linux-kernel - Re: [PATCH 2/5] writeback: dirty position control

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Date:	Fri, 12 Aug 2011 13:45:28 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] writeback: dirty position control

> > Making our final function look like:
> > 
> >                s - x 3
> >  f(x) :=  1 + (-----)
> >                l - s
> 
> Very intuitive reasoning, thanks!
> 
> I substituted real numbers to the function assuming a mem=2GB system.
> 
> with limit=thresh:
> 
>         gnuplot> set xrange [60000:80000]
>         gnuplot> plot 1 +  (70000.0 - x)**3/(80000-70000.0)**3

I'll use the above one, which is more simple and elegant: 

        f(freerun)  = 2.0
        f(setpoint) = 1.0
        f(limit)    = 0

Code is

        unsigned long freerun = (thresh + bg_thresh) / 2;

        setpoint = (limit + freerun) / 2;
        pos_ratio = abs(dirty - setpoint);
        pos_ratio <<= BANDWIDTH_CALC_SHIFT;
        do_div(pos_ratio, limit - setpoint + 1);
        x = pos_ratio;
        pos_ratio = pos_ratio * x >> BANDWIDTH_CALC_SHIFT;
        pos_ratio = pos_ratio * x >> BANDWIDTH_CALC_SHIFT;
        if (dirty > setpoint)
                pos_ratio = -pos_ratio;
        pos_ratio += 1 << BANDWIDTH_CALC_SHIFT;

Thanks,
Fengguang
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