Query regarding physically discontiguous memory

Valdis.Kletnieks at vt.edu Valdis.Kletnieks at vt.edu
Wed Dec 25 10:37:35 EST 2013

On Wed, 25 Dec 2013 14:45:01 +0530, Pritam Bankar said:

> This is what I don't understand what does it mean by
> containing large holes?
> physical discontiguous memory?
> Is it like when I put 2 RAM chips in a system then memory for them will be
> discontiguous?

> Per my understanding RAM memory will ALWAYS be contiguous whether its 512
> MB or 4 GB.

Your understanding is wrong.  That may be true on most x86-based desktops
and laptops and the like, but it's often very wrong on other architectures.
In fact, given that the x86 architecture is just one of like 27 archs that
Linux supports, it's a good bet that there's more archs that *don't* enforce
contiguous memory than do. Counting through the defconfigs to get a number
is left as an exercise for the student. :)

A common case for embedded systems is "256M of RAM at address 0, and
256M of RAM at address 0x80000000" (or whatever the halfway mark for the
architecture is).

NUMA boxes are sometimes "each node's RAM is at <node number>*4G" or similar,
which means lots of holes if each node is partly populated with only 2G.
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