Re: [ANN] LuaProfiler 2.0

[Chris Marrin <chris@...>]

Diego Nehab wrote:
> Hi,
>
> > Have you considered  to use an RDTSC timer for intel platforms?
> > The code is in some ways more portable than times().
> >
> > For win32
> >
> > _int64 t;
> >
> > #define RDTSC_TIMER(x) {__asm rdtsc  __asm mov DWORD PTR [x],EAX
> > __asm  mov DWORD PTR [x+4],EDX}
> >
> >  RDTSC_TIMER(t)
>
>
> Or, you can use QueryPerformanceCounter, if you don't like __asm.  If
> you don't need that much precision, you can also use
> GetSystemTimeAsFileTime.

I have gotten recent experience with QueryPerformanceCounter(). I 
naively tried to get its value and divide it by the value in 
QueryPerformanceFrequency(), first converting both to doubles. I found 
that this conversion reduced the precision down to about 100ms. This is 
because on my shiny new 3GHz P4 machine, the frequency is 3GHz! That 
means the counter increments by 3 billion every second. Since it's a 64 
bit counter it will still not rollover for about 200 years. But if your 
machine has been running for a month or so like mine was, the number 
gets too big to be represented very precisely by a double!

This never troubled me before I found because my older 1.7GHz P4 ran at 
a frequency of only 3.5MHz.

My solution was to snapshot a count value when my app started and 
subtract this from the current value. Now I get nice sub-microsecond 
accruracy. The problem there, of course is that, if my app runs for many 
days, the precision will slowly be reduced back to the intolerable 100ms 
level. I plan to solve that by periodically taking another snapshot of 
the counter as the initial value.

I can post my final solution when I have it, if anyone is interested...

--
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"As a general rule,don't solve puzzles that open portals to Hell"'