Kernel Memory

[michi1 at michaelblizek.twilightparadox.com]

Hi!

On 17:57 Thu 21 Jun     , Vijay Chauhan wrote:
> Hello,
> 
> I am newbie.
> It has been said "kernel memory is not pageable"
> What does it mean? There is no concept of kernel virtual address?
> 
> Any simple explanation will help me to udnerstand.

The right term is actually "kernel memory is not swappable". Swapping means
writing inactive memory to disk and then using it for something else. Kernel
memory not being swappable is a design decicion made in the early linux days.
Operating systems which swap kernel memory need to isolate everything which
should not be swappd out (e.g. things needed for swap-in, realtime stuff,
security sensitive data, ...). This is quite a bit of work. I also guess it is
pretty pointless nowadays. Installed memory and is getting so huge that virtual
memory developers have a hard time trying to keep cpu-usage overhead for
swapping user space memory low.

> There is no concept of kernel virtual address?

Kernel memory uses virtual addresses as well. However, these the entire system
memory is continuously mapped somewhere in the virtual address space. The
drawback is that fragmentation turns allocation of large continuous memory
regions into a game of luck.

There is also an virtual address area (vmalloc) which is used to dynamically
map multiple scattered pages to a continuous region. But this is rather slow
and rarely used.

You might want to take a look at: http://lwn.net/Kernel/LDD3/

	-Michi
-- 
programing a layer 3+4 network protocol for mesh networks
see http://michaelblizek.twilightparadox.com