[nSLUG] Software Freedom Day

Ian Campbell ian at slu.ms
Sat Sep 12 21:14:28 ADT 2009


On Sat, Sep 12, 2009 at 08:54:25PM -0300, jwark at eastlink.ca wrote:
> I have a laptop with an AMD64 processor and 1GB of RAM on which
> I run Debian 64bit. I use gkrellm to monitor the system. I have tried
> various 32bit distros on this system and find that CPU utiliztion is
> significantly *less* with the 64bit distro.

Saying I'm skeptical is perhaps a bit harsh given that... I mean
you're seeing what you're seeing, but this:

http://www.phoronix.com/scan.php?page=article&item=616&num=1

shows approximately what you'd (general you, not you specifically I
suppose) expect.

64bit gives you more address space and more general purpose registers.
The latter, under some circumstances, *can* give you a performance
boost although in most cases it won't. As you can see from the charts,
GCC gets a bump, with everything everything else it has a minimal
effect (positive or negative.)

On the other hand, the former can easily worsen performance.  Pointers
are now twice the size, with predictable effects... I have a client
with a perl script that fetches all of a poorly designed database
(think hundreds of columns, tens of thousands of rows) into an array
of hashes. The machine he was on was replaced with a 64bit install and
his RAM usage absolutely exploded.

I'm not sure why you would see a lower load.

Actually that's not true, one possibility is that the x86 packages for
most distros are compiled with a 486 or 686 target, which means none
of the newer instruction set extensions. x86_64 guarantees that at
least SSE and SSE2 etc. will be there.



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