[gentoo-user] For what is emerge waiting for...?

Hi,

no critism or anything else negative implied here...
,..I am just asking out of coriosity:

This day I am busy compiling all the application,
which are installed on my old system.

I gave emerge 12 threads and 6 cores to do its work
(The CPU is a 6 core one).

I start emerge with a bunch of packages to compile and
install.

And it happens from time to time that all cores seem to be 
on holiday: No load at all. And emerge sits there and waits...
And the harddisk is not busy at all...no blinkenlights,
nothing.

Would it be possible, that there is kind of temporary
dead lock of some kind...when emerge is started with more
then one core?

What else than writing to disk and using the cores?

Cheers!
Meino