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Offloading Computational Tasks of Hybrid MPI + OpenMP/OmpSs-2 Applications to GPUs

Table of contents:

Quick Overview

N-Body Benchmark

Users can clone or download this examples from the ?https://pm.bsc.es/gitlab/DEEP-EST/apps/NBody repository and transfer it to a DEEP working directory.

Description

An N-Body simulation numerically approximates the evolution of a system of bodies in which each body continuously interacts with every other body. A familiar example is an astrophysical simulation in which each body represents a galaxy or an individual star, and the bodies attract each other through the gravitational force.

N-body simulation arises in many other computational science problems as well. For example, protein folding is studied using N-body simulation to calculate electrostatic and Van der Waals forces. Turbulent fluid flow simulation and global illumination computation in computer graphics are other examples of problems that use N-Body simulation.

Requirements

The requirements of this application are shown in the following lists. The main requirements are:

Versions

The N-Body application has several versions which are built in different binaries. All of them divide the particle space into smaller blocks. MPI processes are divided into two groups: GPU processes and CPU processes. GPU processes are responsible for computing the forces between each pair of particles blocks, and then, these forces are sent to the CPU processes, where each process updates its particles blocks using the received forces. The particles and forces blocks are equally distributed amongst each MPI process in each group. Thus, each MPI process is in charge of computing the forces or updating the particles of a consecutive chunk of blocks.

The available versions are:

Building & Executing on DEEP

The simplest way to compile this application is: 

# Clone the benchmark's repository
$ git clone https://pm.bsc.es/gitlab/DEEP-EST/apps/NBody.git
$ cd NBody

# Load the required environment (MPI, CUDA, OmpSs-2, OpenMP, etc.)
# Needed only once per session
$ source ./setenv_deep.sh

# Compile all N-Body variants
$ make

The benchmark versions are built with a specific block size, which is decided at compilation time (i.e., the binary names contain the block size). The default block size is 2048. Optionally, you can indicate a different block size when compiling by doing: 

$ make BS=1024

The next step is the execution of the benchmark on the DEEP system. Since this benchmarks targets the offloading of computational tasks to the Unified Memory GPU devices, we must execute it in a DEEP partition that features this kind of devices. A good example is the dp-dam partition, where each nodes features:

In this case, we are going to request an interactive job in a dp-dam node. All we need to is: 

$ srun -p dp-dam -N 1 -n 8 -c 12 -t 01:00:00 --pty /bin/bash -i

With that command, we will be prompted to an interactive session in an exclusive dp-dam node. We indicated that we want to launch 8 processes with 12 CPUs per process at the moment of launching a binary in the allocated node through srun. However, you should be able to change the configuration (without overtaking the initial number of resources) when executing the binaries passing a different configuration to the srun command.

At this point, we are ready to execute the benchmark with multiple MPI processes. The benchmark accepts several options. The most relevant options are the number of total particles with -p, the number of timesteps with -t, and the maximum number of GPU processes with -g. More options can be seen passing the -h option. An example of an execution is: 

$ srun -n 8 -c 12 ./nbody.tampi.ompss2.cuda.2048bs.bin -t 100 -p 16384 -g 4

in which the application will perform 100 timesteps in 8 MPI processes with 12 cores per process (used by the OmpSs-2's runtime system). The maximum number of GPU processes is 4, so there will be 4 CPU processes and 4 GPU processes (all processes have access to GPU devices). Since the total number of particles is 16384, each process will be in charge of computing/updating 4096 forces/particles, which are 2 blocks.

In the CUDA variants, a process can belong to the GPU processes group if it has access to at least one GPU device. However, in the case of the non-CUDA versions, all processes can belong to the GPU processes group (i.e., the GPU processes are simulated). For this reason, the application provides -g option in order to control the maximum number of GPU processes. By default, the number of GPU processes will be half of the total number of processes. Also note that the non-CUDA variants cannot compute kernels on the GPU. In these cases, the structure of the application is kept but the CUDA tasks are replaced by regular CPU tasks.

Similarly, the OpenMP variants can be executed following the same steps but setting the OMP_NUM_THREADS to the corresponding number of CPUs per process. As an example, we could execute the following command: 

$ OMP_NUM_THREADS=24 srun -n 4 -c 24 ./nbody.tampi.omp.2048bs.bin -t 100 -p 8912 -g 2

Finally, if you want to execute the benchmark without using an interactive session, you can modify the submit.job script and submit it into the job queue through the sbatch command.

References