linux-kernel - questions on calc_delta

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Message-ID: <481A2BF6.2070406@austin.ibm.com>
Date:	Thu, 01 May 2008 15:45:42 -0500
From:	Joel Schopp <jschopp@...tin.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: questions on calc_delta_mine() in sched.c

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?
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.
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?

-Joel Schopp

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