linux-kernel - Re: [PATCH 1/4] sched/fair: Optimize sum computation with a lookup table

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Date:	Tue, 12 Apr 2016 03:17:12 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	peterz@...radead.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, bsegall@...gle.com, pjt@...gle.com,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	juri.lelli@....com
Subject: Re: [PATCH 1/4] sched/fair: Optimize sum computation with a lookup
 table

On Mon, Apr 11, 2016 at 05:59:11PM +0100, Dietmar Eggemann wrote:
> On 10/04/16 23:36, Yuyang Du wrote:
> > __compute_runnable_contrib() uses a loop to compute sum, whereas a
> > table loopup can do it faster in a constant time.
> > 
> > The following python script can be used to generate the constants:
> > 
> > print " #:     yN_inv   yN_sum"
> > print "-----------------------"
> > y = (0.5)**(1/32.0)
> > x = 2**32
> > xx = 1024
> > for i in range(0, 32):
> > 	if i == 0:
> > 		x = x-1
> > 		xx = xx*y
> > 	else:
> > 		x = x*y
> > 		xx = int(xx*y + 1024*y)
> > 	print "%2d: %#x %8d" % (i, int(x), int(xx))
> > 
> > print " #:  sum_N32"
> > print "------------"
> > xxx = xx
> > for i in range(0, 11):
> > 	if i == 0:
> > 		xxx = xx
> > 	else:
> > 		xxx = xxx/2 + xx
> > 	print "%2d: %8d" % (i, xxx)
> >
> 
> IMHO, it would be nice to add this to the existing tool from the patch
> header of commit 5b51f2f80b3b
> ("sched: Make __update_entity_runnable_avg() fast") simply because people
> already use this one to tweak their pelt tables. Maybe something like
 
I'd prefer not, and recommend switching from the C program for this
kind of job. :)