Virtual Memory Settings in Mac OS

[mol-general@lists.maconlinux.org]

On Wed, Aug 07, 2002 at 02:31:37PM -0400, Robert Vogt IV wrote:
> 	Hmmm ... my guess is probably not while idle, although there might
> be a performance hit when running.  If VM works at all under MacOS, I'm
> assuming MOL is pretending to be the MMU (unless its kernel module is
> directory editing MOL's PTE's), which would be pretty slow.  Samuel -
> which method does the current version use for VM?

Hrm... I would say that MOL does both :-)

MOL presents a fully working MMU to the client OS (in this case 
MacOS). I guess one could say the MOL pretends to be the MMU. MOL
does not hook into any VM functions of MacOS though.

In order to provide a fully working MMU, MOL takes full control
over the PTEs (and other MMU facilities). Oversimplified, MOL maps
VSIDs used by the client OS into VSIDs reserved by MOL.

I have recently done extensive benchmarking of the MOL engine. The
overhead due to the MMU-wrapping is quite small. So small in fact that
further optimizations in this area are hardly justified.

For a truly multi-tasking client OS (i.e. not MacOS), the main 
overhead seems to be related to the privileged instructions 
issued during (client OS) context. This is a problem which 
can be worked around though.

/Samuel