PAE kernel and 4GB of memory

Roger Heflin rogerheflin at
Fri Jul 25 21:30:39 UTC 2008

Howard Wilkinson wrote:
> 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.

Typically about the only cards that remap memory on a large scale (now) are 
video boards, I don't think that even the high-end IO (disk,infiniband) boards 
result in memory remapping, for them DMA to and from main memory is fast enough, 
for the video boards applications need high speed access to their memory so 
things can be displayed.   On my desktop board here if I have a nvidia 6200 
plugged in I lose 256MB (I don't even know if it is remapped to someplace I can 
even touch it if I were using a PAE kernel), without that board I have all of 
the ram, and I *only* have 3GB.

> 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.

When I last tried a PAE vs non-PAE I could not see a big enough difference in 
speed to matter, I take it those machines aren't 64 bit capable?

How much memory does "cat /proc/meminfo" if you are getting within 256MB of 
4096M it is probably not worth messing with.

> 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 :-)

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