Question on Credit accounting in Credit Scheduler

Question on Credit accounting in Credit Scheduler

[Thomas Pfeuffer]

Hello, 

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?


Later, the credits a vcpu gets is calculated by


     credit_fair = ( credit_fair + (sdom->active_vcpu_count -1)
                ) / sdom->active_vcpu_count -1

Again, I would expect that the credits are calculated by

     credit_fair =  credit_fair / sdom->active_vcpu_count


I do not understand why the term (sdom->active_vcpu_count -1 ) is
neccessary.

Best regards, 

Thomas Pfeuffer

Re: Question on Credit accounting in Credit Scheduler

[Tim Deegan]

At 14:21 +0100 on 29 Jul (1280413287), Thomas Pfeuffer wrote:
> Hello, 
> 
> 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?

It makes the integer division round up instead of rounding down. 

Tim.

-- 
Tim Deegan <Tim.Deegan@citrix.com>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

AW: Question on Credit accounting in Credit Scheduler

[Heiko Wundram]

> -----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.

Re: Question on Credit accounting in Credit Scheduler

[George Dunlap]

I'd thought of writing a div_round_up() macro that would make this a
bit more clear, something like:

/* Divide x by y, rounding up */
#div_round_up(x, y) (((x)+((y)-1))/(y))

 -George

On Thu, Jul 29, 2010 at 2:35 PM, Tim Deegan <Tim.Deegan@citrix.com> wrote:
> At 14:21 +0100 on 29 Jul (1280413287), Thomas Pfeuffer wrote:
>> Hello,
>>
>> 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?
>
> It makes the integer division round up instead of rounding down.
>
> Tim.
>
> --
> Tim Deegan <Tim.Deegan@citrix.com>
> Principal Software Engineer, XenServer Engineering
> Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
>

Re: AW: Question on Credit accounting in Credit Scheduler

[Thomas Pfeuffer]

[-- Attachment #1.1: Type: text/plain, Size: 1803 bytes --]

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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