Re: [Xen-devel] [PATCH 2/2] credit2: track residual from divisions done during accounting

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.02.13 at 13:31, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Wed, Feb 27, 2013 at 11:35 AM, George Dunlap <George.Dunlap@xxxxxxxxxxxxx 
>> wrote:
> 
>> On Wed, Feb 27, 2013 at 9:40 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>
>>> >>> On 26.02.13 at 18:08, George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>>> wrote:
>>> > @@ -271,16 +272,24 @@ struct csched_dom {
>>> >
>>> >  /*
>>> >   * Time-to-credit, credit-to-time.
>>> > + *
>>> > + * We keep track of the "residual" time to make sure that frequent
>>> short
>>> > + * schedules still get accounted for in the end.
>>> > + *
>>> >   * FIXME: Do pre-calculated division?
>>> >   */
>>> > -static s_time_t t2c(struct csched_runqueue_data *rqd, s_time_t time,
>>> struct csched_vcpu *svc)
>>> > +static void t2c_update(struct csched_runqueue_data *rqd, s_time_t time,
>>> > +                          struct csched_vcpu *svc)
>>> >  {
>>> > -    return time * rqd->max_weight / svc->weight;
>>> > +    uint64_t val = time * rqd->max_weight + svc->residual;
>>> > +
>>> > +    svc->residual = do_div(val, svc->weight);
>>> > +    svc->credit -= val;
>>> >  }
>>> >
>>> >  static s_time_t c2t(struct csched_runqueue_data *rqd, s_time_t credit,
>>> struct csched_vcpu *svc)
>>> >  {
>>> > -    return credit * svc->weight / rqd->max_weight;
>>> > +    return (credit * svc->weight) / rqd->max_weight;
>>>
>>> So you dropped the subtraction of svc->residual here - why?
>>>
>>> And if indeed intended, the insertion of parentheses here could be
>>> dropped?
>>>
>>
>> Well for one I think the equation was wrong -- the residual is units of
>> "time", so you should have subtracted the residual after dividing by
>> max_weight.  (From a mathematical perspective, svc->weight /
>> rqd->max_weight is the ratio that changes units of "credit" into units of
>> "time; if we were using floating point ops I might put parentheses around
>> that ratio to make it more clear that's what's going on.)
>>
> 
> 
> Hmm, on further investigation, I think I've convinced myself that this
> reasoning is wrong, and your original equation was correct.
> 
> However, the difference will only ever end up to be one nanosecond, which
> (if used) will be subtracted from the credit after the reset; so I think we
> might as well do without the subtraction here.

Oh, I didn't realize this would only be a nanosecond - my
understanding was that it would be up to (but not including) a
credit unit.

Jan


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