Messages in this thread | | | Subject | Re: [RFC PATCH 14/14] sched: implement usage tracking | From | Peter Zijlstra <> | Date | Thu, 15 Mar 2012 11:52:21 +0100 |
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On Wed, 2012-03-14 at 08:47 -0700, Paul Turner wrote: > On Wed, Mar 14, 2012 at 8:01 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> 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.)
Only for the single entry, right? For the aggregated case the runnable count can be nr_running times your limit, and IIRC (but my brain is fuzzy since its been a while since I looked at this stuff) you use the same structure in both cases.
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