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.
Roger
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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