Currently i have script as my PORTAGE_IONICE_COMMAND which sets cpu scheduling policy to idle, and scheduling class also to idle.
>chrt --idle --pid 0 $1
>ionice --class 3 --pid $1
I think PORTAGE_NICENESS and PORTAGE_IONICE_COMMAND are not very well suited to what they suppoosed to do, because first only sets niceness but not scheduling class, and second does not allow to run multiple commands without use of external script.
I just recently noticed that PORTAGE_NICENESS was not working anymore. Apparently because of the autogroup feature. I was not able to make it work using autogroup. Now I aliased emerge to "chrt --idle 0 emerge", which is effective.
It would be nice if portage could do that via configuration, potentially replacing the old variant.
I think this could improve the user experience greatly. Because without this, I could not even watch video streams while emerge was active.