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Date: Fri, 03 Jun 2011 15:12:52 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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To: John Baldwin <jhb@FreeBSD.org>
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Cc: freebsd-hackers@FreeBSD.org, Jung-uk Kim <jkim@FreeBSD.org>
Subject: Re: [RFC] Enabling invariant TSC timecounter on SMP
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on 03/06/2011 14:50 John Baldwin said the following:
> On Friday, June 03, 2011 2:03:55 am Andriy Gapon wrote:
>>> Consecutive RDTSCs used on a same CPU is always incremental but we 
>>> cannot 100% guarantee that on two cores, even if TSC is derived from 
>>> the same clock.  I am hoping at least latency difference (I believe 
>>> it's about few tens of cycles max) is "eaten up" by lowering 
>>> resolution.  It's not perfect but it's better than serialization 
>>> (Linux) or heuristics (OpenSolaris), just because there are few rare 
>>> conditions to consider.  Thoughts?
>>
>> I am still not sure which case this code should solve.
>>
>> Thread T1: x1 = rdtsc() on CPU1;
>> Thread T1: x2 = rdtsc() on CPU2;
>> x2 < x1 ?
>> Or?
> 
> Yes, that can happen.

Well, I think that the test based on smp_rendezvous should ensure that
difference in TSC values is "small enough"; that is, I expect that cost (in TSC
ticks) of migrating a thread from CPU to CPU should be larger than that
difference if the test was passed.  Is this an unreasonable expectation?

-- 
Andriy Gapon