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

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On 04Nov2009 09:56, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
| On Wed, 4 Nov 2009 14:22:22 +1100
| Cameron Simpson <cs@xxxxxxxxxx> wrote:
| > On 04Nov2009 14:01, I wrote:
| > | On 03Nov2009 23:45, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
| > | | Such as the kernel ... which is much happier in 64bit mode with over 1GB
| > | | of RAM. [...]
| > | 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. [...]
| 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.

Thank you! Cheers,
Cameron Simpson <cs@xxxxxxxxxx> DoD#743

Like dogs and muggers, transistors can sense fear.      - Norman Yarvin

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