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

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Message-Id: <20120512.175217.1632102067268101115.davem@davemloft.net>
Date:	Sat, 12 May 2012 17:52:17 -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 23:48:44 +0200

> On Sat, 2012-05-12 at 16:45 -0400, David Miller wrote:
>> Using a u16 would also work for me.
> 
> I tried it but it gives noticeable errors for count > 16000, and no
> speed gain.
 ...
> And if a flow is really agressive, count can grow above 10^6
> 
>> > 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?
> 
> The load is "shr %edi" as in :
> and the store an "or %ecx,%esi"

Ok, fair enough.
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