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Re: Measuring short times

Dennis Clarke schrieb:
Dennis, those Solaris functions were not useful for me because I got
no idea how to make it work on Windows.

How do you know that those functions are monotonic and consistent ?
On IA-32 CPUs (Pentium or K5 and above) rdts returns the content of the 64 bit CPU-internal clock counter, which is set to 0 at system start and incremented by one at every clock tick. This is guaranteed to be not only monotonic, but even strongly monotonic. Moreover, even on a 4.2 GHz system it would take 2^32 seconds (>130 years) until the counter overflows. For all practical purposes, this should be fine :-)

Only caveat is that it depends on the CPU clock speed. Not only that the basic clock speed is different on every system, it may even change over time due to power saving strategies. Nevertheless, for short time periods this is on Intel the best method to go.

Unfortunately, the rdts instruction is not a pipeline sequence point. This means that the CPU is free to reorder it with other instructions in the pipeline. Thus, if you want to measure really short portions of code, the results could be "surprising". To prevent this, one has to manually insert a sequence point. The cpuid instruction does a good job here. However, this clearly is only necessary for really short periods, periods way below the 500ns the OP asked for.

So how to determine the clock speed?

1) On windows, this is measured by the system and maintained somewhere in the registry. I don't know the exact key, but if you look a bit around or goggle for it you should be able to find it.

2) An alternative would be to wait for a defined amount of time (let's say 100 msec) and get the rdtsc-value before and afterwards to calculate the CPU frequency.
For this you have to:
a) give your thread a high priority: SetThreadPriority(...),
b) set the systems-internal timer to high precision: timeBeginPeriod(1), and
c) do a Sleep(100).
With the timeBeginPeriod(1), the systems-internal timers can be expected to be precise by one msec, so the Sleep(100) should wait for 100 msec with an error of < 1 Percent.


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