Messages in this thread | | | From | Paul Turner <> | Date | Wed, 14 Mar 2012 08:47:46 -0700 | Subject | Re: [RFC PATCH 14/14] sched: implement usage tracking |
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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.) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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