ORTE_HOSTS(7)

NAME
OpenRTE Hostfile and HOST Behavior - Overview of OpenRTE's support for user-supplied hostfiles and comma-delimited lists of hosts

DESCRIPTION

OpenRTE supports several levels of user-specified host lists based on an established precedence order. Users can specify a default hostfile that contains a list of nodes available to all app_contexts given on the command line. Only one default hostfile can be provided for any job. In addition, users can specify a hostfile that contains a list of nodes to be used for a specific app_context, or can provide a comma-delimited list of nodes to be used for that app_context via the -host command line option.

The precedence order applied to these various options depends to some extent on the local environment. The following table illustrates how host and hostfile directives work together to define the set of hosts upon which a job will execute in the absence of a resource manager (RM):

 default
 hostfile      host        hostfile       Result
----------    ------      ----------      -----------------------------------------
 unset        unset          unset        Job is co-located with mpirun
 unset         set           unset        Host defines resource list for the job
 unset        unset           set         Hostfile defines resource list for the job
 unset         set            set         Hostfile defines resource list for the job,
                                          then host filters the list to define the final
                                          set of nodes available to each application
                                          within the job
  set         unset          unset        Default hostfile defines resource list for the job
  set          set           unset        Default hostfile defines resource list for the job,
                                          then host filters the list to define the final
                                          set of nodes available to each application
                                          within the job
  set          set            set         Default hostfile defines resource list for the job,
                                          then hostfile filters the list, and then host filters
                                          the list to define the final set of nodes available
                                          to each application within the job

This changes somewhat in the presence of a RM as that entity specifies the initial allocation of nodes. In this case, the default hostfile, hostfile and host directives are all used to filter the RM's specification so that a user can utilize different portions of the allocation for different jobs. This is done according to the same precedence order as in the prior table, with the RM providing the initial pool of nodes.

RELATIVE INDEXING

Once an initial allocation has been specified (whether by an RM, default hostfile, or hostfile), subsequent hostfile and -host specifications can be made using relative indexing. This allows a user to stipulate which hosts are to be used for a given app_context without specifying the particular host name, but rather its relative position in the allocation.

This can probably best be understood through consideration of a few examples. Consider the case where an RM has allocated a set of nodes to the user named "foo1, foo2, foo3, foo4". The user wants the first app_context to have exclusive use of the first two nodes, and a second app_context to use the last two nodes. Of course, the user could printout the allocation to find the names of the nodes allocated to them and then use -host to specify this layout, but this is cumbersome and would require hand-manipulation for every invocation.

A simpler method is to utilize OpenRTE's relative indexing capability to specify the desired layout. In this case, a command line of:

mpirun -pernode -host +n1,+n2 ./app1 : -host +n3,+n4 ./app2

would provide the desired pattern. The "+" syntax indicates that the information is being provided as a relative index to the existing allocation. Two methods of relative indexing are supported:

+n<#>
    A relative index into the allocation referencing the <#> node. OpenRTE will substitute the <#> node in the allocation

+e[:<#>]
    A request for <#> empty nodes - i.e., OpenRTE is to substitute this reference with <#> nodes that have not yet been used by any other app_context. If the ":<#>" is not provided, OpenRTE will substitute the reference with all empty nodes. Note that OpenRTE does track the empty nodes that have been assigned in this manner, so multiple uses of this option will result in assignment of unique nodes up to the limit of the available empty nodes. Requests for more empty nodes than are available will generate an error.

Relative indexing can be combined with absolute naming of hosts in any arbitrary manner, and can be used in hostfiles as well as with the -host command line option. In addition, any slot specification provided in hostfiles will be respected - thus, a user can specify that only a certain number of slots from a relative indexed host are to be used for a given app_context.

Another example may help illustrate this point. Consider the case where a user has a default hostfile containing:

dummy1 slots=4
dummy2 slots=4
dummy3 slots=4
dummy4 slots=4
dummy5 slots=4

This may, for example, be a hostfile that describes a set of commonly-used resources that the user wishes to execute applications against. For this particular application, the user plans to map byslot, and wants the first two ranks to be on the second node of any allocation, the next ranks to land on an empty node, have one rank specifically on dummy4, the next rank to be on the second node of the allocation again, and finally any remaining ranks to be on whatever empty nodes are left. To accomplish this, the user provides a hostfile of:

+n2 slots=2
+e:1
dummy4 slots=1
+n2
+e

The user can now use this information in combination with OpenRTE's sequential mapper to obtain their specific layout:

mpirun --default-hostfile dummyhosts -hostfile mylayout -mca rmaps seq ./my_app

which will result in:

rank0 being mapped to dummy3
rank1 to dummy1 as the first empty node
rank2 to dummy4
rank3 to dummy3
rank4 to dummy2 and rank5 to dummy5 as the last remaining unused nodes

Note that the sequential mapper ignores the number of slots arguments as it only maps one rank at a time to each node in the list.

If the default round-robin mapper had been used, then the mapping would have resulted in:

ranks 0 and 1 being mapped to dummy3 since two slots were specified
ranks 2-5 on dummy1 as the first empty node, which has four slots
rank6 on dummy4 since the hostfile specifies only a single slot from that node is to be used
ranks 7 and 8 on dummy3 since only two slots remain available
ranks 9-12 on dummy2 since it is the next available empty node and has four slots
ranks 13-16 on dummy5 since it is the last remaining unused node and has four slots

Thus, the use of relative indexing can allow for complex mappings to be ported across allocations, including those obtained from automated resource managers, without the need for manual manipulation of scripts and/or command lines.

SEE ALSO
  orterun(1)