Maximum physical memory on i386 platform

Maximum physical memory on i386 platform

[Ravi K]

Hi,
  The configuration help for HIGHMEM feature on i386
platform states that 'Linux can use up to 64 Gigabytes
of physical memory on x86 systems'. I see a problem
with this:
 - page structures needed to support 64GB would take
up 1GB memory (64 bytes per page of size 4k) 
 - but the kernel can only use 896MB memory, unless
PAGE_OFFSET is changed to a lower value
 - enabling 64GB support does not automatically change
the value of PAGE_OFFSET
   So how are the page structures created if a machine
has 64GB memory? Or is it necessary to change 
PAGE_OFFSET (to 0x80000000) in such a configuration?

Thanks,
Ravi.

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Re: Maximum physical memory on i386 platform

[Joseph A Knapka]

"H. Peter Anvin" wrote:
> 
> Ravi K wrote:
> 
> > Hi,
> >   The configuration help for HIGHMEM feature on i386
> > platform states that 'Linux can use up to 64 Gigabytes
> > of physical memory on x86 systems'. I see a problem
> > with this:
> >  - page structures needed to support 64GB would take
> > up 1GB memory (64 bytes per page of size 4k)
> 
> 64GB is physical memory, not virtual memory.

And at approx. 64 bytes per strct page in mem_map, that's
1G worth of page structs, which is Ravi's point.

Cheers,

-- Joe
"I should like to close this book by sticking out any part of my neck
 which is not yet exposed, and making a few predictions about how the
 problem of quantum gravity will in the end be solved."
 --- Physicist Lee Smolin, "Three Roads to Quantum Gravity"
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Re: Maximum physical memory on i386 platform

[H. Peter Anvin]

Ravi K wrote:

> Hi,
>   The configuration help for HIGHMEM feature on i386
> platform states that 'Linux can use up to 64 Gigabytes
> of physical memory on x86 systems'. I see a problem
> with this:
>  - page structures needed to support 64GB would take
> up 1GB memory (64 bytes per page of size 4k) 


64GB is physical memory, not virtual memory.

	-hpa



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