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Re: should I go for 64bit version of Fedora 11 ?

On Wed, 4 Nov 2009 14:22:22 +1100
Cameron Simpson <cs zip com au> wrote:

> On 04Nov2009 14:01, I wrote:
> | On 03Nov2009 23:45, Alan Cox <alan lxorguk ukuu org uk> wrote:
> | | Such as the kernel ... which is much happier in 64bit mode with over 1GB
> | | of RAM.
> | 
> | Is there some URL I could visit that qualifies this?
> | I'm not doubting you, but I would like to have a mental model of roughly
> | why and how 64-bit mode benefits a system. [...]
> BTW, I found this:
>   http://forums.amd.com/devblog/blogpost.cfm?threadid=93648&catid=317
> which is interesting but doesn't give me much clue about why the kernel
> might like it.

If you have more than 1GB of memory then the kernel in 32bit mode has to
do extra work because it needs to maintain access to both virtual
mappings and physical mappings

Normally 32bit memory is laid out as

[0-3GB]    User application address mapping (as the user space sees it)
[3G-3.xG]  Mapping of almost 1GB RAM of physical ram
[3.xG-4G]  Vmalloc/io mappings/etc

which takes all the 4GB. To support > 1GB of RAM the kernel has to create
and destroy mmu mappings and access them indirectly which has a big cost.

In 64bit mode there is plenty of space for all the application and
mappings of main memory so that isn't required.


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