problem using cpu_time

[kkajino]

I have Visual Fortran Prof. edition 6.6A on a Windows XP machine. I am having problems using the cpu_time function. The returned value only seems to update at a rate of 1.5625e-2 second, and it's not giving me the microsecond resolution that I need. I found a two year old post related to cpu_time, with a response to the effect of "it's the fault of the OS". I wrote a test program and got the same problem, shown below.

program test_cpu_time

real gettime0,gettime1

call cpu_time(gettime0)
do i=1,100,1

print *, i

end do
call cpu_time(gettime1)
deltatime=gettime1-gettime0
print *, gettime0,gettime1,deltatime

end

The output is as follows:

95
96
97
98
99
100
1.5625000E-02 1.5625000E-02 0.0000000E+00
Press any key to continue

Thanks for any help.

Kent

[Steven_L_Intel1]

Same response. The system time that CPU_TIME looks at is updated by the OS at about a 10ms interval. Microsecond timing may not be available without specialized hardware, but look at the Win32 Platform SDK documentation topics on "multimedia timers".

Steve

[Jugoslav_Dujic]

There are also QueryPerformanceCounter/QueryPerformanceFrequency APIs, but I forgot what their resolution was. (Oh, got it:

The Win32 API QueryPerformanceCounter() returns the resolution of a high- resolution performance counter if the hardware supports one. For x86, the resolution is about 0.8 microseconds (0.0008 ms). You need to call QueryPerformanceFrequency() to get the frequency of the
high-resolution performance counter.
)

You'll need some integer(8) arithmetic to get that right.

Jugoslav

[TimP]

In order to get microsecond resolution, you give up the ability to collect process time, and basically count CPU cycles. If you have an up to date Microsoft C compiler, there should be direct support for the rdtsc intrinsic. Up to now, we link in a function which goes through a number of #ifdef's to select the appropriate asm code. This gives better resolution than the QueryPerformance API's, but you also have the problem of finding independently the CPU clock rate. It varies if you have thermal or power saver throttling.