Namd on BGQ
A parameter study was undertaken to test simulation performance and efficiency of NAMD on the Blue Gene/Q cluster, BGQ, with attention to NAMD performance tuning documentation. Determining optimal parameters for a NAMD simulation on this system is more difficult as there are only certain simulation sizes that have optimal topologies (512, 1024, etc). The system of study is a 246,000 atom membrane protein simulation (Cytochrome c Oxidase embedded in a TIP3P solvated DPPC bilayer) using the CHARMM36 forcefield (protein and lipids). The unit cell is cubic with box dimensions 144 x 144 x 117 Angstroms.
Performance Tuning Benchmarks
Efficiency is measured with respect to the 16 ranks-per-node 512 core simulation. All simulations are started using a restart file from a pre-equilibrated snapshot. Performance in nanoseconds per day is based on the geometric mean of the three "Benchmark time" lines at the beginning of the simulation's standard output. In this section, the PME patch grid was manually doubled in either the X, Y, or Z directions. Default PME patch doubling in NAMD 2.9 is generally recommended (twoAway parameters need not be specified in the configuration file).
| Ranks | Cores | NAMD Config Options | ns/day | Efficiency |
| 16 | 512 | 2.79 | 1.00 | |
| 16 | 1024 | 5.05 | 0.91 | |
| 16 | 1024 | twoAwayX (default) | 5.62 | 1.01 |
| 16 | 2048 | twoAwayX (default) | 10.07 | 0.90 |
| 16 | 2048 | twoAwayXY | 10.59 | 0.95 |
| 16 | 4096 | twoAwayX | 14.32 | 0.64 |
| 16 | 4096 | twoAwayXY (default) | 17.63 | 0.79 |
| 16 | 4096 | twoAwayXYZ | 16.79 | 0.75 |
| 16 | 8192 | twoAwayX | 23.52 | 0.53 |
| 16 | 8192 | twoAwayXY (default) | 25.00 | 0.56 |
| 16 | 16384 | twoAwayX | 23.67 | 0.27 |
| 16 | 16384 | twoAwayXY | 28.31 | 0.32 |
| 16 | 16384 | twoAwayXYZ (default) | 27.98 | 0.31 |
PME Pencils
A "pencil-based" PME decomposition may be more efficient than the default "slab-based decomposition". In this study PME pencil grids are created for both dedicated PME nodes (lblUnload=yes) and non-dedicated PME nodes. Fine-tuning of PMEPencils resulted in insignificant performance gains for this study.
| Ranks | Cores | NAMD Config Options | ns/day | Efficiency |
| 16 | 4096 | twoAwayXY, PMEPencils=8, lblUnload=yes | 12.93 | 0.58 |
| 16 | 4096 | twoAwayXY, PMEPencils=12, lblUnload=yes | 17.27 | 0.77 |
| 16 | 4096 | twoAwayXY, PMEPencils=16, lblUnload=yes | 16.02 | 0.72 |
| 16 | 4096 | twoAwayXY, PMEPencils=20, lblUnload=yes | 15.41 | 0.69 |
| 16 | 4096 | twoAwayXY, PMEPencils=12 | 16.21 | 0.73 |
| 16 | 4096 | twoAwayXY, PMEPencils=16 | 17.92 | 0.80 |
| 16 | 4096 | twoAwayXY, PMEPencils=20 | 17.99 | 0.81 |
| 16 | 4096 | twoAwayXY, PMEPencils=24 | 17.83 | 0.80 |
| 16 | 4096 | twoAwayXY, PMEPencils=36 | 16.97 | 0.76 |
| 8 | 4096 | twoAwayXY, PMEPencils=20 | 18.24 | 0.82 |
| 16 | 4096 | twoAwayXY, PMEPencils=20 | 17.99 | 0.81 |
| 32 | 4096 | twoAwayXY, PMEPencils=20 | 13.94 | 0.63 |
Ranks-Per-Node Study
The "ranks-per-node" or simply the number of processes per compute node is a Blue Gene/Q runjob command parameter. For some simulations increasing the number of processes beyond the normal 16 may be more effective. In this study, memory requirements were too large to use 64 and decreased performance using 32. Ranks per node of less than 16 were also tested, although this would be less than optimal for cluster efficiency. Additionally, one study was conducted with the flag "ldbUnloadZero yes" which supposedly ensures no work is scheduled for processor zero, with no measurable effect.
| Ranks | Cores | NAMD Config Options | ns/day | Efficiency |
| 4 | 512 | 2.86 | 1.03 | |
| 8 | 512 | 2.84 | 1.02 | |
| 16 | 512 | 2.79 | 1.00 | |
| 32 | 512 | 2.29 | 0.82 | |
| 16 | 512 | ldbUnloadZero=yes | 2.79 | 1.00 |
Incorrect Particle-Mesh Ewald Grid
Long-range electrostatics are computed using PME for all simulations above with PME grid spacing set to be generated automatically with the "pmeGridSpacing 1.0" setting. A poor choice in PME grid spacing (i.e. not a multiple of 2,3, and 5) can result in increasingly large performance degradation due to the matrix size requirements in the FFT algorithm. Below is an example of the type of performance degradation that one may expect.
| Ranks | Cores | NAMD Config Options | ns/day | Efficiency |
| 16 | 512 | Poor PME Multiple (144x144x111) | 2.70 | 0.97 |
| 16 | 1024 | Poor PME Multiple (144x144x111) | 5.13 | 0.92 |
| 16 | 2048 | Poor PME Multiple (144x144x111) | 8.61 | 0.77 |
| 16 | 4096 | Poor PME Multiple (144x144x111) | 13.93 | 0.62 |
| 16 | 8192 | Poor PME Multiple (144x144x111) | 17.08 | 0.38 |
| 16 | 16384 | Poor PME Multiple (144x144x111) | 17.64 | 0.20 |