RE: [GD-Windows] More timer fun!

[Phil T. <ph...@mi...>]

Not only will QPC sometimes jump ahead by one to four seconds because of
bus traffic, on some processors due to a known manufacturing defect QPC
can actually jump backwards and forwards in time significantly.  As far
as I know this only happens on a small percentage of manufactured CPUs
(I have observed it at least once on a P3).

This has bitten me in the past.  For example one of the programs I
debugged had logic that looked something like this:

	// start timer:

	__int64 freq;
	QueryPerformanceFrequency(&freq);
	__int64 start =3D QueryPerformanceCounter();
=09

	// once a frame:

	if (QueryPerformanceCounter() - start > freq)
	{
		// change a state
	}

Basically someone was waiting for a second to elapse before moving on in
a state.


The program in question was "hung" in a menu waiting to continue.  When
I hooked up a debugger to this it turned out that QPC would reset (jump
backwards) every 6 minutes.  The reset just happened to be between the
initial call to QPC and before the one second could elapse.

Painful indeed,
Phil

-----Original Message-----
From: Jon Watte [mailto:hp...@mi...]=20
Sent: Thursday, August 29, 2002 9:51 PM
To: bri...@py...; gam...@li...
Subject: RE: [GD-Windows] More timer fun!


timGetTime() / GetTickCount() only return 32 bit values, AFAIK.

There are some problems with keeping time on PC hardware. timeGetTime()=20
will lag behind wallclock time when there is lots of bus traffic (=3D=3D =

graphics-heavy application running). QueryPerformanceCounter() will=20
somtimes jump ahead by 1.3 or 4.1 seconds when there is lots of bus=20
traffic, in a quantum leap. RDTSC ticks once per cycle, but mobile=20
CPUs will adjust their CPU frequency quite often, so you can't get a=20
good bead on that, either.

Also, QPC and TGT/GTC each take a bus transaction to execute, and thus=20
run in 1-2 microseconds, which is an eternity if you need to call them=20
more than once or twice per frame rendered.

I ended up with an unholy concoction of all three methods, which ends=20
up getting good overall relation to wallclock time, cheap calls, but=20
some local mis-match where my clock will lag behind wall-clock "locally"

but get adjusted "globally" when the CPU goes into powersave mode.

Yuck.

Cheers,

			/ h+


> -----Original Message-----
> From: gam...@li...
> [mailto:gam...@li...]On Behalf Of
> brian hook
> Sent: Thursday, August 29, 2002 9:40 PM
> To: gam...@li...
> Subject: [GD-Windows] More timer fun!
>=20
>=20
> Well, in the process of trying to figure out why on a very few systems

> (maybe less than 1%) my timing code was messed up, I stumbled across=20
> this tid bit at the Pest Patrol Web site:
>=20
> "A Range Check Error can occur on DELL computers running Windows ME=20
> with software using the Windows function GetTickCount. In such
systems,=20
> this function is not returning the elapsed time since system power on,

> and the value returned can be enormous, causing a range check error.=20
> Dell's Range Check Error affects timeGetTime as well as GetTickCount.
A=20
> third timing function, therefore, has been used in PestPatrol,=20
> PestPatrolCL, and PPUpdater."
>=20
> Well, sure enough, the two users I know that were having this problem=20
> have Dell Dimension L's with Windows ME.  I'm not sure what the=20
> specifics of this problem are (I'm using 64-bit ints already), but I
do=20
> some more range checking now and hopefully this problem goes away.
>=20
> -Hook
>=20
>=20
>=20
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>=20


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