priority-- curious about cpu resouce share

[Zhang Weiwu]

Today I did't experiment on {rt|id}prio(1), just to be interesting. I am 
trying to understand FreeBSD priority mechenism. IMHO realtime/idle 
priority only starvs processes when there is no cpu resource at all. 

To test, I run mpg321 on realtime priority 3, run ppp on realtime priority 
4, a little lower than mpg321, and scp over the ppp tunnel on normal 
priority. 

Top(1) shows an average about:
55.2% user,  0.0% nice, 21.5% system, 12.2% interrupt, 11.1% idle
* mpg321 takes average about 70%;
* ppp is seriously slowed down, it takes about 1.5% (normally 17% on full 
speed);
* sshd almost starves; it takes 0.00% (normally 2%) and only transfer 
several bytes once several minutes; the otherside scp prompts "stalled".

I'm the kind of newbie think 11.1% idle cpu time to be free cpu resource, 
because I don't have idle process. I wish to know why, as there are still 
11.1% free cpu resource, ppp and sshd doesn't get this share? 

Just to be curious. Thank you.

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