Re: [Boost-users] date_time clocks, microsec_clock, etc

From: Aaron W. LaFramboise (aaronrabiddog51_at_[hidden])
Date: 2004-05-06 18:25:32

Jeff Garland wrote:

>Very interesting.  I'll play around with this.  If the last 2 measurements are
>correct they would very precise clocks for timing.
>  
>

Yes, except I am very concerned about some things.

According to official documentation, QueryPerformanceCounter() does not 
garentee any particular precision.  I'm not entirely sure, in fact, of 
what timing mechanism it uses myself.  In addition, the docs mention 
that it may not even be implemented.

rdtsc is excellent, and I think it has the most potiential of any of the 
clocks, except that there seems to be some considerable sentiment (Matt 
Gruenke on this list reminded me of this privately) that it is not 
terribly reliable as anything other than a cycle counter.  In 
particular, there is rumor of laptops and similar that have the ability 
to alter their clock frequency runtime.  (Compare to 
QueryPerformanceFrequency, where the documenation garentees the 
frequency will not change.)  Also--I am not sure if this applies to any 
version of Windows yet or not--the operating system has the ability to 
completely disable rdtsc, supposedly for use in enforcing security in 
'managed' environments.  I think Linux does this.  This fact somewhat 
limits the total applicability of rdtsc, especially on future hardware, 
where rdtsc may be routinely disabled.

In both cases, at least in Win32, we can check if they are implemented 
before we start using them, but then we get this unfortunate situation 
with names.  If a use asks for a nanosec_clock or whatever, but we can't 
determine until runtime that it isn't availible, what do we do?  Throw 
an exception?  Bet they won't be expecting that.  Silently use 
millisec_clock instead?  Big difference there.

Just some things I am thinking about.

Aaron W. LaFramboise