x86_64 mail servers

Matt Kettler mkettler at evi-inc.com
Wed Feb 8 19:00:43 GMT 2006

Mark Nienberg wrote:
> I've seen comments on this list that the x86_64 didn't seem to make much
> difference and I admit it is simpler to use the plain x86 version, but
> it bothers me a little to intentionally not use the software that is
> specifically configured for the chip.

Why does it bother you?

Theoretically x86-64 should be slightly slower for most uses unless you:

1) have a process that needs > 4gb of virtual address space
2) does a lot of 64 bit math that can't be performed with SSE

The ability to have huge processes and large amounts of physical ram is the
primary benefit of using a 64 bit computing architecture. The drawback is that
pointers become larger, taking up more memory, and causing more memory I/O than
would be needed if the app was 32bit. Unless you're actually using the larger
memory space you're increasing overhead without any benefit whatsoever. Very few
apps have such large memory footprints outside the realm of scientific
simulation or massive database crunching.

The other benefit of a 64bit computing architecture is the ability to do 64 bit
math operations in one instruction instead of a series of 32 bit operations.
However, very few applications regularly have any use for 64 bit operations
outside of crypto, some games, and high-end engineering/physics. Even these
regularly get their needs filled by using SSE, so the 64-bit math benefit is
very limited.

There's some benefit here to apps using 64-bit file offsets or 64 bit time
format, but I've never seen a "regular" application where either kind of
calculation was performed often enough to have a noticeable impact on
performance. Some scientific simulations may do a lot of 64bit time
calculations, but most of those could readily use SSE for it.

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