linux-kernel - Re: [PATCH RFC] reciprocal_divide: correction/update of the algorithm

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Message-ID: <52D5A3DC.9030107@gmail.com>
Date:	Tue, 14 Jan 2014 15:53:48 -0500
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	netdev@...r.kernel.org, dborkman@...hat.com,
	linux-kernel@...r.kernel.org, darkjames-ws@...kjames.pl
Subject: Re: [PATCH RFC] reciprocal_divide: correction/update of the algorithm

On 2014-01-14 14:50, Eric Dumazet wrote:
> On Tue, 2014-01-14 at 14:22 -0500, Austin S Hemmelgarn wrote:
> 
>> I disagree with the statement that current CPU's have reasonably fast
>> dividers.  A lot of embedded processors and many low-end x86 CPU's do
>> not in-fact have any hardware divider, and usually provide it using
>> microcode based emulation if they provide it at all.  The AMD Jaguar
>> micro-architecture in particular comes to mind, it uses an iterative
>> division algorithm provided by the microcode that only produces 2 bits
>> of quotient per cycle, even in the best case (2 8-bit integers and an
>> integral 8-bit quotient) this still takes 4 cycles, which is twice as
>> slow as any other math operation on the same processor.
> 
> I doubt you run any BPF filter with a divide instruction in it on these
> platform.
> 
> Get real, do not over optimize things where it does not matter.
> 
Actually, I have three Jaguar based routers, and use BPF regularly as
part of their iptables rules to log certain packet types.

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