netdev - Re: [Bloat] tc linklayer ADSL calc broken after commit 56b765b79 (htb: improved accuracy at high rates)

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Message-ID: <20130530095253.GA8461@uio.no>
Date:	Thu, 30 May 2013 11:52:53 +0200
From:	"Steinar H. Gunderson" <sgunderson@...foot.com>
To:	Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Toke Høiland-Jørgensen <toke@...e.dk>,
	Mike Frysinger <vapier@...too.org>,
	Jiri Benc <jbenc@...hat.com>, Jiri Pirko <jpirko@...hat.com>,
	netdev@...r.kernel.org, bloat@...ts.bufferbloat.net,
	Patrick McHardy <kaber@...sh.net>,
	Steven Barth <cyrus@...nwrt.org>,
	"russell-tcatm@...art.id.au" <russell-tcatm@...art.id.au>,
	David Miller <davem@...hat.com>,
	Jussi Kivilinna <jussi.kivilinna@...et.fi>,
	Felix Fietkau <nbd@....name>, Michal Soltys <soltys@....info>
Subject: Re: [Bloat] tc linklayer ADSL calc broken after commit 56b765b79
 (htb: improved accuracy at high rates)

On Thu, May 30, 2013 at 11:15:47AM +0200, Jesper Dangaard Brouer wrote:
>  int pkt_len = skb->len + (encap_overhead * gso_segments);
>  int wire_sz = DIV_ROUND_UP(pkt_len,48)*53;
> 
> (I suspect, that the compiler might even optimize and remove any
> real divisions, I bet Eric can tell us.)

FWIW, GCC can change divisions by integer constants (even signed divisions)
to some multiplies and shifts, by way of some number theory magic.

/* Steinar */
-- 
Homepage: http://www.sesse.net/
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