[ltp] Kernel only see 3 of 4GB
Bernard Tremblay
linux-thinkpad@linux-thinkpad.org
Mon, 4 Jun 2007 19:43:54 +0200 (CEST)
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On Mon, 4 Jun 2007 15:36:52 +0200, "Florian Reitmeir"
<florian@reitmeir.org> said:
> On Mon, 04 Jun 2007, r8scq7b02@sneakemail.com wrote:
>
> > On Monday 04 June 2007 04:56, Michael Gaber wrote:
> > > yup, you'll have to use a 64 bit system because a 32bit can only address
> > > 4GB
> >
> > Are you sure about that?
> >
> > linux-image-2.6-686-bigmem:
> > This package depends on the latest binary image for Linux kernel 2.6 on
> > Pentium Pro/Celeron/Pentium II/Pentium III/Pentium 4 with 4-64G RAM machines.
>
> not all chipsets support more than 3gb, even on amd64 architectures.
> and on 32 bit there is a limit of 3gb _per_ process.
>
> --
> Florian Reitmeir
> --
> The linux-thinkpad mailing list home page is at:
> http://mailman.linux-thinkpad.org/mailman/listinfo/linux-thinkpad
Hi,
Both are true. 2**32 = 4GB thus you cannot address more than that. But there is ways to make the 32 bits machine to use more than 32 bits in a virtual address... It's call AWE (Advanced Windowing Extensions). The idea is to add a 3rd level to the calculation from virtual to real address. You add 4 more bits to the addressing and you then have 2**4=16 windows of 4GB each that you can address wich give your exactly 64GB. This is a similar process that we used before in 16 bits addressing to go over the 64KB limitation. This AWE feature is available at high price from M$ if you can afford to buy a datacenter licence. This feature, as one would expect, is free with Linux big memory kernel. Linux hackers please correct me if I'm making wrong assumptions about that AWE feature.
We have some Compaq servers here at my work place with 4GB that report to have 3,6GB... I don't know if it's because of PCI mapping but it makes some sense to me. But why would this PCI mapping not calculated as being part of the physical memory ?
Anybody would have a clearer explanation about how all this works and what is the BIOS role in this ?
Regards,
BT
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<div>On Mon, 4 Jun 2007 15:36:52 +0200, "Florian Reitmeir"<BR><<A href="mailto:florian@reitmeir.org">florian@reitmeir.org</A>> said:<BR>> On Mon, 04 Jun 2007, <A href="mailto:r8scq7b02@sneakemail.com">r8scq7b02@sneakemail.com</A> wrote:<BR>> <BR>> > On Monday 04 June 2007 04:56, Michael Gaber wrote:<BR>> > > yup, you'll have to use a 64 bit system because a 32bit can only address<BR>> > > 4GB<BR>> > <BR>> > Are you sure about that?<BR>> > <BR>> > linux-image-2.6-686-bigmem:<BR>> > This package depends on the latest binary image for Linux kernel 2.6 on <BR>> > Pentium Pro/Celeron/Pentium II/Pentium III/Pentium 4 with 4-64G RAM machines. <BR>> <BR>> not all chipsets support more than 3gb, even on amd64 architectures.<BR>> and on 32 bit there is a limit of 3gb _per_ process.<BR>> <BR>> -- <BR>> Florian Reitmeir<BR>> -- <BR>> The linux-thinkpad mailing list home page is at:<BR>>
<A href="http://mailman.linux-thinkpad.org/mailman/listinfo/linux-thinkpad">http://mailman.linux-thinkpad.org/mailman/listinfo/linux-thinkpad</A></div> <div> </div> <div>Hi,</div> <div> </div> <div>Both are true. 2**32 = 4GB thus you cannot address more than that. But there is ways to make the 32 bits machine to use more than 32 bits in a virtual address... It's call AWE (Advanced Windowing Extensions). The idea is to add a 3rd level to the calculation from virtual to real address. You add 4 more bits to the addressing and you then have 2**4=16 windows of 4GB each that you can address wich give your exactly 64GB. This is a similar process that we used before in 16 bits addressing to go over the 64KB limitation. This AWE feature is available at high price from M$ if you can afford to buy a datacenter licence. This feature, as one would expect, is free
with Linux big memory kernel. Linux hackers please correct me if I'm making wrong assumptions about that AWE feature.</div> <div> </div> <div>We have some Compaq servers here at my work place with 4GB that report to have 3,6GB... I don't know if it's because of PCI mapping but it makes some sense to me. But why would this PCI mapping not calculated as being part of the physical memory ? </div> <div> </div> <div>Anybody would have a clearer explanation about how all this works and what is the BIOS role in this ? </div> <div> </div> <div>Regards,</div> <div> BT<BR></div><p> 
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