HPX - High Performance ParalleX


How to Use HPX Applications with PBS

Most HPX applications are executed on parallel computers. These platforms typically provide integrated job management services that facilitate the allocation of computing resources for each parallel program. HPX includes out of the box support for one of the most common job management systems, the Portable Batch System (PBS).

All PBS jobs require a script to specify the resource requirements and other parameters associated with a parallel job. The PBS script is basically a shell script with PBS directives placed within commented sections at the beginning of the file. The remaining (not commented-out) portions of the file executes just like any other regular shell script. While the description of all available PBS options is outside the scope of this tutorial (the interested reader may refer to in-depth documentation for more information), below is a minimal example to illustrate the approach. As a test application we will use the multithreaded hello_world program, explained in the section Hello World Example.

#PBS -l nodes=2:ppn=4


pbsdsh -u $APP_PATH --hpx:nodes=`cat $PBS_NODEFILE` $APP_OPTIONS

The #PBS -l nodes=2:ppn=4 directive will cause two compute nodes to be allocated for the application, as specified int the option nodes. Each of the nodes will dedicate four cores to the program, as per the option ppn, short for "processors per node" (PBS does not distinguish between processors and cores). Note that requesting more cores per node than physically available is pointless and may prevent PBS from accepting the script.

APP_PATH and APP_OPTIONS are shell variables that respectively specify the correct path to the executable (hello_world in this case) and the command line options. Since the hello_world application doesn't need any command line options, APP_OPTIONS has been left empty. Unlike in other execution environments, there is no need to use the --hpx:threads option to indicate the required number of OS threads per node; the HPX library will derive this parameter automatically from PBS.

Finally, pbsdsh is a PBS command that starts tasks to the resources allocated to the current job. It is recommended to leave this line as shown and modify only the PBS options and shell variables as needed for a specific application.

[Important] Important

A script invoked by pbsdsh starts in a very basic environment: the user's $HOME directory is defined and is the current directory, the LANG variable is set to C, and the PATH is set to the basic /usr/local/bin:/usr/bin:/bin as defined in a system-wide file pbs_environment. Nothing that would normally be set up by a system shell profile or user shell profile is defined, unlike the environment for the main job script.

Another choice is for the pbsdsh command in your main job script to invoke your program via a shell, like sh or bash, so that it gives an initialized environment for each instance. We create a small script runme.sh which is used to invoke the program:

# Small script which invokes the program based on what was passed on its
# command line.
# This script is executed by the bash shell which will initialize all
# environment variables as usual.

Now, we invoke this script using the pbsdsh tool:

#PBS -l nodes=2:ppn=4


pbsdsh -u runme.sh $APP_PATH --hpx:nodes=`cat $PBS_NODEFILE` $APP_OPTIONS

All that remains now is submitting the job to the queuing system. Assuming that the contents of the PBS script were saved in file pbs_hello_world.sh in the current directory, this is accomplished by typing:

$ qsub ./pbs_hello_world_pbs.sh

If the job is accepted, qsub will print out the assigned job ID, which may look like:

$ 42.supercomputer.some.university.edu

To check the status of your job, issue the following command:

$ qstat 42.supercomputer.some.university.edu

and look for a single-letter job status symbol. The common cases include:

The example qstat output below shows a job waiting for execution resources to become available:

Job id                    Name             User            Time Use S Queue
------------------------- ---------------- --------------- -------- - -----
42.supercomputer          ...ello_world.sh joe_user               0 Q batch

After the job completes, PBS will place two files, pbs_hello_world.sh.o42 and pbs_hello_world.sh.e42, in the directory where the job was submitted. The first contains the standard output and the second contains the standard error from all the nodes on which the application executed. In our example, the error output file should be empty and standard output file should contain something similar to:

hello world from OS-thread 3 on locality 0
hello world from OS-thread 2 on locality 0
hello world from OS-thread 1 on locality 1
hello world from OS-thread 0 on locality 0
hello world from OS-thread 3 on locality 1
hello world from OS-thread 2 on locality 1
hello world from OS-thread 1 on locality 0
hello world from OS-thread 0 on locality 1

Congratulations! You have just run your first distributed HPX application!