Which mpi to use

The --host option to mpirun takes a comma-delimited list of hosts on which to run. If the --host -specified nodes are not in the already-provided host list, mpirun will abort without launching anything.

In this case, the --host option acts like an exclusionary filter — it limits the scope of where processes will be scheduled from the original list of hosts to produce a final list of hosts. Finally, note that in exclusionary mode, processes will only be executed on the --host -specified hosts, even if it causes oversubscription.

Inclusionary: If a list of hosts has not been provided by another source, then the hosts provided by the --host option will be used as the original and final host list. In this case, --host acts as an inclusionary agent; all --host -supplied hosts become available for scheduling processes. Note, too, that --host is essentially a per-application switch.

The short version is that if you are not oversubscribing your nodes i. If you're oversubscribing, the issue gets much more complicated — keep reading. The more complete answer is: Open MPI schedules processes to nodes by asking two questions from each application on the mpirun command line: How many processes should be launched?

Where should those processes be launched? The "how many" question is directly answered with the -np switch to mpirun. The "where" question is a little more complicated, and depends on three factors: The final node list e.

The default number of slots on any machine, if not explicitly specified, is 1 e. Max slot counts, however, are rarely specified by schedulers. The max slot count for each node will default to "infinite" if it is not provided meaning that Open MPI will oversubscribe the node if you ask it to — see more on oversubscribing in this FAQ entry. In this mode, Open MPI will schedule processes on a node until all of its default slots are exhausted before proceeding to the next node.

In this mode, Open MPI will schedule a single process on each node in a round-robin fashion looping back to the beginning of the node list as necessary until all processes have been scheduled. Nodes are skipped once their default slot counts are exhausted. Nodes are skipped in this process if their maximum slot count is exhausted.

If the maximum slot count is exhausted on all nodes while there are still processes to be scheduled, Open MPI will abort without launching any processes. If you don't like how this scheduling occurs, please let us know. If you are using a supported resource manager, Open MPI will get the slot information directly from that entity.

If you are using the --host parameter to mpirun , be aware that each instance of a hostname bumps up the internal slot count by one. This tells Open MPI that host "node0" has a slot count of 4. Specifically, Open MPI assumes that you are oversubscribing the node. But be very careful to ensure that Open MPI knows that you are oversubscibing your node! If Open MPI is unaware that you are oversubscribing a node, severe performance degradation can result.

See this FAQ entry for more details on oversubscription. However, it is critical that Open MPI knows that you are oversubscribing the node, or severe performance degradation can result. The short explanation is as follows: never specify a number of slots that is more than the available number of processors.

For example, if you want to run 4 processes on a uniprocessor, then indicate that you only have 1 slot but want to run 4 processes. Here's the full explanation: Open MPI basically runs its message passing progression engine in two modes: aggressive and degraded. With some network transports, this means that Open MPI will spin in tight loops attempting to make message passing progress, effectively causing other processes to not get any CPU cycles and therefore never make any progress.

This is actually a lie there is only 1 processor — not 4 , and can cause extremely bad performance. It does not affect behavior of non-MPI processes, nor does it affect the behavior of a process that is not inside an MPI library call. Users are cautioned against setting this parameter unless you are really, absolutely, positively sure of what you are doing.

So if you wanted to run a 4-process MPI job of your a. It is possible to get TotalView to recognize that mpirun is simply a "starter" program and should be effectively ignored. Specifically, TotalView can be configured to skip mpirun and mpiexec and orterun and jump right into your MPI application. You can simply use the built-in support to launch, monitor, and kill MPI jobs.

If you are using an older version of DDT that does not have this built-in support, keep reading. Assuming that you are using Open MPI v1. See the official DDT documentation for more details. The documentation contained in the Open MPI tarball will have the most up-to-date information, but as of v1.

There is no need to specify what nodes to run on. For users of Open MPI 1. In the 1. Version 1. The default configuration of Open MPI uses dlopen internally to load its support components. These components rely on symbols available in libmpi. Beginning with the v1. For example, if four processes in a job share a node, they will each be given a local rank ranging from 0 to 3.

Note that this may be different than the number of processes in the job. Open MPI guarantees that these variables will remain stable throughout future releases.

This is done since a Open MPI has no idea what connections an application process will really use, and b creating the connections takes time. Once the connection is established, it remains "connected" until one of the two connected processes terminates, so the creation time cost is paid only once.

This is accomplished in a somewhat scalable fashion to help minimize startup time. Open MPI has two main components for Libfabric a. See each Lifabric provider man page e. Open MPI Team. Source Code Access. Bug Tracking. Regression Testing. Version Information. You must have static libraries available for everything that your program links to.

This includes Open MPI; you must have used the --enable-static option to Open MPI's configure or otherwise have available the static versions of the Open MPI libraries note that Open MPI static builds default to including all of its plugins in its libraries — as opposed to having each plugin in its own dynamic shared object file. Note that some popular Linux libraries do not have static versions by default e. Open MPI must have been built without a memory manager. This means that Open MPI must have been configured with the --without-memory-manager flag.

This is irrelevant on some platforms for which Open MPI does not have a memory manager, but on some platforms it is necessary Linux. It is harmless to use this flag on platforms where Open MPI does not have a memory manager. On some systems Linux , you may see linker warnings about some files requiring dynamic libraries for functions such as gethostname and dlopen. These are ok, but do mean that you need to have the shared libraries installed.

You can disable all of Open MPI's dlopen behavior i. This will eliminate the linker warnings about dlopen. See this FAQ entry for the details. But it is possible. First, you must read this FAQ entry. Both libibverbs and your verbs hardware plugin must be available in static form. This example shows the steps for the GNU compiler suite, but other compilers will be similar.

Finally, some installations use the library directory "lib64" while others use "lib". Adjust your directory names as appropriate. Specifically, these added arguments do the following: -static : Tell the linker to generate a static executable. You can either add these arguments in manually, or you can see this FAQ entry to modify the default behavior of the wrapper compilers to hide this complexity from end users but be aware that if you modify the wrapper compilers' default behavior, all users will be creating static applications!

This is unfortunately due to a design flaw in the MPI F90 bindings themselves. After selecting the appropriate starting Bmake. The example below is from the Bmake. The MPI section is commented out. Uncomment it. The rest are correct as is.

We built this example on Solaris, hence the name below. I configured Open MPI with --enable-static flag. I used the VASP supplied makefile. Other MPI language bindings and application-level programming interfaces have been been written by third parties.

Here are a link to some of the available packages But the list changes over time; projects come, and projects go. Your best bet these days is simply to use Google to find MPI bindings and application-level programming interfaces. Starting with v4. See this FAQ category for much more information. They provide some details on building dependencies and installation steps as well as some relevant notes with regards to Slurm Support. Slurm support for PMIx was first included in Slurm It has since been updated to support the PMIx v2.

This Slurm version specifically does not support PMIx v2. Slurm If running PMIx v1, it is recommended to run at least 1. It may work, but there is no guarantee that it will. For the purpose of these instructions let's imagine PMIx v2. If PMIx isn't installed in any of the previous locations, the Slurm configure script can be requested to point to the non default location.

Here's an example:. For instance to build against 1. The default for pmix will be the highest version of the library:. Inspecting the generated config. After configuration, we can proceed to install Slurm using make or rpm accordingly :. If support for PMI2 version is also needed, it can also be installed from the contribs directory:. It is planned to alleviate that by putting these libraries in a separate libpmi-slurm package.

Here's an example setup for two nodes named compute[]:. Here's an example indicating that both components work properly:. This is the preferred mode of operation. Starting with Open MPI version 3. Open MPI version 4. In Open MPI version 2.