RE: i686 SMP systems with more then 12 GB ram with 2.4.x kernel ?

From: M. Edward Borasky (
Date: Tue Jan 01 2002 - 13:15:59 EST

> Because much of the memory cannot be used for kernel objects there is an
> imbalance in available resources and its very hard to balance them sanely.

As I understand it, in a Linux / i686 system, there are three zones: DMA
(0 - 2^24-1), low (2^24 - 2^30-1) and high (2^30 and up). And the hardware
(PAE) apparently distinguishes memory addresses above 2^32-1 as well.

1. Shouldn't there be *four* zones: (DMA, low, high and PAE)?

2. Isn't the boundary at 2^30 really irrelevant and the three "correct"
zones are (0 - 2^24-1), (2^24 - 2^32-1) and (2^32 - 2^36-1)?

3. On a system without ISA DMA devices, can DMA and low be merged into a
single zone?

4. It's pretty obvious exactly which functions require memory under 2^24 --
ISA DMA. But exactly which functions require memory under 2^30 and which
functions require memory under 2^32? It seems relatively easy to write a
Perl script to truck through the kernel source and figure this out; has
anyone done it? It would seem to me a valuable piece of information -- what
the demands are for the relatively precious areas of memory under 1 GB and
under 4 GB.

M. Edward Borasky

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