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

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Message-Id: <20120512.164513.1156706853054390966.davem@davemloft.net>
Date:	Sat, 12 May 2012 16:45:13 -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 22:40:56 +0200

> 24 bit of precision for the reciprocal value is more than enough (Van
> suggested 16 bits in fact), so we have actually room for 7 bits if
> needed.

Using a u16 would also work for me.

> By the way, gcc on x86 generates nice "and 0xfffffffe,%eax" instruction
> for (vars->rec_inv_sqrt << 1).

Yeah but what do stores of ->rec_inv_sqrt look like?
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