linux-kernel - Re: A little coding style nugget of joy

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Date:	Wed, 19 Sep 2007 17:39:46 -0400
From:	"Andrew Lutomirski" <luto@...ealbox.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, kernel1@...erdogtech.com
Subject: Re: A little coding style nugget of joy

On 9/19/07, Andi Kleen <andi@...stfloor.org> wrote:
> > This is a terrible assumption in general (i.e. if filesize % blocksize
> > is close to uniformly distributed).  If you remove one byte and the data
> > is stored with blocksize B, then you either save zero bytes with
> > probability 1-1/B or you save B bytes with probability 1/B.  The
> > expected number of bytes saved is B*1/B=1.  Since expectation is linear,
> > if you remove x bytes, the expected number of bytes saved is x (even if
> > there is more than one byte removed per file).
>
> You didn't calculate the probability of actually saving a full block
> or not (that's the only thing that matters). I assumed it's relatively
> small and can be ignored in practice since the amount of end white
> space is negligible compared to total file size.

Sure I did.  It's roughly 1/B per byte removed ( = 1/4096 ).

--Andy
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