# Multilocale Chapel Execution¶

This document outlines the steps to get started with multilocale Chapel using GASNet-based communication. This configuration is fully functional on every platform that supports multilocale Chapel. However, there are also other communication configurations that work in specific situations. On Cray XC/XE/XK systems, using native communication as described in Using Chapel on Cray Systems will probably give the best performance. For instructions on using the preliminary OpenFabrics Interfaces libfabric-based ofi communication layer, see Using Chapel with libfabric.

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

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

2. Inspect your current communication configuration:

$CHPL_HOME/util/printchplenv  3. Configure the Chapel runtime appropriately. For most configurations, set: export CHPL_COMM=gasnet  Then, consider changing these variables: Note that the runtime libraries used by the Chapel compiler are based on these settings. 4. Re-make the compiler and runtime from CHPL_HOME (see Building Chapel): cd$CHPL_HOME
make

5. Compile your Chapel program as usual:

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

6. 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 ..."

7. 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 command line option is a good way to learn about what the launcher is doing.

## What is GASNet?¶

GASNet is a one-sided communication and active message library being developed by Lawrence Berkeley National Laboratory and UC Berkeley. For details, refer to the GASNet website.

## Setting CHPL_COMM_SUBSTRATE¶

Users can set CHPL_COMM_SUBSTRATE to indicate the GASNet conduit that they wish to use. Conduits are alternative implementations of the GASNet library. GASNet uses different conduits to support different networks. Novice users can leave this unset and Chapel will make a choice for them.

Most settings for CHPL_COMM_SUBSTRATE rely on the particular network hardware. The options include:

ibv
OpenIB/OpenFabrics Verbs for InfiniBand (see Using Chapel with InfiniBand)
udp
UDP - portable conduit, works on any network with a TCP/IP stack (see Using the Portable UDP Conduit)
gemini
Gemini for Cray XE and XK series (see Using Chapel on Cray Systems)
aries
Aries for Cray XC series (see Using Chapel on Cray Systems)
mpi
MPI - portable conduit, works on any network with MPI 1.1 or newer
mxm
Mellanox MXM for InfiniBand (mxm is deprecated -- the ibv substrate is recommended)
pami
PAMI for IBM Power 775, BlueGene/Q and others
portals4
Portals4 (BETA) for the Portals 4.x API
smp
Simulates multiple locales on a single shared-memory machine

See the GASNet website for more information on each of these conduits.

Current defaults are:

CHPL_TARGET_PLATFORM CHPL_COMM_SUBSTRATE
cray-cs ibv
cray-xc aries
cray-xe gemini
cray-xk gemini
pwr6 ibv
other udp

## Setting CHPL_GASNET_SEGMENT¶

Users can set CHPL_GASNET_SEGMENT to choose a memory segment to use with GASNet. A GASNet segment is a region of memory that is expected to be used for remote memory access. The GASNet library works to make memory in this segment available for accelerated memory access supported directly by network hardware. The options are:

everything
All memory is available for remote memory access.
fast
A limited portion of memory is available and optimized for fastest remote memory access
large
As with fast, but a larger amount of memory is available for communication

Each choice of segment has different tradeoffs. For the fast segment, the Chapel heap is entirely in memory that can be directly accessed over the network, but the drawback is that the size of the heap must be specified at program start-up. For everything, accessing memory over the network is generally slower, but there is no need to worry about the size of the heap or whether some memory is registered with the network. The large segment offers a compromise between these two options, where some portion of the heap will be available for faster network access.

Current defaults are:

CHPL_COMM_SUBSTRATE CHPL_GASNET_SEGMENT
aries fast
gemini fast
ibv large
portals fast
smp fast
other everything

## Troubleshooting¶

If you are trying to debug job launch, try adding -v to your program's command line and set:

export GASNET_VERBOSEENV=1


We've had best results with console I/O using:

export GASNET_ROUTE_OUTPUT=0


Advanced users may want to set CHPL_COMM_DEBUG in order to enable GASNet's internal sanity checking. (It is off by default.) Note that you'll need to re-make GASNet and runtime when changing this setting.