.. _readme-multilocale: ============================ 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. For instructions on using the OpenFabrics Interface's libfabric-based ``ofi`` communication layer---the preferred option on HPE Cray EX systems---see :ref:`readme-libfabric`. On Cray XC systems, using native ``ugni`` communication as described in :ref:`Using Chapel on Cray Systems ` typically gives the best performance. 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 :ref:`platform-specific ` documentation. #. Inspect your current communication configuration: .. code-block:: bash $CHPL_HOME/util/printchplenv #. Configure the Chapel runtime appropriately. For most configurations, set: .. code-block:: bash export CHPL_COMM=gasnet Then, consider changing these variables: * :ref:`CHPL_LAUNCHER ` controls job launch * :ref:`CHPL_COMM_SUBSTRATE ` selects a GASNet conduit * :ref:`CHPL_GASNET_SEGMENT ` indicates a memory segment Note that the runtime libraries used by the Chapel compiler are based on these settings. #. .. _remake-the-compiler: Re-make the compiler and runtime from ``CHPL_HOME`` (see :ref:`readme-building`): .. code-block:: bash cd $CHPL_HOME make #. Compile your Chapel program as usual: .. code-block:: bash chpl -o hello $CHPL_HOME/examples/hello6-taskpar-dist.chpl #. Set any environment variables necessary to control the launcher. See :ref:`readme-launcher` or documentation for your platform. For example, to specify which servers the UDP conduit should use as described in :ref:`using-udp`: .. code-block:: bash export GASNET_SPAWNFN=S export GASNET_SSH_SERVERS="host1 host2 host3 ..." # or SSH_SERVERS #. Specify the number of locales on the command line. For example: .. code-block:: bash ./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 :ref:`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. .. _what-is-gasnet: 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 `_. .. _set-comm-conduit: 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 :ref:`Using Chapel with InfiniBand `) udp UDP - portable conduit, works on any network with a TCP/IP stack (see :ref:`Using the Portable UDP Conduit `) mpi MPI - portable conduit, works on any network with MPI 1.1 or newer 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 pwr6 ibv other udp ==================== =================== .. _set-comm-segment: 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. This option is not supported with the ``smp`` or ``ucx`` substrates. 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 =================== ==================== ibv large smp fast other everything =================== ==================== .. index:: single: colocale single: co-locale .. _readme-colocale: 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. For ``CHPL_COMM=gasnet`` runs, you may also want to consider setting one or both of: .. code-block:: bash export GASNET_SPAWN_VERBOSE=1 export GASNET_VERBOSEENV=1 where the first prints more information about GASNet's job launch actions, and the second is used to audit environment variable settings. When running ``CHPL_COMM=gasnet`` programs using the ``udp`` conduit, we've had best results with console I/O using: .. code-block:: bash export GASNET_ROUTE_OUTPUT=0 .. _set-comm-debugging: Advanced users may also 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-build GASNet and runtime when changing this setting (i.e., re-run the ``make`` command you used for your initial Chapel install).