linux-kernel - Re: [PATCH] tcp_cubic: use 32 bit math

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Message-ID: <20070308041628.GA24530@1wt.eu>
Date:	Thu, 8 Mar 2007 05:16:28 +0100
From:	Willy Tarreau <w@....eu>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
Cc:	David Miller <davem@...emloft.net>,
	rkuhn@....physik.tu-muenchen.de, andi@...stfloor.org,
	dada1@...mosbay.com, jengelh@...ux01.gwdg.de,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] tcp_cubic: use 32 bit math

On Wed, Mar 07, 2007 at 07:10:47PM -0800, Stephen Hemminger wrote:
> David Miller wrote:
> >From: Stephen Hemminger <shemminger@...ux-foundation.org>
> >Date: Wed, 7 Mar 2007 17:07:31 -0800
> >
> >  
> >>The basic calculation has to be done in 32 bits to avoid
> >>doing 64 bit divide by 3. The value x is only 22bits max
> >>so only need full 64 bits only for x^2.
> >>
> >>Signed-off-by: Stephen Hemminger <shemminger@...ux-foundation.org>
> >>    
> >
> >Applied, thanks Stephen.
> >
> >What about Willy Tarreau's supposedly even faster variant?
> >Or does this incorporate that set of improvements?
> >  
> That's what this is:
>    x = (2 * x + (uint32_t)div64_64(a, (uint64_t)x*(uint64_t)x)) / 3;

Confirmed, it's the same. BTW, has someone tested on a 64bit system if
it brings any difference ?

Willy

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