[coyotos-dev] Optional floating state

[Norman Hardy]

Shap and I were talking on the phone about an idea that did not get  
into the Keykos kernel but is described at <http://cap-lore.com/ 
CapTheory/KK/float.html>. (The 370 couldn't use it.)
I described it as omitting floating state (and such other optional  
user state such as vector registers) from domains that didn't need them.
He objected that it was difficult to know if that state was used and  
testing could not prove that it was unused.
It is better than that.
Include the state in each domain and code the kernel as described in  
the link above.
Then the overhead of loading or saving that state is paid only when  
and if the domain actually uses that state.
Indeed the optional state is not even swapped into RAM until when and  
if it is used.
Further if a period of time occurs where the domain is running but  
temporarily not using the state, the overhead is avoided.
In a period of time where only one domain is using the state, the  
overhead is also avoided, even while time sharing with other domains.

I can''t see a single pattern where you loose!
That's a rare optimization!