[svlug] Maximum memory (debian 3.1)

[Joe Buck]

On Mon, Mar 19, 2007 at 03:23:38PM -0700, David Ljung Madison wrote:
> Anyways, does anyone have any thoughts on why my kernel is only
> showing ~3G of RAM?  The BIOS recognizes 4G (as does my boot
> loadable 'memtest').

Your memory is all there, but any given process can only use 3Gb.

It has to do with the way the address space is mapped.  The "standard"
configuration maps the kernel into 1/4 the address space, leaving only 3G
per-process.  There is an alternate kernel available from Red Hat (you
presumably can also get it via Centos) that makes all 4G available, but they
say

"NOTE: To provide a 4GB address space for both kernel and user space, the
kernel must maintain two separate virtual memory address mappings. This
introduces overhead when transferring from user to kernel space; for
example, in the case of system calls and interrupts. The impact of this
overhead on overall performance is highly application dependent."

This suggests that you don't want it if you're at all I/O bound (though
it's considered a win for huge EDA applications).