[ILUG] Large amounts of RAM

[Paul Kelly]

Gavin McCullagh wrote:

> I remember not that long ago that to get linux to use more than (I
> think) 64MB RAM, you had to pass "mem=64M" to the kernel at boot.

The original PC AT BIOS specification allowed for at most 64MB of RAM to 
be reported. To discover any more than this you need to delve into 
'extended' BIOS calls, of which there are several flavours for finding RAM.

> My assumption is that this was the 2.0 kernel or something like that.

Some of the recent 2.0 kernels (past .36 I think) were fairly good at 
memory detection. 2.2 kernels are better on the whole. It will either 
work and give you all your RAM or not work and give you 64MB, detected 
through the original BIOS call.

> This is no longer necessary.  Is it the case that the ceiling was raised
> ie if you're above XGB RAM do you now need to pass it to the kernel??

Most precompiled Linux kernels support at most 1GB of RAM for reasons of 
efficiency (and some historical design choices). There are kernel 
compile options (and patches depending on your choice of kernel) which 
allow support of almost 4GB.

The 2.4 kernel extends this to the full 36-bit (64GB) address space 
supported by current Intel processors. There is a performance penalty 
for this mode, but I'm sure someone with reason enough to install an x86 
machine with 64GB RAM ('I want my MP3s to load faster' :) will be aware 
of this.

In a 2.4 kernel the option is in Processor type and features->High 
Memory Support.

> I ask because we have a machine here with 3 512MB sticks and top reads:
> 
> Mem:   906360K av,  313568K used,  592792K free,   72008K shrd,  133904K 

That'll be hitting the aforementioned 1GB limit. I don't think you lose 
much by allowing the kernel to address 1.5GB.

Paul.
Trying to work out how many watts 64GB would need...