Re: [RFC PATCH 14/14] sched: implement usage tracking

[Paul Turner]

On Wed, Mar 14, 2012 at 8:01 AM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> On Tue, 2012-03-13 at 17:57 +0100, Vincent Guittot wrote:
>> >  struct sched_avg {
>> >        u64 runnable_avg_sum, runnable_avg_period;
>> >        u64 last_runnable_update, decay_count;
>> > +       u32 usage_avg_sum;
>>
>> Why usage_avg_sum is 32bits whereas runnable_avg_sum and
>> runnable_avg_period are 64bits long ? You are doing the same
>> computation on these 3 variables. Only the computation need to be done
>> in 64bits but the result could be saved in 32bits ?
>
> Since you can never use more than 100% of cpu time, usage_avg_sum is
> bound to 100% of the period, which (assuming your period < ~4s) should
> thus still fit in the ~4s u32 provides.
>
> Runnable otoh is not bound by that and thus we cannot guarantee the
> value stays within the ~4s value range.

Actually for runnable we can also make such a guarantee since:

runnable_sum <= \Sum 1024 * k^p --> 1024/(1-k) [geometric series, k<1]
--> ~48k for our choice of k.

(We do however need 64-bits on any values that accumulate sums of
loads, e.g. removed_load and *_load_sum.)
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