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- Subject: Re: [ANN] LuaProfiler 2.0
- From: Chris Marrin <chris@...>
- Date: Wed, 15 Jun 2005 07:15:04 -0700
Rici Lake wrote:
On 14-Jun-05, at 7:19 PM, Chris Marrin wrote:
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!
I find this a bit puzzling. A double has 53 bits of precision. A day has
86400 seconds; log2 of 86400*3e9 is 47.88; in other words, a double can
accurately represent a number of 3GHz ticks up to a bit over 34 days.
After that, it should lose one bit of precision, which should be barely
noticeable. In a year, you would lose less than four bits of precision.
If you are reduced to a precision of 100ms, you are losing something
like 28 bits of precision. This suggests that you were doing arithmetic
with floats rather than doubles; floats have 24 bits of precision, or 29
less than doubles, which would work out to about the loss you are
Yeah, I did the same calculation. I figured the counter must be
incrementing by more than 1 per tick, but I hadn't yet investigated
that. Then I saw Glenn Maynard's post and realized that you are right.
D3D is resetting precision to single precision! Thanks...
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"As a general rule,don't solve puzzles that open portals to Hell"'