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Date: Mon, 26 Aug 2002 13:19:24 -0700
From: "Martin J. Bligh" <Martin.Bligh@us.ibm.com>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
cc: Pavel Machek <pavel@elf.ucw.cz>, Andrea Arcangeli <andrea@suse.de>,
       Mikael Pettersson <mikpe@csd.uu.se>, john stultz <johnstul@us.ibm.com>,
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       lkml <linux-kernel@vger.kernel.org>, Leah Cunningham <leahc@us.ibm.com>,
       wilhelm.nuesser@sap.com, paramjit@us.ibm.com, msw@redhat.com
Subject: Re: [PATCH] tsc-disable_B9
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>> It's not correlating it to real time that's the problem. It's getting resceduled
>> inbetween calls that hurts. Take your example.
>> 
>> rdtsc
>> mov %eax,%ebx
>> 			<- get rescheduled here
>> rdtsc
>> 
>> Broken. May even take negative "time".
> 
> Statistically irrelevant. When you have 100,000 samples all the
> pre-emption ones drop into the dud sample filter with IRQ disturbance
> and so on.
 
OK, so let's take a better example. People are (I think) mainly using rdtsc
to provide a monotonically increasing counter for database "transaction
order" stamping. I think that's really the case we're worried about. 

M.

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