Re: [PATCH 6/9] clockpro-clockpro.patch

[IWAMOTO Toshihiro]

At Sat, 31 Dec 2005 12:29:06 +0100,
Peter Zijlstra wrote:
> 
> Forgot one in the previous mail.
> 
> On Fri, 2005-12-30 at 22:24 -0200, Marcelo Tosatti wrote:
> > Please make it easier for others to understand why the hands 
> > swap, and when, and why.
> 
> Its not the hands that swap, its the lists. The hands will lap each
> other, like the minute hand will lap the hour hand every ~65 minutes.
> 
> Let me try some ascii art.
> 
>    ====
>   ^---<>---v
>        ====
> 
> '='	a page
> '^---<' hand cold
> '>---v' hand hot
> 
> now let hand cold move 4 pages:
> 
>    
>   ^---<>---v
>    ========
> 
> ie. hand hot and hand cold have the same position.
> now if we want to move hand cold one more position this happens:
> 
>    =======
>   ^---<>---v
>           =
> 
> see the swap?

Is it okay to allow Hcold to lap Hhot?
In my understanding of CLOCK-Pro, such lapping causes sudden increase
in the distance between Hhot and Hcold.  As that distance is an
important parameter of page aging/replacement decisions, I'm afraid
that such lapping would result in incorrect page aging and bad
performance.

I guess the alternative is to advance Hhot together with Hcold, but
I'm not sure this is correct, either.

Please enlighten me.

--
IWAMOTO Toshihiro
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