Re: AW: Question on Credit accounting in Credit Scheduler

[Thomas Pfeuffer <thomas.pfeuffer@mytum.de>]

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Hello,

thanks for your answer.

Best regards,

Thomas


Heiko Wundram wrote:

>>-----Ursprüngliche Nachricht-----
>>Von: xen-devel-bounces@lists.xensource.com [mailto:xen-devel-
>>bounces@lists.xensource.com] Im Auftrag von Thomas Pfeuffer
>>Gesendet: Donnerstag, 29. Juli 2010 15:21
>>An: xen-devel@lists.xensource.com
>>Betreff: [Xen-devel] Question on Credit accounting in Credit Scheduler
>>
>>I have looked through the source code of Credit Scheduler.
>>
>>In csched_acct(), the number of credits a domain gets (i.e
>>credit_fair),
>>is calculated as follows:
>>
>>     credit_fair = ( ( credit_total * sdom->weight) + ( weight_total -
>>1)
>>                  ) / weigth_total
>>
>>But I would expect, that the Credits are calculated by
>>
>>     credit_fair = (credit_total * sdom->weight) / weigth_total
>>
>>Does anybody know, what function the term (weight_total -1) has?
>>    
>>
>
>Without knowing details of the Xen scheduling algorithm (i.e., I'm guessing
>that the above is integer-only math), all it does is "round up" the (fair)
>credits a domain gets [(x+y-1)/y means division with always rounding up,
>think of the "Gaußklammer", don't know the german name, in reverse];
>basically, if the above calculation is done for all domains, the respective
>sum of all calculated credit_fair will always be equal to or higher than
>credit_total, which wouldn't be the case (because of truncation when just
>doing x/y) when using your function, i.e. the sum of all credit_fair might
>be less than credit_total.
>
>For a scheduling algorithm, you'll want to always give away _at least_ all
>available credits, and this is a simple enough method to do just that
>without resorting to floating point calculations.
>
>--- Heiko.
>
>
>  
>


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