Multilocale Chapel Execution

This document outlines the steps to get started with multilocale Chapel. Chapel provides several multilocale configurations. See Communication Layers for more information on the different communication layers available in Chapel.

Steps 2-3 describe how to build a multilocale Chapel, and steps 4-6 cover compiling and running multilocale Chapel programs.

0. Check for instructions more relevant to your platform in platform-specific documentation.

1. Inspect your current communication configuration:

   $CHPL_HOME/util/printchplenv
   

2. Configure the Chapel runtime appropriately. This should be done based on what network you intend to target.

   Generally speaking, the following table describes the best way to configure the Chapel runtime for different networks. See the documentation for networks for more information.

   Network	Chapel Communication Layer
   Slingshot	libfabric
   InfiniBand	GASNet
   EFA	libfabric
   Ethernet	GASNet or libfabric
   Aries	UGNI
   Omni-Path	GASNet

   If you wish to emulated multilocale execution on a single machine, you should use GASNet.

   • To build GASNet, set the following:

     export CHPL_COMM=gasnet
     

     Then, consider changing these variables to target your specific system. For most systems, these are inferred to sensible defaults automatically and do not need to be set:

     • CHPL_LAUNCHER controls job launch

     • CHPL_COMM_SUBSTRATE selects a GASNet conduit

     • CHPL_GASNET_SEGMENT indicates a memory segment

   • To build libfabric, set the following:

     export CHPL_COMM=ofi
     

     Then, consider changing these variables to target your specific system. For most systems, these are inferred to sensible defaults automatically and do not need to be set:

     • CHPL_LAUNCHER controls job launch

     • CHPL_COMM_OFI_OOB selects the out-of-band communication method

   Note that the runtime libraries used by the Chapel compiler are based on these settings.

3. Re-make the compiler and runtime from CHPL_HOME (see Building Chapel):

   cd $CHPL_HOME
   make
   

4. Compile your Chapel program as usual:

   chpl -o hello $CHPL_HOME/examples/hello6-taskpar-dist.chpl
   

5. Set any environment variables necessary to control the launcher. See Chapel Launchers or documentation for your platform.

   For example, to specify which servers the UDP conduit should use as described in Using the Portable UDP Conduit:

   export GASNET_SPAWNFN=S
   export GASNET_SSH_SERVERS="host1 host2 host3 ..."  # or SSH_SERVERS
   

6. Specify the number of locales on the command line. For example:

   ./hello -nl 2
   

   runs our Hello World program on 2 locales.

What is this _real program?

When you compile a Chapel program for multiple locales, you should typically see two binaries (e.g., hello and hello_real). The first binary contains code to launch the Chapel program onto the compute nodes as specified by the CHPL_LAUNCHER variable. The second contains the program code itself. The -v and --dry-run command line options are a good way to learn about what the launcher is doing.

Co-locales

On some platforms Chapel can run multiple locales on the same node without oversubscription (i.e., without sharing cores). For example, on a node with multiple sockets performance may be improved by running one locale in each socket to avoid inter-socket memory latencies. We refer to this functionality as co-locales. Chapel supports co-locales in the following configurations:

CHPL_COMM	CHPL_LAUNCHER
gasnet	gasnetrun_*
gasnet	pbs-gasnetrun_ibv
gasnet	slurm-gasnetrun_*
gasnet	slurm-srun
gasnet	smp
ofi	slurm-srun

There are two ways to create co-locales. The first is to set the CHPL_RT_LOCALES_PER_NODE environment variable. If set, Chapel will run the specified number of locales per node. The second way to create co-locales is to use the command-line argument -nl NxLt, where N is the number of nodes, L is the number of locales per node, and t is an optional suffix indicating the architectural feature to which the co-locales should be bound. The L is optional; if it isn’t specified then Chapel will run the “ideal” number of locales based on the node architecture. Currently this is limited to the value of CHPL_RT_LOCALES_PER_NODE; in future releases we plan to include more sophisticated heuristics such as automatically running one locale per socket on nodes with multiple sockets.

Note

The -nl NxLt syntax is considered unstable and may change in the future.

By default, Chapel will try to bind co-locales to an architectural feature. For example, launching a Chapel program with the argument -nl 1x2 on a node with two sockets will bind each co-locale to its own socket. Chapel looks at the number of sockets, NUMA domains, caches, and cores on the node, in that order, to determine if the co-locales can be bound to an architectural feature. If the number of co-locales requested does not match the number of any feature then Chapel simply assigns an equal number of cores to each co-locale. Any remaining cores are unused.

You can force Chapel to bind co-locales to an architectural feature with a suffix to the -nl argument. The valid suffixes and their bindings are:

Suffix	Binding
s or socket	socket
numa	NUMA domain
llc	last-level cache
c or core	core

It is an error to specify a number of co-locales greater than the number of the specified architectural feature. For example, specifying -nl 1x2s on a node with a single socket is an error. Any remaining cores are unused; for example, specifying -nl 1x1s on a node with two sockets will leave the cores in one socket unused.

Troubleshooting

If you are trying to debug job launch, try adding -v or --dry-run to your program’s command-line to see the command(s) that the Chapel launcher is executing to get things running.

Note

For GASNet-based multilocale runs, see Troubleshooting for additional tips on debugging GASNet.

Advanced users may also want to set CHPL_COMM_DEBUG. This enables a number of debugging features in the Chapel runtime. After setting this variable, you will need to rebuild the Chapel runtime and any third-party comm layers (e.g., GASNet, libfabric, etc.) that you are using (i.e., re-run the make command you used for your initial Chapel install).

Note

When using GASNet, setting CHPL_COMM_DEBUG will also enable some of GASNet’s internal sanity checking (off by default).