In some email I received from Eduardo Horvath, sie wrote:
[...]
> Let's say I want to wait on a particular `vm_page_t'.  The `vm_page_t'
> needs to contain a `condvar_t'.  Or more precisely *every* `vm_page_t' in
> the system needs to contain a `condvar_t' even if it's only used in one
> piece of code.  If you have 1 byte `condvar_t' and 1000 pages of RAM
> that's 1K.  If the `condvar_t' is a pointer in LP64 that's 8K.  (Ignoring
> structure alignment and packing issues that may waste more space.)

Ah.  I've never used condvar_t's in such quantity :)  I do see a tsleep()
on a "map" which is a `vm_map_t', so you're not far off.  I understand now
how keeping a tsleep(), which uses its own lock, has its advantages.

Darren