Using Chapel on SGI Altix¶

The Chapel team does not have access to an SGI Altix for the purposes of testing, so this file describes the experiences that other users have had in hopes that it will be useful to future Altix users. This file assumes a certain amount of familiarity building and using Chapel in single- and multi-locale settings such as desktops or commodity clusters.

As an opening note, performance on the Altix was not quite as good as we’d expected based on what we see on other platforms. We conjecture that the cause is that our runtime pthreads are not bound to specific cores/processors and expect that some overhead creeps in as the OS tries to schedule them – that perhaps it ends up moving them around more than is ideal. The Altix represents a larger core count per node (not to mention a different OS) than the platforms that we’ve focused on to date, so it may be more sensitive to this. As a result, a possible area of future work would be to look into how our runtime might be modified to do this binding and/or to utilize a user-level tasking library that multiplexes logical tasks onto a fixed set of pthreads equal to (and bound to) the number of cores.

Note

This performance characterization was written when fifo was the default tasking layer. It is likely that qthreads performs substantially better on Altix as it provides all of the desired runtime capabilities listed above. Additionally, previous studies done by the qthreads team have shown strong performance on Altix. See Chapel Tasks for more information on Chapel’s tasking layers.

Due to its support for shared memory, an Altix machine can either be viewed as a large single locale, or as a multi-locale machine. Our experience is that better performance is obtained when treating it as a single-locale machine. However, since most Altix installations are organized as multi-partition machines, you will need to treat it as a multi-locale machine if you want to span multiple partitions.

For the single-locale version, you should be able to build Chapel in the standard way as described in the Quick Start Instructions, making sure that the CHPL_COMM environment variable is unset or set to none.

For multi-locale executions, Chapel uses GASNet as our means of specifying inter-locale communication. Left to its own devices, GASNet prefers to use its SHMEM conduit on the SGI Altix because it’s considered the lowest overhead communication layer with which to implement GASNet. Unfortunately, GASNet’s SHMEM conduit is not thread-safe (because SHMEM itself is apparently not thread-safe); or, to use GASNet terminology, it does not support PAR mode. Chapel’s execution model requires PAR mode, and therefore the suboptimal MPI conduit needs to be used for GASNet executions.

This can be done using the following settings:

# cd to chapel source directory
export CHPL_HOME=$(pwd)
export CHPL_COMM=gasnet
export CHPL_COMM_SUBSTRATE=mpi
# edit make/Makefile.base to include
CHPL_GASNET_CFG_OPTIONS += --disable-aligned-segments

In our experience, different Altix installations support MPI in different ways. If your Altix machine supports MPI via an mpicc compiler command and launches using an mpiexec command, then make the following settings:

export MPI_CC=mpicc
export MPI_LDFLAGS=-mt
export MPI_RUNCMD="mpiexec -n %N %C"
export MPI_LIBS=

If on the other hand, your Altix machine supports MPI via a normal compiler invocation and by linking in the mpi libraries, make the following settings:

export MPI_CC=cc   # or whatever compiler you wish to use
export MPI_LIBS=-lmpi

Then build the Chapel compiler/runtime again using make or gmake.