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

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Message-ID: <20140114201042.GA13804@order.stressinduktion.org>
Date:	Tue, 14 Jan 2014 21:10:42 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Austin S Hemmelgarn <ahferroin7@...il.com>, 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 Tue, Jan 14, 2014 at 11:50:32AM -0800, 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.

If I read the instruction tables correctly, we could half the latency with
reciprocal divide even on haswell.

What a pitty that the Intel Architecture Code Analyzer does not support imul
nor div instruction. :(

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