linux-kernel - Re: [PATCH] reserve_early end-of-conventional-memory to 1MB II

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Date:	Wed, 27 Feb 2008 08:44:40 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <ak@...e.de>
CC:	Ingo Molnar <mingo@...e.hu>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander van Heukelum <heukelum@...lshack.com>
Subject: Re: [PATCH] reserve_early end-of-conventional-memory to 1MB II -
 some numbers to put it into perspective

Andi Kleen wrote:
> 
> Just to give some perspective of this:
> 
> On my laptop here
> 
>  BIOS-e820: 000000000009dc00 - 00000000000a0000 (reserved)
>  BIOS-e820: 00000000000d2000 - 0000000000100000 (reserved)
> 
> This means it reserves only ~193KB in the 640k-1MB area
> 
> With this patch it will reserve 384KB instead. This means 191KB
> are lost. While that doesn't sound too much it worth as much as
> 382 patches that reduce kernel code size by 512bytes or
> worth 3820 patches that reduce kernel code by 100 bytes in terms
> of memory consumption.
> 
> Now such kernel code size patches are always popular, but why undo that
> work by throwing away perfectly good memory elsewhere?
> 
> Or also the laptop kernel does
> 
> Freeing unused kernel memory: 340k freed
> 
> This means the 193KB now lost are worth 56% of the complete
> memory that is freed by __initdata/__init. Just maintaining
> these annotations is a lot of work, but why do all that if we
> then throw away than half as much memory as they save so easily?
> 

It doesn't waste any memory at all.

It arguably wastes some *address space*, but that's an entirely 
different thing.

Unless you have chipset-specific drivers to enable memory in the 
640-1024K memory area, there is nothing there.

	-hpa
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