Re: [PATCH 4/5] Light Fragmentation Avoidance V20: 004_percpu

[Mel Gorman]

On Tue, 15 Nov 2005, Paul Jackson wrote:

> Mel wrote:
> > -		mark -= mark / 2;			[A]
> > +		mark /= 2;				[B]
> >  	if (alloc_flags & ALLOC_HARDER)
> > -		mark -= mark / 4;			[C]
> > +		mark /= 4;				[D]
>
> Why these changes?  For each of [A] - [D] above, if I start with a
> value of mark == 33 and recycle that same mark through the above
> transformation 16 times, I get the following sequence of values:


This change by me is totally totally wrong. I shouldn't have modified how
the calculation is made at all. Fix made.

>  A:  33  17   9   5   3   2   1   1   1   1   1   1   1   1   1   1
>  B:  33  16   8   4   2   1   0   0   0   0   0   0   0   0   0   0
>  C:  33  25  19  15  12   9   7   6   5   4   3   3   3   3   3   3
>  D:  33   8   2   0   0   0   0   0   0   0   0   0   0   0   0   0
>
> Comparing [A] to [B], observe that [A] converges to 1, but [B] to 0,
> due to handling the underflow differently.
>
> Comparing [C] to [D], observe that [D] converges to 0, due to the
> different underflow, and converges much faster, since it is taking off
> 3/4's instead of 1/4 each iteration.
>
> I doubt you want this change.
>

And you'd be right.

-- 
Mel Gorman
Part-time Phd Student                          Java Applications Developer
University of Limerick                         IBM Dublin Software Lab
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