Meeting update - Nov. 18, 2013

Last week was a short week for me — I left town late Thursday for the weekend to celebrate my grandmother's 70th birthday. I spent the time I had learning about estimating memory usage for simulations (following this page here- ​http://astrobear.pas.rochester.edu/trac/astrobear/wiki/Tutorials/JobSizes). It was basically straightforward in retrospect, but got a little confused on the wording/explanation on the wiki page for it — so spent a bit of time learning about that. I plan on editing that page to make it clearer. After many emails back and forth, I think this is how it goes - ,

  1. The amount of allocated variables for MHD vs. Hydro simulations are different than I expected. For hydro simulations, you only count the fluid variables you need for the simulation (e.g. in 2D this would be rho, px, py, E - 4 variables). For MHD , there are *always* 8 variables in the q-array (cell-centered quantities), no matter the dimension of the problem - (rho, px, py, pz, E, Bx, By, Bz). In addition to these 8 variables, you also count the variables in the aux arrays - these include the EMF's on cell edges, and the B fields at cell faces. There are different arrays for EMF parents/children and the accumulated EMF over the steps. Together in 2D these give another 5 'auxillary' variables - Bx, By, Ez, Ez_child, Ez_parent. In 3D there are 12 - Bx, By, Bz, Ex, Ey, Ez, Ex_child, Ey_child, Ez_child, Ex_parent, Ey_parent, Ez_parent. What about 1D? And should we put that on the wiki page too?
  1. GB is a misnomer for RAM, where the unit is actually the GiB in *most* usages of the word. 1 GiB = 10243 bytes. So, when I was doing my memory calculations using the GB = 1e9 bytes, I was off from the values quoted on the wiki page. I didn't know RAM is actually in units of GiB, even though the CIRC pages quote them in GB. This should be explained on the wiki page for memory estimates as well.

Also just a tip — we need 9 credit hours to be full-time graduate students now — not the usual 12, according to the department website and verified by the graduate student coordinator.

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