[Users] GRHydro Multipatch announcement

[Roland Haas]

Hello Ian, all,

> Do you know if there is any performance penalty when multipatch is switched off?
I did not do extensive testing, but for the simple TOV only test that I
did, I found no measurable slowdown when multipatch was switchted off
compared not compiling in Multipatch.

I ran the TOVSolver testsuite test_tov_carpet.par with HLLE solver and
TVD reconstruction and 64^3 grid for 100 iterations. The differences
between using pointers and straight arrays was about 1%, sometimes (when
running the same run more than once) pointers were faster, sometimes
arrays (this is for gcc 4.6, on my laptop on a single core).

If you look at what was done then essentially pointers were introduced
which point either to the local tensor variables (gaa) or to the global
ones (gxx). Apparently using pointers still lets the compiler do all the
optimizations and keeps track of aliasing. I do not know if this might
change once we use cctk_lsh in the flesh to declare array dimensions
rather than passing three individual integers per grid function.

So, no to my knowledge there is no performance penalty. Things will
likely be different if one actually were doing the transformations but
with a trivial jacobian (which would make the code a bit shorter and
easier to read in some parts in part. GRHydro_Transform*.c).

Yours,
Roland

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