linux-kernel - Re: Does Linux have plan to support memory hole remapping?

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Message-ID: <20080409091307.GG19010@one.firstfloor.org>
Date:	Wed, 9 Apr 2008 11:13:07 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Zhao Forrest <forrest.zhao@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>, discuss@...-64.org,
	linux-kernel@...r.kernel.org, yhlu.kernel@...il.com, mingo@...e.hu,
	ak@...e.de
Subject: Re: Does Linux have plan to support memory hole remapping?

On Wed, Apr 09, 2008 at 05:04:24PM +0800, Zhao Forrest wrote:
> On 4/9/08, Andi Kleen <andi@...stfloor.org> wrote:
> > "Zhao Forrest" <forrest.zhao@...il.com> writes:
> > >
> > > As we can see from above information that the physical memory in system
> > > is 32768MB(32GB). However OS is only using about
> > > (32768-512)MB(MemTotal:     33010240 kB). Does this mean that this
> > > linux kernel can't use the physical memory remapped
> > > from (4G-512M, 4G) to (32G, 32G+512M)?
> >
> > The linux kernel can only use the memory passed to it by the BIOS.
> > Sometimes they need special BIOS setup options to enable remapping. If
> > there are no such options and you can't upgrade it you're out of luck
> 
> Yes, I have enabled mem hole remapping in BIOS SETUP.
> After looking into dmesg, I found that all 32GB physical memory has
> been allocated to 8 nodes, but why MemTotal shows a less amount of
> memory than 32GB? Interesting......

There is some loss of memory before MemTotal, both from the BIOS
(SMM, Frame Buffer for integrated graphics etc.) and from the kernel
(mem_map which costs a few percent and some other data structures
which are allocated early) 

I covered this in detail in http://halobates.de/memorywaste.pdf

> Does MemTotal means the amount of physical memory allocated to all
> nodes? Or it has different meanings?

Amount of memory left over after early boot on all nodes and visible
to the kernel (so e.g. minus reservations for kdump kernels) 

-Andi
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