# Using Chapel on Cray Systems¶

The following information is assembled to help Chapel users get up and running on Cray® systems including the Cray XC™, XE™, XK™, and CS™ series systems.

## Getting Started with Chapel on Cray X-Series Systems¶

Chapel is available as a module for Cray X-series systems. When it is installed on your system, you do not need to build Chapel from the source release (though you can). To use Chapel with the default settings and confirm it is correctly installed, do the following:

module load chapel

2. Compile an example program using:

chpl -o hello6-taskpar-dist $CHPL_HOME/examples/hello6-taskpar-dist.chpl  3. Execute the resulting executable (on four locales): ./hello6-taskpar-dist -nl 4  This may be all that is necessary to use Chapel on a Cray X-Series system. If the installation setup by your system administrator deviates from the default settings, or you are interested in other configuration options, see Using Chapel on a Cray System below. If instead you wish to build Chapel from source, continue on to Building Chapel for a Cray System from Source just below. For information on obtaining and installing the Chapel module please contact your system administrator. ## Getting Started with Chapel on Cray CS Systems¶ On Cray CS systems, Chapel is not currently available as a module due to the wide diversity of software packages that Cray CS customers may choose to install on their system. For this reason, Chapel must be built from source on Cray CS systems using the Building Chapel for a Cray System from Source instructions just below. ## Building Chapel for a Cray System from Source¶ 1. If using a XC, XE, or XK system, continue to step 2. If using a CS series system, set CHPL_HOST_PLATFORM to cray-cs. For example: export CHPL_HOST_PLATFORM=cray-cs  These are the supported systems and strings. Note that these values are used by default when building on the given systems. They can also be set manually. Also note that the cray-xe configuration covers Cray XK systems as well as Cray XE systems. System CHPL_HOST_PLATFORM CS series cray-cs XC series cray-xc XE series cray-xe XK series cray-xe 2. Optionally, set the CHPL_LAUNCHER environment variable to indicate how Chapel should launch jobs on your system: On a Cray CS system, to... set CHPL_LAUNCHER to... ...run jobs interactively on your system gasnetrun_ibv ...queue jobs using PBSPro (qsub) pbs-gasnetrun_ibv ...queue jobs using SLURM (sbatch) slurm-gasnetrun_ibv ...queue jobs using LSF (bsub) lsf-gasnetrun_ibv On a Cray X-series system, to... set CHPL_LAUNCHER to... ...run jobs interactively on your system aprun ...queue jobs using PBS (qsub) pbs-aprun ...queue jobs using SLURM (sbatch) slurm-srun You can also set CHPL_LAUNCHER to none if you prefer to manually manage all queuing and job launch commands yourself. On Cray CS systems, CHPL_LAUNCHER defaults to gasnetrun_ibv. On Cray X-Series systems, CHPL_LAUNCHER defaults to aprun if aprun is in your path, slurm-srun if srun is in your path and none otherwise. For more information on Chapel's launcher capabilities and options, refer to Chapel Launchers. 3. Select the target compiler that Chapel should use when compiling code for the compute node: On a Cray CS series system, set the CHPL_TARGET_COMPILER environment variable to indicate which compiler to use (and make sure that the compiler is in your path). To request... set CHPL_TARGET_COMPILER to... ...the GNU compiler (gcc) gnu (default) ...the Intel compiler (icc) intel On a Cray X-series system, ensure that you have one of the following Programming Environment modules loaded to specify your target compiler: PrgEnv-cray PrgEnv-gnu PrgEnv-intel  4. Make sure you're in the top-level chapel/ directory and make/re-make the compiler and runtime: gmake  Note that a single Chapel installation can support multiple configurations simultaneously and that you can switch between them simply by changing any of the above settings. However, each configuration must be built separately. Thus, you can change any of the settings in the steps before this, and then re-run this step in order to create additional installations. Thereafter, you can switch between any of these configurations without rebuilding. ## Using Chapel on a Cray System¶ 1. If you are working from a Chapel module: 1. Load the module using module load chapel 2. Optionally select a launcher, as in step 2 above 3. Select a target compiler, as in step 3 above If you are working from a source installation: 1. Set your host platform as in step 1 above 2. Optionally select a launcher, as in step 2 above 3. Select a target compiler, as in step 3 above 4. Set CHPL_HOME and your paths by invoking the appropriate util/setchplenv script for your shell. For example: source util/setchplenv.bash  2. Compile your Chapel program. For example: chpl -o hello6-taskpar-dist$CHPL_HOME/examples/hello6-taskpar-dist.chpl


See Compiling Chapel Programs or man chpl for further details.

3. If CHPL_LAUNCHER is set to anything other than none, when you compile a Chapel program for your Cray system, you will see two binaries (e.g., hello6-taskpar-dist and hello6-taskpar-dist_real). The first binary contains code to launch the Chapel program onto the compute nodes, as specified by your CHPL_LAUNCHER setting. The second contains the program code itself; it is not intended to be executed directly from the shell prompt.

You can use the -v flag to see the commands used by the launcher binary to start your program.

If CHPL_LAUNCHER is pbs-aprun or pbs-gasnetrun_ibv:

1. You can optionally specify a queue name using the environment variable CHPL_LAUNCHER_QUEUE. For example:

export CHPL_LAUNCHER_QUEUE=batch


If this variable is left unset, no queue name will be specified. Alternatively, you can set the queue name on your Chapel program command line using the --queue flag.

2. You can also optionally set a wall clock time limit for the job using CHPL_LAUNCHER_WALLTIME. For example to specify a 10-minute time limit, use:

export CHPL_LAUNCHER_WALLTIME=00:10:00


Alternatively, you can set the wall clock time limit on your Chapel program command line using the --walltime flag.

If CHPL_LAUNCHER is slurm-gasnetrun_ibv:

You must set the amount of time to request from SLURM. For example, the following requests 15 minutes:

export CHPL_LAUNCHER_WALLTIME=00:15:00


4. Execute your Chapel program. Multi-locale executions require the number of locales (compute nodes) to be specified on the command line. For example:

./hello6-taskpar-dist -nl 2


Requests the program to be executed using two locales.

5. If your Cray system has compute nodes with varying numbers of cores, you can request nodes with at least a certain number of cores using the variable CHPL_LAUNCHER_CORES_PER_LOCALE. For example, on a Cray system in which some compute nodes have 24 or more cores per compute node, you could request nodes with at least 24 cores using:

export CHPL_LAUNCHER_CORES_PER_LOCALE=24


This variable may be needed when you are using the aprun launcher and running Chapel programs within batch jobs you are managing yourself. The aprun launcher currently creates aprun commands that request the maximum number of cores per locale found on any locale in the system, irrespective of the fact that the batch job may have a lower limit than that on the number of cores per locale. If the batch job limit is less than the maximum number of cores per locale, you will get the following error message when you try to run a Chapel program:

apsched: claim exceeds reservation's CPUs


You can work around this by setting CHPL_LAUNCHER_CORES_PER_LOCALE to the same or lesser value as the number of cores per locale specified for the batch job (for example, the mppdepth resource for the PBS qsub command). In the future we hope to achieve better integration between Chapel launchers and workload managers.

6. If your Cray system has compute nodes with varying numbers of CPUs per compute unit, you can request nodes with a certain number of CPUs per compute unit using the variable CHPL_LAUNCHER_CPUS_PER_CU. For example, on a Cray XC series system with some nodes having at least 2 CPUs per compute unit, to request running on those nodes you would use:

export CHPL_LAUNCHER_CPUS_PER_CU=2


Currently, the only legal values for CHPL_LAUNCHER_CPUS_PER_CU are 0 (the default), 1, and 2.

...CHPL_* environment settings Setting up Your Environment for Chapel
...Compiling Chapel programs Compiling Chapel Programs
...Launcher options Chapel Launchers
...Executing Chapel programs Executing Chapel Programs
...Running multi-locale Chapel programs Multilocale Chapel Execution

## Cray File Systems and Chapel execution¶

For best results, it is recommended that you execute your Chapel program by placing the binaries on a file system shared between the login node and compute nodes (typically Lustre), as this will provide the greatest degree of transparency when executing your program. In some cases, running a Chapel program from a non-shared file system will make it impossible to launch onto the compute nodes. In other cases, the launch will succeed, but any files read or written by the Chapel program will be opened relative to the compute node's file system rather than the login node's.

## Special Notes for Cray XC, XE, and XK Series Systems¶

### Controlling the Heap Size¶

The "heap" is an area of memory used for dynamic allocation of everything from user data to internal management data structures. When running on Cray XC/XE/XK systems using either of 2 particular comm layer configurations, the runtime needs to know the maximum size the heap will grow to during execution. One of these is the default configuration, with the ugni comm layer:

CHPL_COMM=ugni
with a craype-hugepages module loaded


In this configuration, the heap is used for all dynamic allocations except for data space for arrays larger than 2 hugepages. (See Using the ugni Communications Layer, below, for more about hugepages.) The heap can be fairly small, because in nearly all programs that need a lot of memory, large arrays will drive the space requirements. The default size of the heap in this case is 16 GiB.

The other configuration is when the gasnet comm layer is used with a native substrate:

CHPL_COMM=gasnet
CHPL_COMM_SUBSTRATE=gemini or aries
CHPL_GASNET_SEGMENT=fast or large


In this case, the heap is used for all dynamic allocations, arrays and otherwise. By default the heap will occupy as much of the free memory on each compute node as the runtime can acquire, less some amount to allow for demands from other (system) programs running there.

In either of these configurations, advanced users may want to make the heap smaller than the default. Programs start more quickly with a smaller heap, and in the unfortunate event that you need to produce core files, those will be written more quickly if the heap is smaller. However, note that if you reduce the heap size to less than the amount your program actually needs and then run it, it will terminate prematurely due to not having enough memory.

To change the heap size, set the CHPL_RT_MAX_HEAP_SIZE environment variable. Set it to just a number to specify the size of the heap in bytes, or to a number with a k or K, m or M, or g or G suffix with no intervening spaces to specify the heap size in KiB (2**10 bytes), MiB (2**20 bytes), or GiB (2**30 bytes), respectively. Any of the following would set the heap size to 1 GiB, for example:

export CHPL_RT_MAX_HEAP_SIZE=1073741824
export CHPL_RT_MAX_HEAP_SIZE=1048576k
export CHPL_RT_MAX_HEAP_SIZE=1024m
export CHPL_RT_MAX_HEAP_SIZE=1g


Note that the value you set in CHPL_RT_MAX_HEAP_SIZE may get rounded up internally to match the page alignment. How much, if any, this will add depends on the hugepage size in any craype-hugepage module you have loaded at the time you execute the program.

Note that for CHPL_COMM=gasnet, CHPL_RT_MAX_HEAP_SIZE is synonymous with GASNET_MAX_SEGSIZE, and the former overrides the latter if both are set.

### Native Communication Layer¶

The Multilocale Chapel Execution page describes a variety of communication layers that can be used by Chapel programs. In addition to the standard runtime layers available, Chapel supports a Cray-specific ugni communication layer. The ugni communication layer interacts with the system's network interface very closely through a lightweight interface called uGNI (user Generic Network Interface). On Cray XC or XE systems the default is to use the ugni communication layer.

### Using the ugni Communications Layer¶

To use ugni communications:

1. Set your CHPL_COMM environment variable to ugni (the default):

export CHPL_COMM=ugni


This specifies that you wish to use the Cray-specific communication layer.

2. (Optional) Load an appropriate craype-hugepages module. For example:

module load craype-hugepages16M


The ugni communication layer can be used with or without so-called hugepages. Performance for remote variable references is much better when hugepages are used. However, using hugepages effectively may require setting CHPL_RT_MAX_HEAP_SIZE to a value large enough to encompass the program's memory needs (see Controlling the Heap Size, above), and that quantity can be hard to know. Using hugepages also means that the tasking layer cannot use guard pages to detect task stack overflows (see below).

To use hugepages, you must have a craype-hugepages module loaded both when building your program and when running it. There are several hugepage modules, with suffixes indicating the page size they support. For example, craype-hugepages16M supports 16 MiB hugepages. It does not matter which craype-hugepages module you have loaded when you build your program. Any of them will do. However, which one you have loaded when you run your program does matter. For general use, the Chapel group recommends the craype-hugepages16M module. You can read on for more information about craype-hugepage modules if you would like, but the recommended craype-hugepages16M module will probably give you satisfactory results.

The Cray network interface chips (NICs) can only address memory that has been registered with them, and there are limits on how many pages of memory can be registered. The Gemini NIC used on Cray XE and XK systems can register no more than 16k (2**14) pages of memory. The Aries NIC used on Cray XC systems can register more, but it has an on-board cache of registered page information with 16k entries and performance will be reduced if the number of registered pages exceeds the 16k entries in that cache. Thus for any kind of Cray X* system, you should choose a hugepage module whose page size is large enough that 16k of its hugepages will cover the program's per-node memory requirement or if that is not known, the compute node memory size. For example, the 2 MiB hugepages in the craype-hugepages2M module will cover a 32 GiB Cray XE compute node, but on a Cray XC system with 128 GiB compute nodes at least 8 MiB hugepages will be needed to achieve full coverage. Generally, using larger hugepage sizes results in modest performance benefits, mostly in program startup time. The craype-hugepages16M module will result in slightly faster program startup, and its 16 MiB hugepages will cover the node memory on any Cray X-series system.

Note that when hugepages are used with the ugni comm layer, tasking layers cannot use guard pages for stack overflow detection. Qthreads tasking cannot detect stack overflow except by means of guard pages, so if ugni communications is combined with qthreads tasking, stack overflow detection is unavailable.

#### Communication Layer Concurrency¶

The CHPL_RT_COMM_CONCURRENCY environment variable tells the ugni communication layer how much program concurrency it should try to support. This basically controls how much of the communication resources on the NIC will be used by the program. The default value is the number of hardware processor cores the program will use for Chapel tasks (CHPL_RT_NUM_HARDWARE_THREADS in the next section). Usually this is enough, but for highly parallel codes that do a lot of remote references, increasing it may help the performance. Useful values for CHPL_RT_COMM_CONCURRENCY are in the range 1 to 30. Values specified outside this range are silently increased or reduced so as to fall within it.

## Network Atomics¶

The Gemini(TM) and Aries(TM) networks support remote atomic memory operations (AMOs) on XC, XE, and XK series systems. When the CHPL_NETWORK_ATOMICS environment variable is set to ugni, the following operations on remote atomics are done using the network:

32- and 64-bit signed and unsigned integer types:
32- and 64-bit real types:
write()
exchange()
compareExchange()
sub(), fetchSub()

32- and 64-bit signed and unsigned integer types:
or(),  fetchOr()
and(), fetchAnd()
xor(), fetchXor()


Note that on XE and XK systems, which have Gemini networks, out of the above list only the 64-bit integer operations are done natively by the network hardware. 32-bit integer and all real operations are done using implicit on statements inside the ugni communication layer, accelerated by Gemini hardware capabilities.

On XC systems, which have Aries networks, all of the operations shown above are done natively by the network hardware except 64-bit real add, which is disabled in hardware and thus done using on statements.

## Known Constraints and Bugs¶

• Our PBS launcher explicitly supports PBS Pro, Moab/Torque, and the NCCS site versions of PBS. It may also work with other versions. If our PBS launcher does not work for you, you can fall back on a more manual launch of your program. For example, supposing the program is compiled to myprogram:

• Launch the myprogram_real binary manually using aprun and your own qsub script or command.
• Use ./myprogram --generate-qsub-script to generate a qsub script. Then edit the generated script and launch the myprogram_real binary manually as above.
• Redirecting stdin when executing a Chapel program under PBS/qsub may not work due to limitations of qsub.

• GASNet targets multiple network conduits as the underlying communication mechanism. On certain platforms, the Chapel build will use the mpi conduit as the default. As a result of using the mpi conduit, you may see a GASNet warning message at program start up. To squelch this message, you can set the environment variable GASNET_QUIET=yes.

• For X-series systems, there is a known issue with the Cray MPI release that causes some programs to assert and then hang during exit. A workaround is to set the environment variable, MPICH_GNI_DYNAMIC_CONN to disabled. Setting this environment variable affects all MPI programs, so remember to unset it after running your Chapel program.

• The amount of memory available to a Chapel program running over GASNet with the gemini and aries conduits is allocated at program start up. The default memory segment size may be too high on some platforms, resulting in an internal Chapel error or a GASNet initialization error such as:

node 1 log gasnetc_init_segment() at $CHPL_HOME/third-party/gasnet/gasnet-src/gemini-conduit/gasnet_gemini.c:<line#>: MemRegister segment fault 8 at 0x2aab6ae00000 60000000, code GNI_RC_ERROR_RESOURCE  If your Chapel program exits with such an error, try setting the environment variable CHPL_RT_MAX_HEAP_SIZE or GASNET_MAX_SEGSIZE to a lower value than the default (say 1G) and re-running your program. For more information, refer to the discussion of CHPL_RT_MAX_HEAP_SIZE above and/or the discussion of GASNET_MAX_SEGSIZE here: $CHPL_HOME/third-party/gasnet/gasnet-src/README

• As mentioned in Controlling the Heap Size, with CHPL_COMM=ugni and a hugepage module loaded, large arrays are allocated separately from the heap. This can change how programs report out-of-memory conditions when creating an array. With heap allocation, when a program runs out of memory it prints this message and then halts:

Out of memory allocating "array elements"


For an array allocated outside the heap, however, the allocation may seem to succeed and then the program will get an interrupt later, when it tries to initialize the memory. If this happens a Bus error signal will be reported, either like this with a SLURM workload manager:

srun: error: <nodename>: task <ID>: Bus error


or like this with a PBS Pro or Moab/Torque workload manager:

Process died with signal 7: 'Bus error'


These messages are not perfectly diagnostic because there are other program errors that can cause the same signal, notably misaligned memory references. However, such errors are not at all common in Chapel by the nature of the language. So in general, these messages can be taken as indicating that the program ran out of memory when trying to create space for a large array. We expect to be able to restore the previous behavior in the future.

## NCCS user notes¶

• NCCS Cray systems use a different qsub mechanism in order to enforce their queuing policies. We have attempted to make our pbs-aprun launch code work with this version of qsub, but require a CHPL_LAUNCHER_ACCOUNT environment variable to be set to specify your NCCS account name. For example:

export CHPL_LAUNCHER_ACCOUNT=MYACCOUNTID

• NCCS users either need to specify debug as their queue or set an explicit wall clock time limit using the mechanisms described above.