Provided by: openmpi-doc_4.1.2-2ubuntu1_all 
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)
4.1.2 Nov 24, 2021 ORTE_HOSTS(7)