netdev - Re: [PATCH net-next] codel: use Newton method instead of sqrt() and divides

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Message-Id: <20120512.155259.1178343836887150194.davem@davemloft.net>
Date:	Sat, 12 May 2012 15:52:59 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	dave.taht@...ferbloat.net, netdev@...r.kernel.org,
	nichols@...lere.com, van@...lere.net, codel@...ts.bufferbloat.net,
	ycheng@...gle.com, mattmathis@...gle.com, therbert@...gle.com,
	shemminger@...tta.com, nanditad@...gle.com
Subject: Re: [PATCH net-next] codel: use Newton method instead of sqrt()
 and divides

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sat, 12 May 2012 15:32:13 +0200

> From: Eric Dumazet <edumazet@...gle.com>
> 
> As Van pointed out, interval/sqrt(count) can be implemented using
> multiplies only.
> 
> http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
> 
> This patch implements the Newton method and reciprocal divide.
> 
> Total cost is 15 cycles instead of 120 on my Corei5 machine (64bit
> kernel).
> 
> There is a small 'error' for count values < 5, but we don't really care.
> 
> I reuse a hole in struct codel_vars :
>  - pack the dropping boolean into one bit
>  - use 31bit to store the reciprocal value of sqrt(count).
> 
> Suggested-by: Van Jacobson <van@...lere.net>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied but I never like that bitfield sharing for real integers.

GCC makes a complete mess of it as it extracts and inserts the
integer value into that bit field.  You are guarenteed to get
better code if you do this by hand in a full u32.

Either that or just bite the bullet and use a completely seperate
field, maybe we'll need more boolean states later.
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