[gmx-users] 64-bit vs. 32-bit

[Berk Hess]

>Is it really impossible to have the same gain when going to 64 bit as you
>see for non-assembly-optimized code? I am just curious on what is causing
>this "barrier".

64-bit computation is always slower than 32-bit.
An Athlon 64 can be (much) faster than an Athlon XP/MP,
because of a better architecture.
But with the Athlon 64 you can have direct access to double
the amount of registers in 64-bit mode, not in 32-bit mode.
So programs could run faster in 64-bit mode, not due to the 64 bits,
but due to the amount of registers.
This might make Gromacs a bit faster to, when new innerloops are available.

The main reason why Gromacs is not much faster on an Athlon 64 is
that it heavily uses SSE, which just scales with the clock speed.
Most other software does not use SSE so much, so they will
get much more gain out of the better architecture.

Berk.

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