linux-kernel - Re: [PATCH 14/16] sched: make __update_entity_runnable

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Message-ID: <1341917490.3462.119.camel@twins>
Date:	Tue, 10 Jul 2012 12:51:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Benjamin Segall <bsegall@...gle.com>
Cc:	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org,
	Venki Pallipadi <venki@...gle.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [PATCH 14/16] sched: make __update_entity_runnable_avg() fast

On Mon, 2012-07-09 at 13:18 -0700, Benjamin Segall wrote:
> Correct, the sum is 1..n. The maximum was chosen as 516/46742 because
> that is the point when the approximation of loop + fixed point math
> being used reaches a maximum, even if the ideal y^n series would cap out
> slightly differently.

Can you run a simple kid like me through the math here.. it seems
strange that fixed point math would give a higher order term than the
regular case. And by such a wide margin as well, (516-320)/32 ~ 6, so
that's 1/64-th of the 320-th term.


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