Re: [ESPResSo-users] iccp3m question

[Axel Arnold]

On 07.10.14 22:17, Stefan Kesselheim wrote:
> Here is how the parallelism of ICDP3M works: The algorithm assumes the 
> electrostatics solver, in this case P3M calculates the forces on all particles, 
> including ghost particles, and that it is stored in the force property of the 
> particle. The electric field is calculated by dividing the occurring forces by 
> the particle charge. Then in each iteration, the algorithm calculates the new 
> surface charge density based on the electric field times the normal vector. 
> Multiplied by the area of the corresponding discretisation element, this yields 
> the new charge to assign to the particle. As the forces on real particles and 
> ghost particles are identical (to the bit), the charge update does not have to 
> be communicated, but is consistent on all nodes.
> Then, a new iteration step can be performed.
>
> The iteration in iccp3m_iteration the loop over is executed over the local 
> cells. This would be correct, if the local cells included the ghost cells. It 
> is however possible that this assumption is wrong. Then, of course, the 
> magnitude of the charges becomes inconsistent between the nodes, as it is not 
> communicated. Of the other espresso guys someone should know. Can you help guys?
Things are a bit more complicated. P3M computes the Fourier space forces 
only for physical particles, not for ghosts. Ghosts carry only forces 
originating from the real space forces, such that each actio-reactio 
pair occurs exactly twice. In other words, if a particle has ghost(s), 
parts of the real space interactions are stored with the particle, and 
other parts are stored with its ghosts.

After the normal force computation, the ghost forces are communicated 
back to their primary particles they are copies of, and zeroed out. 
Neither before nor after this step ghost and real forces are the same.

So, to answer the simple question: local_cells only contains physical 
cells. But looping over ghost cells doesn't really help as well, since 
you don't have access to the particles these ghosts are stemming from, 
namely, other processors. There are only very few places where accessing 
the ghosts is justified.

Best,
Axel

--
JP Dr. Axel Arnold
ICP, Universität Stuttgart
Allmandring 3
70569 Stuttgart, Germany
Email: address@hidden
Tel: +49 711 685 67609