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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Ada.Execution_Time
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Date: Wed, 29 Dec 2010 10:07:33 +0100
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Date: 2010-12-29T10:07:33+01:00
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On Tue, 28 Dec 2010 22:39:31 +0000, Simon Wright wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> Yes, the TSC can be read any time, and, if I correctly remember, each
>> reading will give a new value. PPC RT clock is also reliable.
>>
>> There could be certain issues for multi-core processors, you should
>> take care that the task synchronizing the counter with UTC would not
>> jump from core to core. Alternatively you should synchronize clocks of
>> individual cores.
> 
> Using the TSC on a MacBook Pro gives very unreliable results (I rather
> think the core you're using goes to sleep and you may or may not wake
> up using the same core!).

Can it be because the processor changes the TSC frequency when it goes into
the sleep mode? I thought Intel has fixed that bug.

I cannot think out a good time service for a multi-core with unsynchronized
TSC's. Intel should simply fix that mess.

> However, the system clock (Ada.Calendar) is precise to a microsecond.

It likely uses a programmable timer. TSC gives fraction of nanosecond.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de