Using Chapel with libfabric¶

This document describes how to run Chapel across multiple compute nodes using the OpenFabrics Interfaces libfabric-based ofi communication layer. Multilocale Chapel Execution gives general information about running Chapel in a multilocale configuration.

Building Chapel with the ofi Communication Layer¶

Make general, non-communication Chapel configuration settings as described in Setting up Your Environment for Chapel.
Configure the Chapel runtime to select the ofi communication layer
```
export CHPL_COMM=ofi
```
Set the CHPL_LIBFABRIC environment variable indicating whether to use a system-installed libfabric or the bundled one. Current options are:

Value

Description

system

use a system install of libfabric

bundled

use the libfabric bundled with Chapel in third-party

If unset, Chapel will attempt to find a libfabric installation using the command
```
pkg-config --exists libfabric
```
with the specific paths involved filled in using other pkg-config options. Note that in particular on HPE Cray EX systems, we expect to always use a system-installed libfabric, so much so that if the environment variable is set to ‘bundled’ or the default search fails to find a libfabric on such systems, a warning will result.
Select a launcher. On Cray XC and HPE Cray EX systems you can skip this step, because on those systems the automatically-selected aprun or srun launcher settings will work with the ofi communication layer. But on other systems, select the Chapel mpirun4ofi launcher. For more information see The mpirun4ofi Launcher, below.
```
export CHPL_LAUNCHER=mpirun4ofi
```
If you are using the mpirun4ofi launcher, set the variable indicating the path to an OpenMPI installation. It may be possible to skip this step, if your system has OpenMPI already installed and your target compiler can find its include and library files itself. But this is not common, so you will probably need to do:
```
export MPI_DIR=<Place where OpenMPI is installed>
```
The <Place where OpenMPI is installed> should be a directory with an include subdirectory that contains the OpenMPI include files and a lib subdirectory that contains the OpenMPI library files. If your system does not have those installed already, you will need to download OpenMPI and possibly build it.

Note

In the future we hope both to be able to support the MPICH MPI package in addition to supporting the OpenMPI MPI package, and to use the regular Chapel mpirun launcher with the ofi communication layer, but for now OpenMPI and mpirun4ofi are the only options on platforms other than Cray XC and HPE Cray EX systems.

Note: On a Mac OS X system, OpenMPI can be obtained through Homebrew with the following command.
```
brew install open-mpi
```
Re-make the compiler and runtime from CHPL_HOME (see Building Chapel).
```
cd $CHPL_HOME
make
```
Now you are ready to compile and run programs. Compile your Chapel program as usual.
```
chpl $CHPL_HOME/examples/hello6-taskpar-dist.chpl
```
Optionally set any environment variables necessary during execution (see below) and run, specifying the number of locales on the command line. For example, this runs the hello6-taskpar-dist example program on 2 locales:
```
./hello6-taskpar-dist -nl 2
```

Execution Environment¶

Libfabric Providers¶

Libfabric defines an abstract network and operations on it, and so-called providers within libfabric define the concrete instances of the network and operations. The provider used by a program is selected at execution time. The ofi communication layer has been tested with 3 different providers:

gni
The gni provider works only on Cray XC systems. It is built on the Cray native uGNI library and communicates over the Cray proprietary Aries network interface. This is the default provider on Cray XC systems. Note that the libfabric gni provider itself is something of a work-in-progress, and Chapel performance using libfabric and gni will probably never match what can be achieved using the native ugni communication layer.

tcp
The tcp provider should work on all platforms. It is built on TCP sockets and communicates over any network interface on which the OS can provide sockets support. This is the default provider on all systems other than Cray XC and HPE Cray EX. The tcp provider is fully functional, indeed to the extent libfabric has a reference provider the tcp provider is it, but its emphasis is definitely functionality rather than performance.

verbs
The verbs provider works on any system with verbs-based network hardware (Infiniband, iWarp, etc.). It is built on the Linux Verbs API.

(Note for libfabric devotees: when the verbs provider is specified to the ofi communication layer as described below, what is actually used is the verbs;ofi_rxm provider, which is the verbs provider plus a utility provider which supports reliable datagrams for remote memory access operations.)

The CHPL_RT_COMM_OFI_PROVIDER or FI_PROVIDER environment variables can be set to force use of a provider other than the default, with the former overriding the latter if both are set. In particular, this can force use of the tcp provider on Cray XC systems, or the verbs provider on verbs-based systems where the default would otherwise be the tcp provider. For example, the following would force use of the verbs provider:

export CHPL_RT_COMM_OFI_PROVIDER=verbs

The Chapel group has done full testing both on a Cray XC system with the gni and tcp providers, and on InfiniBand-based Cray CS and HPE Cray EX systems with the tcp and verbs providers. Some additional testing has been done with the tcp provider on a MacBook running Mac OS X. All of these configurations are expected to work. Provider settings we have not tested with the ofi communication layer may lead to internal errors. Settings which are at odds with the available networks, such as specifying the gni provider on a vanilla Linux cluster, will definitely lead to internal errors.

Although the discussion here has revolved around selecting providers as such, in reality the selection is capability-based and precisely which provider ends up getting selected doesn’t particularly matter to the communication layer. There is no expectation that users or programmers should concern themselves with providers. Providers are simply the mechanism of portability when using libfabric.

Memory Registration and the Heap¶

Network technologies sometimes require memory registration, meaning that ranges of memory which will be the source or target of communication operations must be made known to the network before any such operations can occur. When the ofi communication layer is used with either the verbs provider on InfiniBand-based platforms including HPE Cray EX systems, or with the gni provider on Cray XC systems, memory has to be registered. This has certain implications for users, the most notable being that the heap must have a fixed size.

The heap is an area of memory used for dynamic allocation of everything from user data to task stacks to internal management data structures. When memory must be registered, the communication layer needs to know the maximum size the heap will grow to during execution. By default, the ofi communication layer creates a fixed heap whose size is 85% of compute node physical memory when it predicts that doing so will result in better network performance.

You can adjust this by setting the CHPL_RT_MAX_HEAP_SIZE environment variable. Set it to a positive number for the desired heap size in bytes optionally followed by k or K for KiB, m or M for MiB, g or G for GiB, or to a positive integer followed by % to indicate a percentage of the node real memory. Either CHPL_RT_MAX_HEAP_SIZE=12g or =20% specifies roughly a 12 GiB heap on a 64 GiB compute node, for example.

Alternatively, you can prevent creation of a fixed heap entirely by setting CHPL_RT_MAX_HEAP_SIZE=0. This may cause the selection of a different provider than the highest-performing one, however.

Note

In the future we hope to be able to reduce the user impact of memory registration and fixed heaps when using the ofi communication layer.

Hugepages on Cray XC Systems¶

We have not yet quantified the effects, but performance with the gni provider may be improved if you have a craype-hugepages module loaded both when you build your program and when you run it. For example:

module load craype-hugepages16M

See Native ugni Communication Layer for more discussion about hugepages, hugepage modules, and the heap size. Note, however, that anything there about a dynamically sized heap does not apply to the ofi communication layer and the libfabric gni provider.

The mpirun4ofi Launcher¶

Programs built with the ofi communication layer on Cray XC and HPE Cray EX systems can use the existing launchers. On other systems, for now they must use the mpirun4ofi launcher, which is a provisional, thin wrapper around OpenMPI mpirun.

The mpirun4ofi launcher can run libfabric-based Chapel programs either with or without slurm. Outside of a slurm job, it will run all of the per-locale Chapel program instances directly on the launch node. In this situation you should be sure to follow the guidance in Overloading system nodes if you are using Qthreads-based tasking. Within a slurm job, the mpirun4ofi launcher will arrange for the per-locale Chapel program instances to be distributed in a cyclic manner across the nodes assigned to the job. Overloading can still be an issue if there are more Chapel locales (program instances) than nodes in the slurm job, however.