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RE: [Xen-devel] Power aware credit scheduler

>From: Tian, Kevin
>Sent: 2008年6月19日 12:52
>c) when cpu's freq is scaled dynamically
>       When cpufreq/Px is enabled, cpu's frequency is adjusted
>to different operation points driven by a on-demand governor. So
>csched_acct may need take frequency difference among cpus into
>consideration and total available credits won't be a simple 300 *
>online cpu_number. 

Not accurate above. Credit scheduler can't anticipate the cpu freq
in next accounting 30ms phase, and thus it can still only assume
total credit budget as 300 * online cpu_number for allocation. The 
question is whether we need substract credit in 10ms vcpu account
multiplying a freq ratio. But it seems that two issues are along with
this approach:

a) total budget is counted inconsistently as vcpu accounting, which 
may bring inaccurate vision to credit scheduler, like balance, etc.

b) it's not easy to get accurate freq ratio on target cpu. Some cpus
may not support, and on-demand governor runs async with credit
tick timer. Also query remote cpu normaly incurs inter cpu traffic.
Maybe on-demand governor can be put align with credit tick timer
with same interval, which may solve the freq query issue.

It looks complex than initial thought. Also considering on-demand
governor will scale freq back immediately when there're more real
work to be done, this may not show a real impact in reality. We'll
keep an eye in future tune to see whether it matters. :-)


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