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

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