Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.programming > #6668

Re: Time measuring on Windows

Show all headers | View raw

On 2016-05-29 11:53, BartC wrote:
> I need to measure how long short processes take to execute.
>
> All the docs point to using QueryPerformanceCounter() (as opposed to
> using, for example, the RDTSC machine instruction).
>
> But I'm getting crazy results.
>
> If I call QPC(), then count to ten million (a task that should take 8ms
> based on how long it took to count to a billion), and call it again, the
> difference between the two readings should give an indication of how
> long it took.
>
> But I've seen results from 180,000 to over 600,000! (Even RDTSC gives me
> 20M to 42M, but that's across two cores; I thought QPC was supposed to
> fix that). I understand values need to be scaled to get actual times,
> but the problem is they are inconsistent.

They are consistent, we are using performance counter for fine time 
measurements. The time in seconds is obtained with 
QueryPerformanceFrequency. Ticks do not make sense. 8ms is too short for 
statistically correct measurements, e.g. when your thread loses 
processor it will not return until next 10ms on a busy machine.

> Is there a more reliable way of measuring execution times in the
> millisecond range?

The method is very reliable though time measurement are tricky, 
especially in presence of time sharing. Under Windows, depending on 
settings, switching threads that do not enter non-busy waiting takes 
10ms to 1ms (which is the minimum), controlled by timer resolution. In 
order to ensure 1ms timer resolution call timeBeginPeriod (1) and see if 
the spread decreases. You can also set the thread priority to 
above-normal or even real-time.

> (This is for a process within a program; I don't want to use an external
> utility to measure the complete runtime of an executable as that will
> include all sorts of overheads.)

Well, you cannot measure thread CPU time under Windows, if that is what 
you mean. Performance counter is for very accurate and fine real-time 
measurements.

Thread's CPU and kernel times can be queried but the result has a 
systematic error in it. Theoretically you can have 100% load with 0% CPU 
time indicated.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Back to comp.programming | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Time measuring on Windows BartC <bc@freeuk.com> - 2016-05-29 10:53 +0100
  Re: Time measuring on Windows "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2016-05-29 17:16 +0200
    Re: Time measuring on Windows BartC <bc@freeuk.com> - 2016-05-29 17:32 +0100
      Re: Time measuring on Windows "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2016-05-29 19:04 +0200

csiph-web