[nSLUG] Software Freedom Day

Ian Campbell ian at slu.ms
Sun Sep 13 16:59:47 ADT 2009


On Sun, Sep 13, 2009 at 03:09:24PM -0300, jwark at eastlink.ca wrote:
> 
> I am too old to want to go investigating hardware architecture details, but I
> believe that the data path widths with 64bit systems are 64 bits wide instead of
> the 32 bits with 32bit systems. That means that data flow rates between
> memory and CPU are higher with 64bit systems. To me, that would adequately
> account for the observed reduction in CPU utilization with the 64bit distro.

The Pentium III had a 64bit bus, so I don't think that's correct.

> I don't see why 64bit pointers would have a slow-down effect. With 64bit pointer
> fetch the time to fetch a 64bit pointer would not be greater than for a 32bit
> pointer to be fetched in a 32bit system.

Time to fetch isn't the issue. 64bit means more memory wasted on
pointers and padding for alignment. It's only a couple bytes per
instance, but it really adds up. At best that means you're just
wasting memory that could have gone to caching, at worst it means
you're swapping more.

Both of those have a measurable effect.



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