Message-ID: <44E09A19.9050205@de.ibm.com>
Date: Mon, 14 Aug 2006 17:43:21 +0200
From: Jan-Bernd Themann <ossthema@de.ibm.com>
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To: Anton Blanchard <anton@samba.org>
CC: =?ISO-8859-1?Q?J=F6rn_Engel?= <joern@wohnheim.fh-wedel.de>,
       netdev <netdev@vger.kernel.org>, linux-ppc <linuxppc-dev@ozlabs.org>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Marcus Eder <meder@de.ibm.com>, Christoph Raisch <raisch@de.ibm.com>,
       Thomas Klein <tklein@de.ibm.com>
Subject: Re: [PATCH 1/6] ehea: interface to network stack
References: <44D99EFC.3000105@de.ibm.com> <20060811205624.GE479@krispykreme> <20060814112656.GC10164@wohnheim.fh-wedel.de> <20060814143842.GM479@krispykreme>
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Hi,

Anton Blanchard wrote:
>> Is a conditional cheaper than a divide?  In case of a misprediction I
>> would assume it to be significantly slower and I don't know the ratio
>> of mispredictions for this branch.
> 
> A quick scan of the web shows 40 cycles for athlon64 idiv, and its
> similarly slow on many other cpus. Even assuming you mispredict every
> branch its going to be a win.
> 
> Anton

as our queue size is always a power of 2, we simply use:
i++;
i &= (ringbufferlength - 1)

So we can get along without the if.

Jan-Bernd


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