CPUs/Compilers for Numerical Simulations?

X43B asks: "I'm building a 'luggable' computer for numerical simulation work (very niche, I know). My goal is to have the best single precision floating point performance for under $1000. I have decided on a Shuttle XPC layout. I can build a AMD 3500+ for ~$80 less than a Prescott 3.4Ghz. I know the AMD is supposed to be a better 'general purpose' CPU however I found this comparison which says the Intels are better for floating point. Additionally, even though the AMD is somewhat cheaper, I have found the free Intel Linux FORTRAN compiler quicker than gfortran. So even if the AMD had similar performance for cross compiling, the Intel would be ~10% faster with the free compiler. Does anyone have any recommendations on AMD vs Intel for single precision floating point operations? If you recommend the AMD, what (cheap or free) compiler can be used that is comparable to the Intel?"

Comments

save the money for tools

[ghostlibrary]

Save your CPU money for the compiler (e.g. PGF90, or what have you), or for development tools, and for more RAM. A good compiler will give you a better handle on numerical accuracy than a 'better' CPU stuck with an off compiler. More RAM will keep you from being stingy in single/double precision allocations.

Also, factor in your work time. A few percentage difference between CPUs won't count nearly as much as the 2 hours you saved because you had a good debugger, or the 4 hours saved because your editor makes it easier to write and jump around your code.

Your accuracy will be pretty much the same, you just have to understand how computers represent floats and plan accordingly. Use accurate representations even if they're slower to get the numerical accuracy you need, then optimize the slow parts.

Only optimize the stuff that runs slow. That means profile, don't just guess. You'll often be surprised by where the bottlenecks are.

Higher accuracy usually means more memory (going with doubles rather than floats) or work (converting to integers within your desired floating range to control floating point accuracy). CPU won't be a biggie, but having lots of RAM will help.

If you have a choice between spending 3 months writing and optimizing code, or spending 1 month writing code that isn't optimized, think of what 2 months of runtime will do. If it takes you a while to write the code, just buy your super-accurate machine _after_ coding, when it's time to do your real runs (since chip speeds already increase).

In fact, if it takes you 3 months to optimize, you'd be better off keeping the slow code, doing another project, and 3 months later just buying a faster PC to run the slow old code :)

All this off the top of my head, hope it helps.