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[Xen-devel] Re: credit scheduler: the policy of credit assignment

On Wed, Jul 05, 2006 at 01:23:50PM +0900, Hirokazu Takahashi wrote:
> Hi Emmanuel,
> I looked into the code of the credit scheduler and one question
> has come up.
> I'm not sure whether it is your intention that once the value of
> credit_balance, which is the sum of credit of all domains on Xen,
> goes to zero it may be stuck at zero. It will happen all of them
> are cpu intensive domains and some of them turn into idle.
> Every 30 msec, the credit scheduler gives them csched_priv.credit
> milliseconds --- 30 msec * number of physical cpus ---. 30 msec later,
> you will find they have consumed all of the time they gave, which
> leads the value of credit_balance will keep zero. This means some
> domains are assigned credit with negative value every time.
> I know even in this case it will be balanced between domains
> based on the weights but it would take quite long time to be balanced.
> I feel the scheduler should give each domains larger credit than now
> when credit_balance is small.
> Am I something wrong about this?

Credit_balance only comes into play when active domains with positive
credit go idle. It's a mechanism to converge the system towards its
stable state.

Are you suggesting that credit_balance, as it is used, should be the
sum of credit *prior* to incrementing active domains' credits?

I'm not really sure I understand what you're concerned about here.
Can you elaborate and use a specific example to illustrate?

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