Re: [Q] about Credit Scheduler Dom0 Scheduling policy.

[Emmanuel Ackaouy <ack@xensource.com>]

On Wed, Oct 25, 2006 at 10:02:35PM +0900, Atsushi SAKAI wrote:
> Hi, Emmanuel
> 
>  Thank you for your patches.
> I tested on my environment
> 
> 1)Credit w/ Boost
> 2)Credit(previous)
> 3)SEDF(previous)
> 
>   1   2    3
>  44  16   33
> 133  66  133
> 533 266  266
> (Kbps)

Wow. This is quite an improvement!

Out of curiosity, what are the numbers like when running this
benchmark with no spinning VCPUs competing?

> With this patches, the CREDIT scheduler changed for I/O aware.
> (At vcpu_wake, the priority changes from UNDER to BOOST,
> At vcpu_acct, the priority changes from BOOST to UNDER.) 
> 
> It seems reasonable fixes!
> But I am afraid many I/O intensive GuestOSes are running.
> (I hope this prospect is needless fear.)

I've been careful to prevent BOOSTed VCPUs from taking over the
system or otherwise impacting fairness:

- Only VCPUs with positive credits can be boosted.
- While boosted, a VCPU is charged for any substential CPU
  resources consumed.
- VCPUs can run uninterrupted with a boosted priority for no
  more than 10ms (1/3-rd of a full time slice).

Only VCPUs which consume a negligeable amount of CPU resources
should get real benefit from boosting. When multiple VCPUs are
boosted, they will round robin or be queued FIFO. The idea is
for a boosted VCPU to preempt spinners but not other boosted
I/O intensive guests. A VCPU cannot use the boosting mechanism
to consume more CPU than its allocated fair share.