bhost (5)

#
# comments
#
<machine> [cpu=<cpucount>] [user=<userid>]
<machine> [cpu=<cpucount>] [user=<userid>]
 ...

A boot schema describes the machines that will combine to form a multicomputer running LAM. It is used by recon(1) to verify initial conditions for running LAM, by lamboot(1) to start LAM, and by lamhalt(1) to terminate LAM (note that lamwipe(1) has been deprecated by the lamhalt(1) command).

The particular syntax of a LAM boot schema is sometimes called the "host file" syntax. It is line oriented. One line indicates the name of a machine, typically the full Internet domain name, an optional number of CPUs available on that machine, and optionally the userid with which to access it.

Common boot schema for a particular site may be created by the system administrator and placed in the installation directory under etc/. They typically start with the prefix bhost. Individual users usually create their own boot schema, especially if the configurations are simple.

Note that lamboot resolves all names listed in bhost on the node in which lamboot was invoked on. The lamboot(1) man page contains information about address resolution, examples on how to handle multiple network interface cards (NICs) in a node, etc.

Here is an example three node boot schema:

#
# example LAM host file
#
server.cluster.example.com schedule=no
beowulf1.cluster.example.com cpu=2
beowulf2.cluster.example.com
beowulf2.cluster.example.com
somewhere.else.example.com user=guest

Note that the "guest" ID is significant, since the user has an alternate login ID on somewhere.else.example.com. Additionally note that beowulf1 has a CPU count of 2 listed (a CPU count of 1 is assumed if it is not given). This value is used by mpirun(1), MPI_Comm_spawn(2), and MPI_Comm_spawn_multiple(2) for the "C" (or CPU) notation that specifies how many ranks to start. This is particularly useful for running on SMP machines.

Note the schedule=no clause. This means that LAM will boot a daemon on that node, but by default, will not launch any MPI processes on that node. This is handy for when you want to control your MPI applications from one node (e.g., a server), but don't want to run any MPI applications on it. In some environments this is the default (e.g., BProc). See the LAM User's Guide for more details.

beowulf2 is listed twice, but has no specific CPU count listed. In this case, LAM will keep a running tally of the total number of CPUs for that host. Hence, LAM will calculate that beowulf2 has two CPUs available for use. Calculating the number of CPUs by counting occurances of a hostname is useful in a batch environment where a hostfile may list the same hostname multiple times, indicating that the batch scheduler has allocated multiple CPUs for a single job (e.g., PBS operates this way).

For the above-mentioned schema, the command "mpirun C foo" would start five instances of the foo program; two on beowulf1, two on beowulf2, and one on somewhere.else.

$LAMHOME/etc/bhost.def

default boot schema file

LAM User's Guide, lamboot(1), lamhalt(1), mpirun(1), MPI_Comm_spawn(1), MPI_Comm_spawn_multiple(1), recon(1), lamwipe(1)