Pros and Cons of amd64 (versus i386).

[Peter Jeremy]

On Thu, 2006-Apr-06 15:38:20 +0100, Pete French wrote:
>I was thinking of moving this to amd64, but was kind of put off by results
>from a test system I setup using an Athlon 64 3700+ to talk to this
>machine. The opteron box is currently running 6.1-PRE/i386, and the 3700 is
>runiing either Windows XP or 61-PRE/amd64. Under Windows I can completely
>saturate the ether comming in, and get 70% bandwidth going out (it's gig
>ether). Under amd64 on the client end I can only get about 55% utilisation
>in both directions. This surprised me a lot as when I was running i386
>on that box it was always faster. Of course a number of variables have
>changed since then (primarily moving from a broadcom gigabit card to using
>the onboard realtek card), but I was concened that the difference was due to
>the 64 bit operating system, as opposed to superior windws drivers, which
>seemed unlikely!

If you want to do a valid benchmark, you really need to use the same
hardware for the testing.  amd64 is not necessarily faster than i386
on the same hardware:
On the downside:
- amd64 has more, larger registers so context switching is slower
- 64-bit long/pointer and lower code density means larger working set size
  and lower cache effectiveness
On the upside:
- more registers means less register spills (better code)
- I think there's more efficient support for PIC code
- 64-bit arithmetic means faster multi-precision math (think RSA/DSA/SSL)
- access to >2GB virtual address space

>I know that what I should do is install i386 on the client and test again, but
>doing that will lose my only 64 bit environment so I am loathe to do so. Any
>comments ?

Backup your amd64 environment and install i386.  You can re-install
the amd64 once the testing is finished.  The best benchmark is always
your own application.

-- 
Peter Jeremy