linux-kernel - Re: [RFC PATCH 13/14] sched: make __update_entity_runnable

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Date:	Mon, 06 Feb 2012 21:48:26 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Turner <pjt@...gle.com>
Cc:	linux-kernel@...r.kernel.org, Venki Pallipadi <venki@...gle.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Mike Galbraith <efault@....de>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ben Segall <bsegall@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 13/14] sched: make __update_entity_runnable_avg()
 fast

On Wed, 2012-02-01 at 17:38 -0800, Paul Turner wrote:
> +/* Precomputed fixed inverse multiplies for multiplication by y^n */
> +const static u32 runnable_avg_yN_inv[] = {
> +       0xffffffff,0xfa83b2db,0xf5257d15,0xefe4b99b,0xeac0c6e7,0xe5b906e7,
> +       0xe0ccdeec,0xdbfbb797,0xd744fcca,0xd2a81d91,0xce248c15,0xc9b9bd86,
> +       0xc5672a11,0xc12c4cca,0xbd08a39f,0xb8fbaf47,0xb504f333,0xb123f581,
> +       0xad583eea,0xa9a15ab4,0xa5fed6a9,0xa2704303,0x9ef53260,0x9b8d39b9,
> +       0x9837f051,0x94f4efa8,0x91c3d373,0x8ea4398b,0x8b95c1e3,0x88980e80,
> +       0x85aac367,0x82cd8698,
> +};

I wonder if modern Intel isn't at the point where computing this thing
is cheaper than the cacheline miss. You can compute y^n in O(log n) time
and with n < 32 that's 5 multiplications (see fixed_power_int). Add to
that the division.

Of course there's platforms, ARM?, where reverse is likely true. Bugger
that.

> +/* Precomputed \Sum y^k { 1<=k<=n } */
> +const static u32 runnable_avg_yN_sum[] = {
> +           0, 1002, 1982, 2941, 3880, 4798, 5697, 6576, 7437, 8279, 9103,
> +        9909,10698,11470,12226,12966,13690,14398,15091,15769,16433,17082,
> +       17718,18340,18949,19545,20128,20698,21256,21802,22336,22859,23371,
> +}; 

Right, can't see a fast way to compute this..

The asymmetry in the tables annoys me though 32 vs 33 entries, hex vs
dec :-)

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