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NAME

       ORTE_HOSTS  -  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)