Re: HIGHMEM4G config for 1GB RAM on desktop?

[Jens Axboe]

On Wed, Aug 04 2004, Con Kolivas wrote:
> Jens Axboe wrote:
> >On Mon, Aug 02 2004, Barry K. Nathan wrote:
> >
> >>On Mon, Aug 02, 2004 at 04:02:34PM -0500, Steve Snyder wrote:
> >>
> >>>There seems to be a controversy about the use of the CONFIG_HIGHMEM4G  
> >>>kernel configuration.  After reading many posts on the subject, I still 
> >>>don't know which setting is best for me.
> 
> No idea what the performance hit is of highmem these days - it seems 
> insignificant compared to 2.4 so I've had it enabled for 1Gb ram.
> 
> >There's also the option of moving the mapping only slightly, so that all
> >of the 1G fits in low memory. That's the best option for 1G desktop
> >machines, imho. Changing PAGE_OFFSET from 0xc0000000 to 0xb0000000 would
> >probably be enough.
> >
> >Then you can have your cake and eat it too.
> 
> Something like this attached patch? Seems to work nicely. Thanks!
> 
> Cheers,
> Con

> Index: linux-2.6.8-rc2-mm2/arch/i386/kernel/vmlinux.lds.S
> ===================================================================
> --- linux-2.6.8-rc2-mm2.orig/arch/i386/kernel/vmlinux.lds.S	2004-05-23 12:54:46.000000000 +1000
> +++ linux-2.6.8-rc2-mm2/arch/i386/kernel/vmlinux.lds.S	2004-08-04 00:20:02.219462913 +1000
> @@ -11,7 +11,7 @@
>  jiffies = jiffies_64;
>  SECTIONS
>  {
> -  . = 0xC0000000 + 0x100000;
> +  . = 0xB0000000 + 0x100000;
>    /* read-only */
>    _text = .;			/* Text and read-only data */
>    .text : {
> Index: linux-2.6.8-rc2-mm2/include/asm-i386/page.h
> ===================================================================
> --- linux-2.6.8-rc2-mm2.orig/include/asm-i386/page.h	2004-08-03 01:29:28.000000000 +1000
> +++ linux-2.6.8-rc2-mm2/include/asm-i386/page.h	2004-08-03 23:58:16.000000000 +1000
> @@ -123,9 +123,9 @@
>  #endif /* __ASSEMBLY__ */
>  
>  #ifdef __ASSEMBLY__
> -#define __PAGE_OFFSET		(0xC0000000)
> +#define __PAGE_OFFSET		(0xB0000000)
>  #else
> -#define __PAGE_OFFSET		(0xC0000000UL)
> +#define __PAGE_OFFSET		(0xB0000000UL)
>  #endif

Yup precisely. I agree that there probably isn't a whole lot of
performance hit on a 1GB, it just seems silly that we need highmem on
such a standard memory configuration these days. Especially when just
moving the offset slightly removes that need.

-- 
Jens Axboe

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