Re: PAE kernel and 4GB of memory
On Thu, 2008-07-24 at 14:40 -0700, Nifty Fedora Mitch wrote:
On Thu, Jul 24, 2008 at 01:49:34PM +0100, Howard Wilkinson wrote:
>
> I am looking for a definitive answer to the question of where the PAE
> kernels become useful. I have seen various articles that mention needing
> PAE kernels if you have more then 4GB of physical memory in a 32-bit
> processor environment. I have also seen statements that say you need
> them if you have 4GB or more of memory. Now which is right? Also, even
> if you need a PAE kernel because the last few bytes are not addressable
> when you have exactly 4GB is this useful or is the trade off of larger
> page tables and pages going to eat any benefit of being able to address
> these few bytes and if so when does the PAE kernel become useful?
>
http://en.wikipedia.org/wiki/Physical_Address_Extension
Can you be more specific and define 'useful'.
In general on a 32 bit system you will have 32 bit pointers by default...
signed arithmetic gives you an effective 2GB process size. Compare and
contrast lseek() and lseek64()... sizeof(off_t something).
32 bits gives you a 4GB address space. Pointers are not signed (and the
off_t type reflects this, which is why it's different from int).
But if you have six 2GB processes running on a 6.x GB system is that
useful?
Are you playing with one Big process.
Do you have a test case or pointer to a test case (best) so folks with
large memory systems can sanity test this for you?
I suspect the question is related to accessing *physical* (not virtual)
memory.
poc