# 11.7.1. Reducing startup time for jobs

Error

TODO This whole section needs to be checked.

There are several ways to reduce the startup time on large clusters. Some of them are described on this page. We continue to work on making startup even faster, especially on the large clusters coming in future years.

Open MPI v5.0.5 is significantly faster and more robust than its predecessors. We recommend that anyone running large jobs and/or on large clusters make the upgrade to the v5.0.x series.

Several major launch time enhancements have been made starting with the v3.0 release. Most of these take place in the background — i.e., there is nothing you (as a user) need do to take advantage of them. However, there are a few that are left as options until we can assess any potential negative impacts on different applications.

Some options are available when launching via `mpirun` or when launching using the native resource manager launcher (e.g., `srun` in a Slurm environment). These are activated by setting the corresponding MCA parameter, and include:

- Setting the `pmix_base_async_modex` MCA parameter will eliminate a global out-of-band collective operation during `MPI_INIT`. This operation is performed in order to share endpoint information prior to communication. At scale, this operation can take some time and scales at best logarithmically. Setting the parameter bypasses the operation and causes the system to lookup the endpoint information for a peer only at first message. Thus, instead of collecting endpoint information for all processes, only the endpoint information for those processes this peer communicates with will be retrieved. The parameter is especially effective for applications with sparse communication patterns — i.e., where a process only communicates with a few other peers. Applications that use dense communication patterns (i.e., where a peer communicates directly to all other peers in the job) will probably see a negative impact of this option.

  Note

  This option is only available in PMIx-supporting environments, or when launching via `mpirun`

- The `async_mpi_init` parameter is automatically set to `true` when the `pmix_base_async_modex` parameter has been set, but can also be independently controlled. When set to `true`, this parameter causes `MPI_Init` to skip an out-of-band barrier operation at the end of the procedure that is not required whenever direct retrieval of endpoint information is being used.

- Similarly, the `async_mpi_finalize` parameter skips an out-of-band barrier operation usually performed at the beginning of `MPI_FINALIZE`. Some transports (e.g., the `usnic` BTL) require this barrier to ensure that all MPI messages are completed prior to finalizing, while other transports handle this internally and thus do not require the additional barrier. Check with your transport provider to be sure, or you can experiment to determine the proper setting.