Message-ID: <49BFA1D7.8020302@windriver.com>
Date: Tue, 17 Mar 2009 09:12:55 -0400
From: Paul Gortmaker <paul.gortmaker@windriver.com>
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To: Yasunori Goto <y-goto@jp.fujitsu.com>
CC: Clemens Ladisch <clemens@ladisch.de>,
       Linux Kernel ML <linux-kernel@vger.kernel.org>, robert.picco@hp.com,
       venkatesh.pallipadi@intel.com, vojtech@suse.cz, mingo@redhat.com
Subject: Re: [Patch] Fix the possibility of insane return value of hpet_calibrate()
 against SMI.
References: <20090316111300.7CE5.E1E9C6FF@jp.fujitsu.com> <49BE215F.7040307@ladisch.de> <20090317185152.47CD.E1E9C6FF@jp.fujitsu.com>
In-Reply-To: <20090317185152.47CD.E1E9C6FF@jp.fujitsu.com>
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Yasunori Goto wrote:
> Sorry for late response.
>
>   
>> Yasunori Goto wrote:
>>     
>>>> I've no sense of feel for how long each calibration run would take.
>>>> Would doing it 5 times show up as a significant increase in the boot
>>>> time for those that care about boot time being as quick as possible?
>>>>         
>>> Hmm. The loop times is trade off against reliable value....
>>> Though SMI is rare interruption, I don't know how frequent
>>> hypervisor's switch is.
>>>       
>> Could you, just for testing, run the calibration five thousand times or
>> so instead of five times, and count how often you get insane values?
>> (And how much delay does such an SMI add?)
>>     
>
> I tried 50000 times. But insane value was nothing.
> I think SMI is very rare.
>
>   
>>> Each calibration of this has 1 milli second.
>>> Do you think 5 msec is too long?
>>>       
>> This shouldn't matter when booting.  Anyway, I think it's possible to
>> increase TICK_CALIBRATE without losing too much accuracy.
>>     
>
> Hmm. If the person who is trying to reduce boot time for fastboot dislikes this impact,
> then I'll try Vojtech-san's way.
>   

That probably makes sense -- if it is extremely rare, and if you get two
values that are the same (within some small delta) then you probably
are 99.99% confident that you have the right data.

Paul.

>
> Thanks.
>   

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