PAE kernel and 4GB of memory

[Howard Wilkinson]

Roger Heflin wrote:
> 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?
>>
>> Howard.
>>
>
> It depends on the bios, you would have to try with and without the PAE 
> kernel and see if the amount of usable ram changes.
>
> Some bioses won't remap any memory below 4GB (that is covered by 
> something else) to over 4GB, if your bios does not remap anything 
> above 4GB when you only have 4GB (or less) then PAE won't buy you 
> anything.    And since often moving the covered memory from below 4GB, 
> sometimes means moving some non-covered memory and therefore lowering 
> the memory usable for an OS that does not support PAE-ie that other 
> OS, often the bios *WON'T* move the covered memory at all because it 
> would lower the usable memory below 4GB for that other OS.
>
> This is almost always true on the Desktop class MB's, and it is 
> sometimes true on the higher end stuff also.
>
>                               Roger
>
>
Hmmm, I was afraid that this would be the answer! So the "definitive" 
answer is there is not one :-(

Given that some of the BIOS comes from ad-on cards and this is going to 
affect each machine differently, then can the resulting map be different 
with different cards, or even with the same cards but in different 
slots, or even as in the bad old days of Windows 3.1 where the different 
value of the MAC address on a network card meant a different memory layout.

The specific case that I have is 20 Intel SRMK4 machines all with 4GB of 
physical memory. Currently all running Fedora 32-bit kernels. I have yet 
to test the PAE kernels because I do not have any suitable benchmarks 
but I will see what happens.

Thanks to all for the responses.

Howard.

NOTE: my idea of useful is that the benefits outweigh the costs and the 
system gains overall. So a very complex calculation all round :-)