Re: 32bit or 64bit?

[Thomas Goirand <thomas@goirand.fr>]

Marek Podmaka wrote:
> Hello all,
> 
> I just yesterday learned that my Xeons are in fact 64bit, so I have
> few questions about it...
> 
> 1) What is the performance gain between running 32bit and 64bit linux
> on 64bit capable CPU? I'm running Etch with apache+php+mysql. Is it
> worth upgrading?

Hi,

I was quite surprised to see so many people saying that 64 bits goes at
the same speed, or is even slower. To what I could "feel", there is some
improvements.

64 bits arch have few hardware advantage:
1/ 8 general purposes registers instead of 4. That alone should justify
to use as much 64 bits binaries as you can.
2/ No more ugly memory pages that are slow to process, coming from an
old age, and really stupid IMHO.

Note that, if I'm not mistaking and as opposite to what I could read
here, this is ALSO an improvement when you have less than 4 GB of RAM
(eg: there is no more page fault interrupt in the processor).

> 2) Performance is gained just by compiling the same source code into
> 64bit binary? Or are the main apps (apache/php/mysql) optimized in
> source code to benefit explicitly from 64bit?
> 
> 2) Is it possible to run 64bit kernel, but 32bit
> application/libraries/everything? (So just recompiling kernel). If
> possible, is it better than 32bit kernel + 32bit apps?

Sure it's possible, but not so wise, IMHO. I think it's best to always
run in 64 bits mode, and "apt-get install ia32-libs" to be able to run
binaries in 32 bits. Sure, it will take more memory. But hey, memory is
cheap these days...

Thomas