differences between FLUENT 32-bit and 64-bit executables
It is well-known that the FLUENT 64-bit executable, which can be started by adding -64 the FLUENT command line, e.g. %> fluent 3d -64, is better than the 32-bit executable for problems in serial that require large amounts of memory - see for instance Solutions 232 and 344. Are there any other known performance differences between the 32-bit and 64-bit executables?
The 32/64 bit refers to the size of the addressing used in the executable.
The size of a pointer in the 64 bit executable is twice that of the 32 bit
executable, which allows you to access more memory space. The size
of the reals (floats) and ints are the same in either executable (storage used
by the data itself is the same size).
So in going to a 64 bit executable, I believe you are just doubling the
size of storage required for the pointers in the code. FLUENT does use
a lot of pointers, but I do not have exact numbers for you. Also, this will
be affected (to a certain degree) by what models are enabled (less so
than total memory useage though). I don't have a rule of thumb for you but a spot check on a 170K hex
mesh turbulent flow with heat transfer used 22% more memory in 64 bit than in
32 bit on an sgi machine.
There should be no difference in execution speed between 32 and 64 bit. The
only caution here is that this assumes that the platform's 32 bit and 64 bit math
and other system libraries should perform the same (usually true).
There should be no difference in parallel performance, however you may not have
the same mpi options since some vendor or third party MPI libraries may not be
ported to 64 bit mode.