libEnsemble has been developed, supported, and tested on systems of highly varying
scales, from laptops to thousands of compute nodes. On multi-node systems, there are
two basic modes of configuring libEnsemble to run and launch tasks (user applications)
on the available nodes.
The first mode we refer to as central mode, where the libEnsemble manager and worker processes
are grouped onto one or more dedicated nodes. Workers launch applications onto
the remaining allocated nodes:
Alternatively, in distributed mode, the libEnsemble (manager/worker) processes
will share nodes with submitted tasks. This enables libEnsemble, using the mpi4py
communicator, to be run with the workers spread across nodes so as to be co-located
with their tasks.
Configurations with multiple nodes per worker or multiple workers per node are both
common use cases. The distributed approach allows the libEnsemble worker to read files
produced by the application on local node storage. HPC systems that allow only one
application to be launched to a node at any one time prevent distributed configuration.
On systems with a job scheduler, libEnsemble is typically run within a single
job submission. All user simulations will run on
the nodes within that allocation.
How does libEnsemble know where to run tasks (user applications)?
The libEnsemble Executor can be initialized from the user calling
script, and then used by workers to run tasks. The Executor will automatically detect the nodes
available on most systems. Alternatively, the user can provide a file called node_list in
the run directory. By default, the Executor will divide up the nodes evenly to each worker.
If the argument libE_specs["dedicated_mode"]=True is used when initializing libEnsemble, then any node
that is running a libEnsemble manager or worker will be removed from the node-list available
to the workers, ensuring libEnsemble has dedicated nodes.
To run in central mode using a 5-node allocation with 4 workers: From the head node
of the allocation:
mpirun-np5pythonmyscript.py
or:
pythonmyscript.py--commslocal--nworkers4
Either of these will run libEnsemble (inc. manager and 4 workers) on the first node. The remaining
4 nodes will be divided among the workers for submitted applications. If the same run was
performed without libE_specs["dedicated_mode"]=True, runs could be submitted to all 5 nodes. The number of workers
can be modified to allow either multiple workers to map to each node or multiple nodes per worker.
To launch libEnsemble distributed requires a less trivial libEnsemble run script.
For example:
mpirun-np5-ppn1pythonmyscript.py
would launch libEnsemble with 5 processes across 5 nodes. However, the manager would have its
own node, which is likely wasteful. More often, a machinefile is used to add the manager to
the first node. In the examples directory, you can find an example submission
script, configured to run libensemble distributed, with multiple workers per node or multiple nodes
per worker, and adding the manager onto the first node.
HPC systems that only allow one application to be launched to a node at any one time,
will not allow a distributed configuration.
Some large systems have a 3-tier node setup. That is, they have a separate set of launch nodes
(known as MOM nodes on Cray Systems). User batch jobs or interactive sessions run on a launch node.
Most such systems supply a special MPI runner that has some application-level scheduling
capability (e.g., aprun, jsrun). MPI applications can only be submitted from these nodes. Examples
of these systems include Summit and Sierra.
There are two ways of running libEnsemble on these kinds of systems. The first, and simplest,
is to run libEnsemble on the launch nodes. This is often sufficient if the worker’s simulation
or generation functions are not doing much work (other than launching applications). This approach
is inherently centralized. The entire node allocation is available for the worker-launched
tasks.
However, running libEnsemble on the compute nodes is potentially more scalable and
will better manage simulation and generation functions that contain considerable
computational work or I/O. Therefore the second option is to use proxy task-execution
services like Balsam.
Running libEnsemble on the compute nodes while still submitting additional applications
requires alternative Executors that connect to external services like Balsam. Balsam
can take tasks submitted by workers and execute them on the remaining compute nodes,
or to entirely different systems.
Users with persistent gen_f functions may notice that the persistent workers
are still automatically assigned system resources. This can be resolved by
fixing the number of resource sets.
libEnsemble can automatically detect system information. This includes resource information, such as
available nodes and the number of cores on the node, and information about available MPI runners.
System detection for resources can be overridden using the resource_info
libE_specs option.
When using the MPI Executor, it is possible to override the detected information using the
custom_info argument. See the MPI Executor for more.
Alternatively to much of the above, if libEnsemble is running on some resource with
internet access (laptops, login nodes, other servers, etc.), workers can be instructed to
launch generator or simulator user function instances to separate resources from
themselves via Globus Compute (formerly funcX), a distributed, high-performance function-as-a-service platform:
This is useful for running ensembles across machines and heterogeneous resources, but
comes with several caveats:
User functions registered with Globus Compute must be non-persistent, since
manager-worker communicators can’t be serialized or used by a remote resource.
Likewise, the Executor.manager_poll() capability is disabled. The only
available control over remote functions by workers is processing return values
or exceptions when they complete.
Users are responsible for authenticating via Globus and maintaining their
Globus Compute endpoints on their target systems.
Users can still define Executor instances within their user functions and submit
MPI applications normally, as long as libEnsemble and the target application are
accessible on the remote system:
# Within remote user functionfromlibensemble.executorsimportMPIExecutorexctr=MPIExecutor()exctr.register_app(full_path="/home/user/forces.x",app_name="forces")task=exctr.submit(app_name="forces",num_procs=64)
Specify a Globus Compute endpoint in either sim_specs or gen_specs via the globus_compute_endpoint
argument. For example: