Re: [SLUG] problem wth memory totals

From: Ian C. Blenke (icblenke@nks.net)
Date: Tue Jun 18 2002 - 16:17:42 EDT


On Tue, 2002-06-18 at 15:46, Mikes work account wrote:
>
> I am running RH7.1 2.4.9-31smp.
>
> I have 1.4 GB of ram and only 899M shows up on top. I have added the
> append="1407M" to the appropriate boot information in lilo.conf and still
> get only 899M showing up on top. /proc/meminfo is also only picking up
> 899M. When I boot, the systems counts down and gives me 1407 M of ram
> available.
>
> What gives and why doesn't the append statement work in 9-31.

The default kernel memory model does not support more than 960M of RAM.

In order to use that extra RAM, you will want to use CONFIG_HIGHMEM with
either the CONFIG_HIGHMEM4G or CONFIG_HIGHMEM64G memory models.

HIGHMEM4G uses a 3G/1G split under x86, or 2G/2G split on some other
architectures.

HIGHMEM64G is for machines with over 4G of RAM (up to 64G). x86 machines
use Intel's Physical Address Extension (PAE) to address this memory.
Only servers really support this - you won't see this in any desktop PC
at the moment.

You proabably want to enable CONFIG_HIGHMEM4G in your .config and build
a kernel with that memory model.

A prime gotcha is the fairly severe performance penalty on x86 for 2.4
and RAM over 1G. It assumes that PCI devices can only access below the
1G mark, so it uses bounce-buffers for DMA. Read this for more info:

         http://www.tldp.org/HOWTO/IO-Perf-HOWTO/overview.html

YMMV. My dual AthlonXP box has a pretty severely hacked up 2.4.18 kernel
with tweaks like this.

> I have 9-31 working on my desktop and by changing the append statement I can
> alter the ram in top. But not on my smp machine. I have looked for patches
> and found not that relate tot his issue.

Changing MEM= isn't going to help you. You need a new kernel built with
CONFIG_HIGHMEM support and the appropriate memory model. Yes, this means
that your kernel will be a special "SMP Highmem" kernel. Most
distributions do NOT enable HIGHMEM by default in their kernels.

Hope this helps.

 - Ian C. Blenke <icblenke@nks.net> <ian@blenke.com>
http://ian.blenke.com



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