linux-kernel - Re: questions on calc_delta

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Date:	Fri, 02 May 2008 14:14:01 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Joel Schopp <jschopp@...tin.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: questions on calc_delta_mine() in sched.c

On Thu, 2008-05-01 at 15:45 -0500, Joel Schopp wrote:
> Ingo,
> 
> I have a few questions regarding  this code in kernel/sched.c
> 
> static unsigned long
> calc_delta_mine(unsigned long delta_exec, unsigned long weight,
>          struct load_weight *lw)
> {
>      u64 tmp;
> 
>      if (unlikely(!lw->inv_weight))
>          lw->inv_weight = (WMULT_CONST-lw->weight/2) / (lw->weight+1);
> 
> 
> Q1) This code is hit often in scenarios I run, is this really unlikely for 
> others?

I think it became a lot more likely recently, perhaps removing that
unlikely is not such a bad idea.

> Q2) The rest of the code in sched.c seems to make inv_weight == 
> WMULT_CONST/weight and I was wondering if you could explain why this 
> instance is different.

because the rest of the code is wrong, there are only 2 other sites, and
I have a patch that removes those div64_64() with =0;

The idea is to use rounding division: (x + y/2) / y
but we can't because 'x' is touching the limits of our modulo space,
hence we do: (x - y/2) / y
which comes in 1 short, that fixup has been lost along the way.

> Q3) That division is pretty expensive, could we sacrifice some accuracy and 
> do a precompute table?  Do you have another idea how we could get rid of 
> the divide?

Is a full memory miss not more expensive on most modern machines?

And, no sadly I have no ideas on how to get rid of it ;-/

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