Hi Trevor, This makes sense. Please note that although memory is cheap on new parallel machines, there needs to be a balance between recalculation and memory. What we have found is that if we are too cavalier about memory usage, the model will get extremely slow due to memory not fitting on cache. This was found when coding the neutral physics scheme as a straightforward port from mom3. The original port resulted in neutral physics costing some 75% of the model! In mom3, adding neutral physics increased the cost by double. When recoding to reduce the amount of memory requirements (a substantial recoding with much reduction in memory), the neutral physics is now some 30% of the model (depending of course on numbers of tracers and other details). The mom4 neutral physics scheme is therefore less expensive, relative, to that encountered in mom3.
So...although it is a call based on no direct testing with the density module (due to my lack of time), I prefer to keep the double calculation of sp5 rather than introduce a new three-dimensional array. I must admit that we have not maintained a strict policy with mom4 regarding these issues. We may find over time more experience allows us to have a better idea how best to code on cache-based machines. If you wish to provide some tests with timings, then I will be happy to be converted. Thanks, Steve p.s. Glad you will attend the MIT meeting. Should be fun! [EMAIL PROTECTED] wrote: > > Hello Steve, > > One more thing. Since square roots are expensive, might > it not make sense to calculate and store the square root of > salinity immediately salinity is updated each time step, > then it would be available as sp5(i,j,k) and so would not > need to be calculated both in the density code and in the > partical derivative code. That is, right now it is calculated > both in the density code and in the partical derivative code, > and this seems a bit of a waste of computer time. > > I guess similar (i,j,k) arrays could also be stored for > p1t1 = p1*t1 and t2 = t1*t1 as well, but these are only simple > multiplcations that would be saved, not the more expensive > square roots. > > I am thinking of attending the z-coordinate modeliing meeting > at MIT in August. Might see you there. > > Trevor -- Dr. Stephen M. Griffies phone: +1-609-452-6672 Geophysical Fluid Dynamics Laboratory FAX: +1-609-987-5063 P.O. Box 308, Forrestal Campus Site B email: [EMAIL PROTECTED] Princeton, NJ 08542 USA http://www.gfdl.noaa.gov
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