Friday, August 6, 2010

Another Huge Simulation

I've talked some in earlier posts about the F ring simulation that I recently started. I've now posted a fair bit from that simulation including plots and movies that are constantly being updated as the simulation advances. Just today I started another simulation. This one is related to my work on Negative Diffusion. I have a paper that I submitted to Icarus and got back a review for on the topic. The reviewers wanted a number of changes that I hope to get done before the beginning of classes.

One of the issues with this work is that I have to use some interesting boundary conditions to use a local cell with a perturbing moon. These boundary conditions wrap particles in the azimuthal direction such that they preserve the gradient in the epicyclic phase induced by the rate of passage of the cell by the moon. These boundary conditions aren't something that everyone uses. One reviewer wanted me to explain them better, which I can certainly do. However, I realized something that I had thought of before which was that to really convince people, I needed to do a large simulation without the periodic local cell, something more global, and show that the same thing happens. I know it will work because I have seen this behavior in global simulations of the F ring and nearly global simulations of the Keeler gap. I know that this isn't a result of the boundary conditions. However, I feel it would be good to do this with a simulation that basically matches what I had done for one of the local cell simulations for a direct confirmation. The only problem is, there was a reason I was using a local cell. With a local cell these simulations only need 10^5 - 10^6 particles. If you don't use a local cell, the requirements get a lot higher.

So I just started the simulation using 14 nodes of my cluster. That's 112 cores and 112 GB of RAM. I tried using a particle size at the top end of what I had used in the paper. Unfortunately, that ran into swapping. It had nearly 400 million particles and the master machine wasn't happy about that. It wasn't so big that it flat out crashed, but it was just big enough that the memory footprint caused it to run too slowly. So I increased the particle size just a bit to a 156 cm diameter. This gives me just under 266 million particles. That's still a huge simulation, but it is just small enough that it runs without the master doing significant swapping.

I started the simulation this morning and I looked at the first output and things seem to be going well. It looks like it will take about 5 hours per orbit and get to the point where conclusions can be made in a little over a month. That's pretty good for the biggest simulation I've ever run.