Re: [PATCH 01/16] sched: track the runnable average on a per-taskentitiy basis

[Paul Turner]

On Wed, Jul 4, 2012 at 8:32 AM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> On Wed, 2012-06-27 at 19:24 -0700, Paul Turner wrote:
>> Instead of tracking averaging the load parented by a cfs_rq, we can track
>> entity load directly.  With the load for a given cfs_Rq then being the sum of
>> its children.
>>
>> To do this we represent the historical contribution to runnable average within each
>> trailing 1024us of execution as the coefficients of a geometric series.
>>
>> We can express this for a given task t as:
>>   runnable_sum(t) = \Sum u_i * y^i ,
>>   load(t) = weight_t * runnable_sum(t) / (\Sum 1024 * y^i)
>>
>> Where: u_i is the usage in the last i`th 1024us period (approximately 1ms) ~ms
>> and y is chosen such that y^k = 1/2.  We currently choose k to be 32 which
>> roughly translates to about a sched period.
>>
>> Signed-off-by: Paul Turner <pjt@xxxxxxxxxx>
>> ---
>>  include/linux/sched.h |    8 +++
>>  kernel/sched/debug.c  |    4 ++
>>  kernel/sched/fair.c   |  128 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 140 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 9dced2e..5bf5c79 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1136,6 +1136,11 @@ struct load_weight {
>>         unsigned long weight, inv_weight;
>>  };
>>
>> +struct sched_avg {
>> +       u32 runnable_avg_sum, runnable_avg_period;
>> +       u64 last_runnable_update;
>> +};
>
>
> So we can use u32 because:
>
>              n         1
> lim n->inf \Sum y^i = --- = ~ 46.66804636511427012122 ; y^32 = 0.5
>              i=0      1-y
>
> So the values should never be larger than ~47k, right?

Yes -- this is made explicit later in the series.
>
> /me goes add something like that in a comment.
>
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