Doug Rabson <dfr@nlsystems.com> writes:
> Overcommit can be used for many reasons. I use it to reserve a large
> linear address space to mmap alpha i/o spaces [...]

Overcommit can be used for many reasons, but unless you've
misdescribed what you're doing, _that's not one of them_.

The mapped I/O pages need no backing store to be allocated for them by
the VM system.  They're backed by hardware.

And if you have 'placeholder' pages (I note that you didn't say you
mmap all of alpha i/o space, just reserve a large linear address space
in which to mmap it), then it should be possible to map them in such a
way (e.g. read-only ZFOD) in which they wouldn't count against backing
store requirements, either.



cgd
-- 
Chris Demetriou - cgd@netbsd.org - http://www.netbsd.org/People/Pages/cgd.html
Disclaimer: Not speaking for NetBSD, just expressing my own opinion.