General command line options

The following general command line options are available.

Launch options

Mapping, ranking, and binding options

Output options

Input options

Specific options

MPI Options

Debugger / tool options

Debug options

Fault Tolerance Options

These options are only available is Open MPI / PRRTE were compiled with Fault Tolerance functionality enabled.

Details of individual command line options

The sections below offer more detail than the abbreviated lists, above.

The --allow-run-as-root option

Allow execution as root (STRONGLY DISCOURAGED).

Running as root exposes the user to potentially catastrophic file system corruption and damage — e.g., if the user accidentally points the root of the session directory to a system required point, this directory and all underlying elements will be deleted upon job completion, thereby rendering the system inoperable.

It is recognized that some environments (e.g., containers) may require operation as root, and that the user accepts the risks in those scenarios. Accordingly, one can override PRRTE’s run-as-root protection by providing one of the following:

Again, we recommend this only be done if absolutely necessary.

The --bind-to option

By default, processes are bound to individual CPUs (either COREs or HWTHREADs, as defined by default or by user specification for the job). On nodes that are OVERSUBSCRIBEd (i.e., where the number of procs exceeds the number of assigned slots), the default is to not bind the processes.

NOTE:

Binding is performed to the first available specified object type within the object where the process was mapped. In other words, binding can only be done to the mapped object or to a resource located beneath that object.

An object is considered completely consumed when the number of processes bound to it equals the number of CPUs within it. Unbound processes are not considered in this computation. Additional processes cannot be mapped to consumed objects unless the OVERLOAD qualifier is provided via the --bind-to command line option.

Note that directives and qualifiers are case-insensitive and can be shortened to the minimum number of characters to uniquely identify them. Thus, L1CACHE can be given as l1cache or simply as L1.

Supported binding directives include:

Any directive can include qualifiers by adding a colon (:) and any combination of one or more of the following to the --bind-to option:

NOTE:

The --debug-daemons option

Debug daemon output enabled. This is a somewhat limited stream of information normally used to simply confirm that the daemons started. Includes leaving the output streams open.

The --debug-daemons-file option

Debug daemon output is enabled and all output from the daemons is redirected into files with names of the form:

output-prted-<daemon-nspace>-<nodename>.log

These names avoid conflict on shared file systems. The files are located in the top-level session directory assigned to the DVM.

The --display option

The display command line directive must be accompanied by a comma-delimited list of case-insensitive options indicating what information about the job and/or allocation is to be displayed. The full directive need not be provided — only enough characters are required to uniquely identify the directive. For example, ALL is sufficient to represent the ALLOCATION directive — while MAP can not be used to represent MAP-DEVEL (though MAP-D would suffice).

Supported values include:

The display command line directive can include qualifiers by adding a colon (:) and any combination of one or more of the following (delimited by colons):

Provided qualifiers will apply to all of the display directives.

The --forward-signals option

Comma-delimited list of additional signals (names or integers) to forward to application processes (none = forward nothing). Signals provided by default include SIGTSTP, SIGUSR1, SIGUSR2, SIGABRT, SIGALRM, and SIGCONT.

The --host option

Host syntax consists of a comma-delimited list of node names, each entry optionally containing a :N extension indicating the number of slots to assign to that entry:

--host node01:5,node02

In the absence of the slot extension, one slot will be assigned to the node. Duplicate entries are aggregated and the number of slots assigned to that node are summed together.

NOTE:

The --hostfile option

PRRTE 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 to be used by the DVM. Only one default hostfile can be provided for a given DVM. In addition, users can specify a hostfile that contains a list of nodes to be used for a DVM, or can provide a comma-delimited list of nodes to be used for that DVM 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 DVM will execute in the absence of a resource manager (RM):

This changes somewhat in the presence of an 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 DVMs. This is done according to the same precedence order as in the prior table, with the RM providing the initial pool of nodes.

Hostfiles (sometimes called “machine files”) are a combination of two things:

Hostfile syntax consists of one node name on each line, optionally including a designated number of “slots”:

# This is a comment line, and will be ignored
node01  slots=10
node13  slots=5
node15
node16
node17  slots=3
...

Blank lines and lines beginning with a # are ignored.

A “slot” is the PRRTE term for an allocatable unit where we can launch a process. See the section on definition of the term slot for a longer description of slots.

In the absence of the slot parameter, PRRTE will assign either the number of slots to be the number of CPUs detected on the node or the resource manager-assigned value if operating in the presence of an RM.

IMPORTANT:

The --leave-session-attached option

Do not discard stdout/stderr of remote PRRTE daemons. The primary use for this option is to ensure that the daemon output streams (i.e., stdout and stderr) remain open after launch, thus allowing the user to see any daemon-generated error messages. Otherwise, the daemon will “daemonize” itself upon launch, thereby closing its output streams.

The --map-by option

Processes are mapped based on one of the following directives as applied at the job level:

Any directive can include qualifiers by adding a colon (:) and any combination of one or more of the following (delimited by colons) to the --map-by option (except where noted):

NOTE:

The type of CPU (core vs hwthread) used in the mapping algorithm is determined as follows:

If your application uses threads, then you probably want to ensure that you are either not bound at all (by specifying --bind-to none), or bound to multiple cores using an appropriate binding level or specific number of processing elements per application process via the PE=# qualifier to the --map-by command line directive.

A more detailed description of the mapping, ranking, and binding procedure can be obtained via the --help placement option.

The --output option

The output command line directive must be accompanied by a comma-delimited list of case-insensitive options that control how output is generated. The full directive need not be provided — only enough characters are required to uniquely identify the directive. For example, MERGE is sufficient to represent the MERGE-STDERR-TO-STDOUT directive — while TAG can not be used to represent TAG-DETAILED (though TAG-D would suffice).

Supported values include:

Supported qualifiers include NOCOPY (do not copy the output to the stdout/err streams), and RAW (do not buffer the output into complete lines, but instead output it as it is received).

The --pmixmca option

Pass a PMIx MCA parameter

Syntax: --pmixmca <key> <value>, where key is the parameter name and value is the parameter value.

The --prefix option

Prefix to be used to look for PRRTE executables. PRRTE automatically sets the prefix for remote daemons if it was either configured with the --enable-prte-prefix-by-default option OR prte itself was executed with an absolute path to the prte command. This option overrides those settings, if present, and forces use of the provided path.

The --pmix-prefix option

Prefix to be used by a PRRTE executable to look for its PMIx installation on remote nodes. This is the location of the top-level directory for the installation. If the installation has not been moved, it would be the value given to “–prefix” when the installation was configured.

Note that PRRTE cannot determine the exact name of the library subdirectory under this location. For example, some systems will call it “lib” while others call it “lib64”. Accordingly, PRRTE will use the library subdirectory name of the PMIx installation used to build PRRTE.

The --app-prefix option

Prefix to be used by an app to look for its PMIx installation on remote nodes. This is the location of the top-level directory for the installation. If the installation has not been moved, it would be the value given to “–prefix” when the installation was configured.

Note that PRRTE cannot determine the exact name of the library subdirectory under this location. For example, some systems will call it “lib” while others call it “lib64”. Accordingly, PRRTE will use the library subdirectory name of the PMIx installation used to build PRRTE.

In the absence of providing an application-specific prefix, the PMIx prefix (if given) used by PRRTE’s own executables will be applied unless the “–no-app-prefix” directive is given.

The --no-app-prefix option

Do not apply any prefix to this application. This is needed when a default PMIx prefix has been given to PRRTE, but the application has been built against a PMIx library that (a) is different from the one used by PRRTE, and (b) was not moved. Otherwise, PRRTE will apply its default prefix to the application.

The --prtemca option

Pass a PRRTE MCA parameter.

Syntax: --prtemca <key> <value>, where key is the parameter name and value is the parameter value.

The --noprefix option

Disable automatic --prefix behavior. PRRTE automatically sets the prefix for remote daemons if it was either configured with the --enable-prte-prefix-by-default option OR prte itself was executed with an absolute path to the prte command. This option disables that behavior.

The --rank-by option

PRRTE automatically ranks processes for each job starting from zero. Regardless of the algorithm used, rank assignments span applications in the same job — i.e., a command line of

-n 3 app1 : -n 2 app2

will result in app1 having three processes ranked 0-2 and app2 having two processes ranked 3-4.

By default, process ranks are assigned in accordance with the mapping directive — e.g., jobs that are mapped by-node will have the process ranks assigned round-robin on a per-node basis. However, users can override the default by specifying any of the following directives using the --rank-by command line option:

The rank-by command line option has no qualifiers.

NOTE:

A more detailed description of the mapping, ranking, and binding procedure can be obtained via the --help placement option.

The --runtime-options option

The --runtime-options command line directive must be accompanied by a comma-delimited list of case-insensitive options that control the runtime behavior of the job. The full directive need not be provided — only enough characters are required to uniquely identify the directive.

Runtime options are typically true or false, though this is not a requirement on developers. Since the value of each option may need to be set (e.g., to override a default set by MCA parameter), the syntax of the command line directive includes the use of an = character to allow inclusion of a value for the option. For example, one can set the ABORT-NONZERO-STATUS option to true by specifying it as ABORT-NONZERO-STATUS=1. Note that boolean options can be set to true using a non-zero integer or a case-insensitive string of the word true. For the latter representation, the user need only provide at least the T character. The same policy applies to setting a boolean option to false.

Note that a boolean option will default to true if provided without a value. Thus, --runtime-options abort-nonzero is sufficient to set the ABORT-NONZERO-STATUS option to true.

Supported values include:

The --runtime-options command line option has no qualifiers.

NOTE:

The --stream-buffering option

Adjust buffering for stdout/stderr. Allowable values:

The --tune option

Comma-delimited list of one or more files containing PRRTE and PMIx MCA params for tuning DVM and/or application operations. Parameters in the file will be treated as generic parameters and subject to the translation rules/uncertainties. See --help mca for more information.

Syntax in the file is:

param = value

with one parameter and its associated value per line. Empty lines and lines beginning with the # character are ignored.

The -x option

Export an environment variable, optionally specifying a value. For example:

The --unset-env option

Unset the named environmental variable. Note --unset-env foo* unsets all current environmental variables starting with “foo”

The --prepend-env option

Prepend the named environment variable with the given value. The “[c]” must be appended to the name to specify the separator to be used when appending the value.

Example: --prepend-envar LD_LIBRARY_PATH[:] foo/lib will result in:

LD_LIBRARY_PATH=foo/lib:$LD_LIBRARY_PATH

The --append-env option

Append the named environment variable with the given value. The “[c]” must be appended to the name to specify the separator to be used when appending the value.

Example: --append-envar LD_LIBRARY_PATH[:] foo/lib will result in:

LD_LIBRARY_PATH=$LD_LIBRARY_PATH:foo/lib

Deprecated command line options

The following command line options are deprecated, and should generally not be used. They may be removed in a future release of Open MPI.

The --bind-to-core option

Bind each process to its own core.

The --display-allocation option

Display the allocation being used by this job.

The --display-devel-allocation option

Display a detailed list (mostly intended for developers) of the allocation being used by this job.

The --display-devel-map option

Display a detailed process map (mostly intended for developers) just before launch.

The --display-map option

Display the process map just before launch.

The --display-topo option

Display the topology as part of the process map (mostly intended for developers) just before launch.

The --gmca option

Syntax: --gmca <key> <value>, where key is the parameter name and value is the parameter value. The g prefix indicates that this parameter is “global”, and to be applied to all application contexts — not just the one in which the directive appears.

Pass generic MCA parameters — i.e., parameters whose project affiliation must be determined by PRRTE based on matching the name of the parameter with defined values from various projects that PRRTE knows about.

The --mca option

Syntax: --mca <key> <value>, where key is the parameter name and value is the parameter value.

Pass generic MCA parameters — i.e., parameters whose project affiliation must be determined by PRRTE based on matching the name of the parameter with defined values from various projects that PRRTE knows about.

The --merge-stderr-to-stdout option

Merge stderr to stdout for each process.

The --output-directory option

Redirect output from application processes into filename/job/rank/std[out,err,diag]. A relative path value will be converted to an absolute path. The directory name may include a colon followed by a comma-delimited list of optional case-insensitive directives. Supported directives currently include NOJOBID (do not include a job-id directory level) and NOCOPY (do not copy the output to the stdout/err streams).

The --output-filename option

Redirect output from application processes into filename.rank. A relative path value will be converted to an absolute path. The directory name may include a colon followed by a comma-delimited list of optional case-insensitive directives. Supported directives currently include NOCOPY (do not copy the output to the stdout/err streams).

The --report-bindings option

Display process bindings to stderr.

The --tag-output option

Tag all output with [job,rank].

The --timestamp-output option

Timestamp all application process output.

The --xml option

Provide all output in XML format.

Environment Variables

Specifying Host Nodes

Host nodes can be identified on the mpirun command line with the --host option or in a hostfile.

For example:

shell$ mpirun -H aa,aa,bb ./a.out

Launches two processes on node aa and one on bb.

Or, consider the hostfile:

shell$ cat myhostfile
aa slots=2
bb slots=2
cc slots=2

Here, we list both the host names (aa, bb, and cc) but also how many slots there are for each.

shell$ mpirun --hostfile myhostfile ./a.out

will launch two processes on each of the three nodes.

shell$ mpirun --hostfile myhostfile --host aa ./a.out

will launch two processes, both on node aa.

shell$ mpirun --hostfile myhostfile --host dd ./a.out

will find no hosts to run on and will abort with an error. That is, the specified host dd is not in the specified hostfile.

When running under resource managers (e.g., Slurm, Torque, etc.), Open MPI will obtain both the hostnames and the number of slots directly from the resource manager.

Specifying Number of Processes

As we have just seen, the number of processes to run can be set using the hostfile. Other mechanisms exist.

The number of processes launched can be specified as a multiple of the number of nodes or processor packages available. For example,

shell$ mpirun -H aa,bb --map-by ppr:2:package ./a.out

launches processes 0-3 on node aa and process 4-7 on node bb (assuming aa and bb both contain 4 slots each).

shell$ mpirun -H aa,bb --map-by ppr:2:node ./a.out

launches processes 0-1 on node aa and processes 2-3 on node bb.

shell$ mpirun -H aa,bb --map-by ppr:1:node ./a.out

launches one process per host node.

mpirun -H aa,bb --pernode ./a.out

is the same as --map-by ppr:1:node and --npernode 1.

Another alternative is to specify the number of processes with the -n option. Consider now the hostfile:

shell$ cat myhostfile
aa slots=4
bb slots=4
cc slots=4

Now run with myhostfile:

shell$ mpirun --hostfile myhostfile -n 6 ./a.out

will launch processes 0-3 on node aa and processes 4-5 on node bb. The remaining slots in the hostfile will not be used since the -n option indicated that only 6 processes should be launched.

Mapping Processes to Nodes: Using Policies

The examples above illustrate the default mapping of process processes to nodes. This mapping can also be controlled with various mpirun options that describe mapping policies.

Consider the same hostfile as above, again with -n 6. The table below lists a few mpirun variations, and shows which MPI_COMM_WORLD ranks end up on which node:

The --map-by node option will load balance the processes across the available nodes, numbering each process in a round-robin fashion.

The --nolocal option prevents any processes from being mapped onto the local host (in this case node aa). While mpirun typically consumes few system resources, --nolocal can be helpful for launching very large jobs where mpirun may actually need to use noticeable amounts of memory and/or processing time.

Just as -n can specify fewer processes than there are slots, it can also oversubscribe the slots. For example, with the same hostfile:

shell$ mpirun --hostfile myhostfile -n 14 ./a.out

will launch processes 0-3 on node aa, 4-7 on bb, and 8-11 on cc. It will then add the remaining two processes to whichever nodes it chooses.

One can also specify limits to oversubscription. For example, with the same hostfile:

shell$ mpirun --hostfile myhostfile -n 14 --nooversubscribe ./a.out

will produce an error since --nooversubscribe prevents oversubscription.

Limits to oversubscription can also be specified in the hostfile itself:

shell$ cat myhostfile
aa slots=4 max_slots=4
bb         max_slots=4
cc slots=4

The max_slots field specifies such a limit. When it does, the slots value defaults to the limit. Now:

shell$ mpirun --hostfile myhostfile -n 14 ./a.out

causes the first 12 processes to be launched as before, but the remaining two processes will be forced onto node cc. The other two nodes are protected by the hostfile against oversubscription by this job.

Using the --nooversubscribe option can be helpful since Open MPI currently does not get max_slots values from the resource manager.

Of course, -n can also be used with the -H or -host option. For example:

shell$ mpirun -H aa,bb -n 8 ./a.out

launches 8 processes. Since only two hosts are specified, after the first two processes are mapped, one to aa and one to bb, the remaining processes oversubscribe the specified hosts.

And here is a MIMD example:

shell$ mpirun -H aa -n 1 hostname : -H bb,cc -n 2 uptime

will launch process 0 running hostname on node aa and processes 1 and 2 each running uptime on nodes bb and cc, respectively.

Mapping, Ranking, and Binding: Oh My!

Open MPI employs a three-phase procedure for assigning process locations and ranks:

The mapping step is used to assign a default location to each process based on the mapper being employed. Mapping by slot, node, and sequentially results in the assignment of the processes to the node level. In contrast, mapping by object, allows the mapper to assign the process to an actual object on each node.

Note that the location assigned to the process is independent of where it will be bound — the assignment is used solely as input to the binding algorithm.

The mapping of process processes to nodes can be defined not just with general policies but also, if necessary, using arbitrary mappings that cannot be described by a simple policy. One can use the “sequential mapper,” which reads the hostfile line by line, assigning processes to nodes in whatever order the hostfile specifies. Use the ---map-by seq option. For example, using the same hostfile as before:

shell$ mpirun -hostfile myhostfile --map-by seq ./a.out

will launch three processes, one on each of nodes aa, bb, and cc, respectively. The slot counts don’t matter; one process is launched per line on whatever node is listed on the line.

Another way to specify arbitrary mappings is with a rankfile, which gives you detailed control over process binding as well. Rankfiles are discussed below.

The second phase focuses on the ranking of the process within the job’s MPI_COMM_WORLD. Open MPI separates this from the mapping procedure to allow more flexibility in the relative placement of MPI processes. This is best illustrated by considering the following cases where we used the --np 8 --map-by ppr:2:package --host aa:4,bb:4 option:

Ranking by fill assigns MCW ranks in a simple progression across each node. Ranking by span and by slot provide the identical result — a round-robin progression of the packages across all nodes before returning to the first package on the first node. Ranking by node assigns MCW ranks iterating first across nodes then by package.

The binding phase actually binds each process to a given set of processors. This can improve performance if the operating system is placing processes suboptimally. For example, it might oversubscribe some multi-core processor packages, leaving other packages idle; this can lead processes to contend unnecessarily for common resources. Or, it might spread processes out too widely; this can be suboptimal if application performance is sensitive to interprocess communication costs. Binding can also keep the operating system from migrating processes excessively, regardless of how optimally those processes were placed to begin with.

The processors to be used for binding can be identified in terms of topological groupings — e.g., binding to an l3cache will bind each process to all processors within the scope of a single L3 cache within their assigned location. Thus, if a process is assigned by the mapper to a certain package, then a --bind-to l3cache directive will cause the process to be bound to the processors that share a single L3 cache within that package.

Alternatively, processes can be mapped and bound to specified cores using the --map-by pe-list= option. For example, --map-by pe-list=0,2,5 will map three processes all three of which will be bound to logical cores 0,2,5. If you intend to bind each of the three processes to different cores then the :ordered qualifier can be used like --map-by pe-list=0,2,5:ordered. In this example, the first process on a node will be bound to CPU 0, the second process on the node will be bound to CPU 2, and the third process on the node will be bound to CPU 5.

Finally, --report-bindings can be used to report bindings.

As an example, consider a node with two processor packages, each comprised of four cores, and each of those cores contains one hardware thread. The --report-bindings option shows the binding of each process in a descriptive manner. Below are some examples.

shell$ mpirun --np 4 --report-bindings --map-by core --bind-to core
[...] Rank 0 bound to package[0][core:0]
[...] Rank 1 bound to package[0][core:1]
[...] Rank 2 bound to package[0][core:2]
[...] Rank 3 bound to package[0][core:3]

In the above case, the processes bind to successive cores.

shell$ mpirun --np 4 --report-bindings --map-by package --bind-to package
[...] Rank 0 bound to package[0][core:0-3]
[...] Rank 1 bound to package[0][core:0-3]
[...] Rank 2 bound to package[1][core:4-7]
[...] Rank 3 bound to package[1][core:4-7]

In the above case, processes bind to all cores on successive packages. The processes cycle through the processor packages in a round-robin fashion as many times as are needed. By default, the processes are ranked in a fill manner.

shell$ mpirun --np 4 --report-bindings --map-by package --bind-to package --rank-by span
[...] Rank 0 bound to package[0][core:0-3]
[...] Rank 1 bound to package[1][core:4-7]
[...] Rank 2 bound to package[0][core:0-3]
[...] Rank 3 bound to package[1][core:4-7]

The above case demonstrates the difference in ranking when the span qualifier is used instead of the default.

shell$ mpirun --np 4 --report-bindings --map-by slot:PE=2 --bind-to core
[...] Rank 0 bound to package[0][core:0-1]
[...] Rank 1 bound to package[0][core:2-3]
[...] Rank 2 bound to package[0][core:4-5]
[...] Rank 3 bound to package[0][core:6-7]

In the above case, the output shows us that 2 cores have been bound per process. Specifically, the mapping by slot with the PE=2 qualifier indicated that each slot (i.e., process) should consume two processor elements. By default, Open MPI defines “processor element” as “core”, and therefore the --bind-to core caused each process to be bound to both of the cores to which it was mapped.

shell$ mpirun --np 4 --report-bindings --map-by slot:PE=2 --use-hwthread-cpus
[...]] Rank 0 bound to package[0][hwt:0-1]
[...]] Rank 1 bound to package[0][hwt:2-3]
[...]] Rank 2 bound to package[0][hwt:4-5]
[...]] Rank 3 bound to package[0][hwt:6-7]

In the above case, we replace the --bind-to core with --use-hwthread-cpus. The --use-hwthread-cpus is converted into --bind-to hwthread and tells the --report-bindings output to show the hardware threads to which a process is bound. In this case, processes are bound to 2 hardware threads per process.

shell$ mpirun --np 4 --report-bindings --bind-to none
[...] Rank 0 is not bound (or bound to all available processors)
[...] Rank 1 is not bound (or bound to all available processors)
[...] Rank 2 is not bound (or bound to all available processors)
[...] Rank 3 is not bound (or bound to all available processors)

In the above case, binding is turned off and are reported as such.

Open MPI’s support for process binding depends on the underlying operating system. Therefore, certain process binding options may not be available on every system.

Process binding can also be set with MCA parameters. Their usage is less convenient than that of mpirun options. On the other hand, MCA parameters can be set not only on the mpirun command line, but alternatively in a system or user mca-params.conf file or as environment variables, as described in the Setting MCA Parameters. These are MCA parameters for the PRRTE runtime so the command line argument --prtemca (yes, prte with a single r, not two r’s) must be used to pass the MCA parameter key/value pair. Alternatively, the MCA parameter key/value pair may be specific on the command line by prefixing the key with PRTE_MCA_ (again, that is not a typo: PRTE not PRRTE). Some examples include:

Defaults for Mapping, Ranking, and Binding

If the user does not specify each of --map-by, --rank-by, and --bind-to option then the default values are as follows:

Consider 2 identical hosts (hostA and hostB) with 2 packages (denoted by []) each with 8 cores (denoted by /../) and 2 hardware threads per core (denoted by a .).

Default of --map-by core --bind-to core --rank-by span when the number of processes is less than or equal to 2.

shell$ mpirun --np 2 --host hostA:4,hostB:2 ./a.out
R0  hostA  [BB/../../../../../../..][../../../../../../../..]
R1  hostA  [../BB/../../../../../..][../../../../../../../..]

Default of --map-by package --bind-to package --rank-by fill when the number of processes is greater than 2.

shell$ mpirun --np 4 --host hostA:4,hostB:2 ./a.out
R0  hostA  [BB/BB/BB/BB/BB/BB/BB/BB][../../../../../../../..]
R1  hostA  [BB/BB/BB/BB/BB/BB/BB/BB][../../../../../../../..]
R2  hostA  [../../../../../../../..][BB/BB/BB/BB/BB/BB/BB/BB]
R3  hostA  [../../../../../../../..][BB/BB/BB/BB/BB/BB/BB/BB]

If only --map-by OBJ is specified, then it implies --bind-to OBJ --rank-by fill. The example below results in --map-by hwthread --bind-to hwthread --rank-by fill

shell$ mpirun --np 4 --map-by hwthread --host hostA:4,hostB:2 ./a.out
R0  hostA  [B./../../../../../../..][../../../../../../../..]
R1  hostA  [.B/../../../../../../..][../../../../../../../..]
R0  hostA  [../B./../../../../../..][../../../../../../../..]
R1  hostA  [../.B/../../../../../..][../../../../../../../..]

If only --bind-to OBJ is specified, then --map-by is determined by the number of processes and --rank-by fill. The example below results in --map-by package --bind-to core --rank-by fill

shell$ mpirun --np 4 --bind-to core --host hostA:4,hostB:2 ./a.out
R0  hostA  [BB/../../../../../../..][../../../../../../../..]
R1  hostA  [../BB/../../../../../..][../../../../../../../..]
R2  hostA  [../../../../../../../..][BB/../../../../../../..]
R3  hostA  [../../../../../../../..][../BB/../../../../../..]

The mapping pattern might be better seen if we change the default --rank-by from fill to span. First, the processes are mapped by package iterating between the two marking a core at a time. Next, the processes are ranked in a spanning manner that load balances them across the object they were mapped against. Finally, the processes are bound to the core that they were mapped againast.

shell$ mpirun --np 4 --bind-to core --rank-by span --host hostA:4,hostB:2 ./a.out
R0  hostA  [BB/../../../../../../..][../../../../../../../..]
R1  hostA  [../../../../../../../..][BB/../../../../../../..]
R2  hostA  [../BB/../../../../../..][../../../../../../../..]
R3  hostA  [../../../../../../../..][../BB/../../../../../..]

Rankfiles

Rankfiles are text files that specify detailed information about how individual processes should be mapped to nodes, and to which processor(s) they should be bound. Each line of a rankfile specifies the location of one process (for MPI jobs, the process’ “rank” refers to its rank in MPI_COMM_WORLD). The general form of each line in the rankfile is:

rank <N>=<hostname> slot=<slot list>

For example:

shell$ cat myrankfile
rank 0=aa slot=1:0-2
rank 1=bb slot=0:0,1
rank 2=cc slot=2-3
shell$ mpirun -H aa,bb,cc,dd --map-by rankfile:file=myrankfile ./a.out

Means that:

Note that only logicical processor locations are supported. By default, the values specifed are assumed to be cores. If you intend to specify specific hardware threads then you must add the :hwtcpus qualifier to the --map-by command line option (e.g., --map-by rankfile:file=myrankfile:hwtcpus).

If the binding specification overlaps between any two ranks then an error occurs. If you intend to allow processes to share the same logical processing unit then you must pass the --bind-to :overload-allowed command line option to tell the runtime to ignore this check.

The hostnames listed above are “absolute,” meaning that actual resolveable hostnames are specified. However, hostnames can also be specified as “relative,” meaning that they are specified in relation to an externally-specified list of hostnames (e.g., by mpirun’s --host argument, a hostfile, or a job scheduler).

The “relative” specification is of the form +n<X>, where X is an integer specifying the Xth hostname in the set of all available hostnames, indexed from 0. For example:

shell$ cat myrankfile
rank 0=+n0 slot=1:0-2
rank 1=+n1 slot=0:0,1
rank 2=+n2 slot=2-3
shell$ mpirun -H aa,bb,cc,dd --map-by rankfile:file=myrankfile ./a.out

All package/core slot locations are specified as logical indexes.

NOTE:

You can use tools such as Hwloc’s lstopo(1) to find the logical indexes of package and cores.

Application Context or Executable Program?

To distinguish the two different forms, mpirun looks on the command line for --app option. If it is specified, then the file named on the command line is assumed to be an application context. If it is not specified, then the file is assumed to be an executable program.

Locating Files

If no relative or absolute path is specified for a file, Open MPI will first look for files by searching the directories specified by the --path option. If there is no --path option set or if the file is not found at the --path location, then Open MPI will search the user’s PATH environment variable as defined on the source node(s).

If a relative directory is specified, it must be relative to the initial working directory determined by the specific starter used. For example when using the ssh starter, the initial directory is $HOME by default. Other starters may set the initial directory to the current working directory from the invocation of mpirun.

Current Working Directory

The --wdir mpirun option (and its synonym, --wd) allows the user to change to an arbitrary directory before the program is invoked. It can also be used in application context files to specify working directories on specific nodes and/or for specific applications.

If the --wdir option appears both in a context file and on the command line, the context file directory will override the command line value.

If the -wdir option is specified, Open MPI will attempt to change to the specified directory on all of the remote nodes. If this fails, mpirun will abort.

If the -wdir option is not specified, Open MPI will send the directory name where mpirun was invoked to each of the remote nodes. The remote nodes will try to change to that directory. If they are unable (e.g., if the directory does not exist on that node), then Open MPI will use the default directory determined by the starter.

All directory changing occurs before the user’s program is invoked; it does not wait until MPI_INIT(3) is called.

Standard I/O

Open MPI directs UNIX standard input to /dev/null on all processes except the MPI_COMM_WORLD rank 0 process. The MPI_COMM_WORLD rank 0 process inherits standard input from mpirun.

NOTE:

Open MPI directs UNIX standard output and error from remote nodes to the node that invoked mpirun and prints it on the standard output/error of mpirun. Local processes inherit the standard output/error of mpirun and transfer to it directly.

Thus it is possible to redirect standard I/O for Open MPI applications by using the typical shell redirection procedure on mpirun. For example:

shell$ mpirun -n 2 my_app < my_input > my_output

Note that in this example only the MPI_COMM_WORLD rank 0 process will receive the stream from my_input on stdin. The stdin on all the other nodes will be tied to /dev/null. However, the stdout from all nodes will be collected into the my_output file.

Signal Propagation

When mpirun receives a SIGTERM and SIGINT, it will attempt to kill the entire job by sending all processes in the job a SIGTERM, waiting a small number of seconds, then sending all processes in the job a SIGKILL.

SIGUSR1 and SIGUSR2 signals received by mpirun are propagated to all processes in the job.

A SIGTSTOP signal to mpirun will cause a SIGSTOP signal to be sent to all of the programs started by mpirun and likewise a SIGCONT signal to mpirun will cause a SIGCONT sent.

Other signals are not currently propagated by mpirun.

Process Termination / Signal Handling

During the run of an MPI application, if any process dies abnormally (either exiting before invoking MPI_FINALIZE(3), or dying as the result of a signal), mpirun will print out an error message and kill the rest of the MPI application.

User signal handlers should probably avoid trying to cleanup MPI state (Open MPI is currently not async-signal-safe; see MPI_INIT_THREAD(3) for details about MPI_THREAD_MULTIPLE and thread safety). For example, if a segmentation fault occurs in MPI_SEND(3) (perhaps because a bad buffer was passed in) and a user signal handler is invoked, if this user handler attempts to invoke MPI_FINALIZE(3), Bad Things could happen since Open MPI was already “in” MPI when the error occurred. Since mpirun will notice that the process died due to a signal, it is probably not necessary (and safest) for the user to only clean up non-MPI state.

Process Environment

Processes in the MPI application inherit their environment from the PRRTE daemon upon the node on which they are running. The environment is typically inherited from the user’s shell. On remote nodes, the exact environment is determined by the boot MCA module used. The rsh launch module, for example, uses either rsh/ssh to launch the PRRTE daemon on remote nodes, and typically executes one or more of the user’s shell-setup files before launching the PRRTE daemon. When running dynamically linked applications which require the LD_LIBRARY_PATH environment variable to be set, care must be taken to ensure that it is correctly set when booting Open MPI.

See the Remote Execution section for more details.

Remote Execution

Open MPI requires that the PATH environment variable be set to find executables on remote nodes (this is typically only necessary in rsh- or ssh-based environments — batch/scheduled environments typically copy the current environment to the execution of remote jobs, so if the current environment has PATH and/or LD_LIBRARY_PATH set properly, the remote nodes will also have it set properly). If Open MPI was compiled with shared library support, it may also be necessary to have the LD_LIBRARY_PATH environment variable set on remote nodes as well (especially to find the shared libraries required to run user MPI applications).

However, it is not always desirable or possible to edit shell startup files to set PATH and/or LD_LIBRARY_PATH. The --prefix option is provided for some simple configurations where this is not possible.

The --prefix option takes a single argument: the base directory on the remote node where Open MPI is installed. Open MPI will use this directory to set the remote PATH and LD_LIBRARY_PATH before executing any Open MPI or user applications. This allows running Open MPI jobs without having pre-configured the PATH and LD_LIBRARY_PATH on the remote nodes.

Open MPI adds the basename of the current node’s $bindir (the directory where Open MPI’s executables were installed) to the prefix and uses that to set the PATH on the remote node. Similarly, Open MPI adds the basename of the current node’s $libdir (the directory where Open MPI’s libraries were installed) to the prefix and uses that to set the LD_LIBRARY_PATH on the remote node. For example:

If the following command line is used:

shell$ mpirun --prefix /remote/node/directory

Open MPI will add /remote/node/directory/bin to the PATH and /remote/node/directory/lib64 to the LD_LIBRARY_PATH on the remote node before attempting to execute anything.

The --prefix option is not sufficient if the installation paths on the remote node are different than the local node (e.g., if /lib is used on the local node, but /lib64 is used on the remote node), or if the installation paths are something other than a subdirectory under a common prefix.

Note that executing mpirun via an absolute pathname is equivalent to specifying --prefix without the last subdirectory in the absolute pathname to mpirun. For example:

shell$ /usr/local/bin/mpirun ...

is equivalent to

shell$ mpirun --prefix /usr/local

Exported Environment Variables

All environment variables that are named in the form OMPI_* will automatically be exported to new processes on the local and remote nodes. Environmental parameters can also be set/forwarded to the new processes using the MCA parameter mca_base_env_list. The -x option to mpirun has been deprecated, but the syntax of the MCA param follows that prior example. While the syntax of the -x option and MCA param allows the definition of new variables, note that the parser for these options are currently not very sophisticated — it does not even understand quoted values. Users are advised to set variables in the environment and use the option to export them; not to define them.

Setting MCA Parameters

The --mca switch allows the passing of parameters to various MCA (Modular Component Architecture) modules. MCA modules have direct impact on MPI programs because they allow tunable parameters to be set at run time (such as which BTL communication device driver to use, what parameters to pass to that BTL, etc.).

The --mca switch takes two arguments: <key> and <value>. The <key> argument generally specifies which MCA module will receive the value. For example, the <key> btl is used to select which BTL to be used for transporting MPI messages. The <value> argument is the value that is passed. For example:

shell$ mpirun --mca btl tcp,self -n 1 my_mpi_app

This tells Open MPI to use the tcp and self BTLs, and to run a single copy of my_mpi_app an allocated node.

shell$ mpirun --mca btl self -n 1 my_mpi_app

Tells Open MPI to use the self BTL, and to run a single copy of my_mpi_app an allocated node.

The --mca 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 --mca switch is simply a shortcut for setting environment variables. The same effect may be accomplished by setting corresponding environment variables before running mpirun. The form of the environment variables that Open MPI sets is:

OMPI_MCA_<key>=<value>

Thus, the --mca switch overrides any previously set environment variables. The --mca settings similarly override MCA parameters set in the $OPAL_PREFIX/etc/openmpi-mca-params.conf or $HOME/.openmpi/mca-params.conf file.

Unknown <key> arguments are still set as environment variable — they are not checked (by mpirun) for correctness. Illegal or incorrect <value> arguments may or may not be reported — it depends on the specific MCA module.

To find the available component types under the MCA architecture, or to find the available parameters for a specific component, use the ompi_info command. See the ompi_info(1) man page for detailed information on this command.

Setting MCA parameters and environment variables from file

The --tune command line option and its synonym --mca mca_base_envar_file_prefix allows a user to set MCA parameters and environment variables with the syntax described below. This option requires a single file or list of files separated by “,” to follow.

A valid line in the file may contain zero or more -x or --mca. The following patterns are supported:

If any argument is duplicated in the file, the last value read will be used.

MCA parameters and environment specified on the command line have higher precedence than variables specified in the file.

Running as root

WARNING:

mpirun will refuse to run as root by default.

To override this default, you can add the --allow-run-as-root option to the mpirun command line, or you can set the environmental parameters OMPI_ALLOW_RUN_AS_ROOT=1 and OMPI_ALLOW_RUN_AS_ROOT_CONFIRM=1. Note that it takes setting two environment variables to effect the same behavior as --allow-run-as-root in order to stress the Open MPI team’s strong advice against running as the root user.

After extended discussions with communities who use containers (where running as the root user is the default), there was a persistent desire to be able to enable root execution of mpirun via an environmental control (vs. the existing --allow-run-as-root command line parameter). The compromise of using two environment variables was reached: it allows root execution via an environmental control, but it conveys the Open MPI team’s strong recommendation against this behavior.

Exit status

There is no standard definition for what mpirun should return as an exit status. After considerable discussion, we settled on the following method for assigning the mpirun exit status (note: in the following description, the “primary” job is the initial application started by mpirun — all jobs that are spawned by that job are designated “secondary” jobs):

By default, the job will abort when any process terminates with non-zero status. The MCA parameter --prtemca state_base_error_non_zero_exit can be set to “false” (or “0”) to cause Open MPI to not abort a job if one or more processes return a non-zero status. In that situation the Open MPI records and notes that processes exited with non-zero termination status to report the appropriate exit status of mpirun (per bullet points above).