the rsh module to use "ssh -x" as the specific agent to launch executables on remote nodes.

And so on. LAM's boot SSI modules are described in lamssi_boot(7). This page should be consulted for specific actions that are taken by, and how to tweak the run-time behavior of each boot module.

The -ssi switch can be used multiple times to specify different <key> and/or <value> arguments. If the same <key> is specified more than once, the <value>s are concatenated with a comma (",") separating them.

Note that the -ssi switch is simply a shortcut for setting environment variables. The same effect may be accomplished by setting corresponding environment variables before running lamboot. The form of the environment variables that LAM sets are: LAM_MPI_SSI_<key>=<value>.

Note that the -ssi switch overrides any previously set environment variables. Also note that unknown <key> arguments are still set as environment variable -- they are not checked (by lamwipe) for correctness. Illegal or incorrect <value> arguments may or may not be reported -- it depends on the specific SSI module.

All tweakable aspects of launching executables on remote nodes during lamboot are discussed in lamssi(7) and lamssi_boot(7). Topics include (but are not limited to): discovery of remote shell, run-time overrides of the agent use to launch remote executables (e.g., rsh and ssh), etc.

The stdio of each LAM daemon on a remote host that is launched by lamboot is closed by default. Normally, the stdio of the LAM daemon launched on the local host is left open so that the internal LAM tstdio(3) package works properly. However, it is sometimes desirable to close the stdio of the local LAM daemon as well. For example:

rsh somenode lamboot -s hostfile

This is because rsh waits for two conditions before exiting: lamboot to exit, and stdout / stderr to be closed. Without -s, stdout / stderr would not be closed, and rsh (and ssh) will hang even though lamboot had completed. -s causes the stdout / stderr of the local LAM daemon to be closed upon invocation, which will allow rsh to complete. Using -s will not affect lamboot in any other way, but it will prevent the tstdio(3) package from working properly.

If the -x option is given, LAM runs in fault tolerant mode. In this mode, nodes exchange ``heart beat'' messages periodically to make sure all nodes are running and the links connecting them are operational. When a node's heart beats stop, it is declared ``dead'' and all LAM nodes (and processes) are notified. This allows users to write fault tolerant applications that can degrade gracefully, or fully recover by replacing the defunct node with another (see lamgrow(1)). Since this mode introduces a performance penalty, it is not activated by default.