Using Chapel with Omni-Path ¶

This document describes how to run Chapel across multiple nodes of an Intel Omni-Path cluster. Multilocale Chapel Execution describes general information about running Chapel in a multilocale configuration.

Note

Only CHPL_COMM=gasnet with the ofi substrate is supported for Omni-Path. CHPL_COMM=ofi is not supported.

Configuring for Omni-Path ¶

To build Chapel with Omni-Path support, set:

export CHPL_COMM=gasnet
export CHPL_COMM_SUBSTRATE=ofi

GASNet should automatically detect the psm2 provider, but to ensure this provider is used FI_PROVIDER=psm2 may optionally be set.

Configuring a Launcher ¶

A gasnetrun_ofi-based launcher should be used to launch jobs and native launchers like srun will not work. Many Omni-Path clusters use a workload manager or queueing system such as Slurm. To select an appropriate Chapel launcher you can set CHPL_LAUNCHER to one of the following values:

Launcher Name	Description
gasnetrun_ofi	run jobs interactively on your system
slurm-gasnetrun_ofi	queue jobs using Slurm (srun/sbatch)

Selecting a Spawner ¶

Under the covers, gasnetrun_ofi-based launchers must figure out how to spawn jobs and get them up and running on the compute nodes. GASNet’s three ways of doing this on Omni-Path clusters are ssh, pmi, and mpi, described just below. When MPI is detected at configure time, it will be selected as the default, but we recommend using one of the other options if possible. This can be done by setting the GASNET_OFI_SPAWNER environment variable, whose legal values are:

ssh: Based on our experience, this is the preferred option, when possible. This requires the ability to ssh to the system’s compute nodes, which is not supported by all systems, depending on how they are configured. See the following sub-section for details on this option.
pmi: When GASNet’s configure step detects a PMI-capable job scheduler like Slurm, pmi can be the next best choice because it often “just works” and can reduce overhead compared to mpi. For more information about this option, including how to configure job launch via PMIRUN_CMD, see the GASNet README for the PMI-based spawner (also available at $CHPL_HOME/third-party/gasnet/gasnet-src/other/pmi-spawner/README).
mpi: When the previous cases are not options, mpi serves as a reasonable last resort. Note that it may, depending on its configuration, incur a performance penalty due to competition between MPI and GASNet for limited communication resources. See the Using MPI for Job Launch section below for best practices when using this option.

Using SSH for Job Launch ¶

To launch Omni-Path jobs with SSH, use the following:

# Specify ssh spawner
export GASNET_OFI_SPAWNER=ssh

# Specify the nodes to run on (only required when using plain
# gasnetrun_ofi outside a Slurm/PBS/LSF reservation)
export GASNET_SSH_SERVERS="nid00001 nid00002 nid00003 ..."

If you receive an error message like:

*** Failed to start processes on nid00001, possibly due to an inability to establish an ssh connection from login-node without interactive authentication.

This indicates passwordless SSH is not set up. You can try copying existing SSH keys or generating new ones with the following:

ssh-keygen -t rsa # use default location and empty passphrase
cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys

If you see the same error message this may indicate ssh connections to compute nodes are not allowed, in which case using the MPI spawner may be your only option.

For further information about environment variables that can be used to control how ssh is used to launch your Chapel program, see the descriptions of GASNET_SSH_CMD and GASNET_SSH_OPTIONS in the GASNet README for the ssh spawner (also available at $CHPL_HOME/third-party/gasnet/gasnet-src/other/ssh-spawner/README).

Using MPI for Job Launch ¶

To launch Omni-Path jobs with mpirun, first make sure that mpicc is available and that MPI programs launch appropriately with mpirun. Then use the following. You’ll want to make sure that GASNet detects MPI in its configuration output.

export GASNET_OFI_SPAWNER=mpi

As mentioned above, a potential downside of using MPI for launching Chapel programs is that the resources that it requires to get the program up and running can interfere with those needed by GASNet. In some cases, this can result in negative impacts on performance. In others, it can prevent GASNet from accessing the network resources it requires at all. For example, the following error is an example of one in which MPI is preventing GASNet from accessing what it needs:

*** FATAL ERROR (proc 0): in gasnetc_ofi_init() at /third-party/gasnet/gasnet-src/ofi-conduit/gasnet_ofi.c:1336: fi_endpoint for rdma failed: -22(Invalid argument)

For tips and best practices about how to configure/use GASNet to avoid such conflicts with MPI, please refer to the section “Build-time Configuration” in the GASNet README for the MPI spawner (also available at $CHPL_HOME/third-party/gasnet/gasnet-src/other/mpi-spawner/README). Within this README, see also the description of the MPIRUN_CMD environment variable as a means of configuring how jobs are started.

Verifying Job Launch ¶

Once the above configuration has been done, checking that jobs are launching properly is recommended. The following Chapel program will print out the locale names and how much parallelism is available per locale. Ideally each locale is running on a unique node (not oversubscribed) and the amount of parallelism matches the number of physical cores on each node.

for loc in Locales do on loc do
  writeln((here.name, here.maxTaskPar));

An example run may look something like the following:

(nid00001, 28)
(nid00002, 28)

If nodes are oversubscribed or the amount of parallelism is far less than expected see Selecting a Spawner and if that does not help consider opening a bug as described in Reporting Chapel Issues.

In some cases, we have found that setting HFI_NO_CPUAFFINITY=1 may be required to get access to all cores.