Re: [Boost-users] A problem on using boost timer (different time intervals were showed). Can any one with kindness help me?

Subject: Re: [Boost-users] A problem on using boost timer (different time intervals were showed). Can any one with kindness help me?
From: Dominique Devienne (ddevienne_at_[hidden])
Date: 2009-09-24 16:08:15

On Thu, Sep 24, 2009 at 3:01 AM, Johan Nilsson
<r.johan.nilsson_at_[hidden]> wrote:
>> Does it happen due to the multi-process and multi-thread mode by
>> windows?
>
> No, it happens because the actual resolution of the timer under Windows is
> platform-dependent, and on your platform ~15ms. If the exection takes less
> time, you'll see 0 elapsed.
>
>> I do not know how to solve it. My program is quite strict on the time
>> consumed by the computer.
>
> Run the program's main body many times and divide the total time with the
> number of executions.

Yes, the around 15ms resolution of the default Windows timer is too coarse IMHO,
given the multi-GHz CPUs of today. If what you time fully happens
between two ticks,
the timer says zero indeed.

http://en.wikipedia.org/wiki/Andrew_Koenig wrote a few years back a
nice 6 article
series on benchmarking (was it Byte, DrDoobs, CUJ, or OOC, I don't recall), and
showed you can spin until the clock ticks (up to 15ms worth..., was
around 30,000
iterations at that time, in Java code), do your short op, then spin
again until the
next clock tick, and by approximating the time a spin iter takes, you can get a
fairly accurate measurement. Also talked about taking N samples, throwing out
the extremes, and then taking the median I think.

Windows does support a high resolution timer, but I could never find a good impl
to re-use, and it's not portable of course. I wish Boost provided such
a high res
timer in a portable manner :) --DD