Re: Threads: New Win32 Implementation

From: pinkfloydhomer_at_[hidden]
Date: 2001-03-03 09:56:17

--- In boost_at_y..., Beman Dawes <beman_at_e...> wrote:
> At 01:30 PM 9/7/2000 +0000, William Kempf wrote:
> 
>  >I considered this, but stuck with GetTickCount() because it is
>  >platform specific code and GetTickCount() returns milliseconds,
>  >whereas clock() would have to be used in a mathematical 
calculation
>  >using CLOCKS_PER_SEC to determine milliseconds.  So GetTickCount()
>  >seemed safe and much easier to deal with for this specific case.
> 
> As a point of information, John Maddock sent me a version of the 
boost 
> timer functions implemented using Win32 native calls to 
> QueryPerformanceCounter rather than std::clock().
> 
> We ran some timings, and the results were quite surprising:
> 
>      *  For a given compiler, clock() was sometimes dramatically 
(10 times) 
> faster that the native function.  This is counter intuitive since 
the 
> native function had better resolution.
>      *  Which was faster varied from compiler to compiler.  On 
John's 
> machine, the fastest clock() was over 70 times faster that the 
slowest.
>      *  Which was faster varied from platform to platform.  The 
> characteristics of the timings on Win 95/98 were different from the 
Win NT 
> timings, even when adjusted for differing hardware.
> 
> The results were so varied that about the only thing you could say 
is that 
> to truly optimize timing, you might have to make the clock() vs 
> QueryPerformanceCounter() trade-off as late as run-time.
> 

Also, std::clock() and GetTickCount() does not perform the same 
function. std::clock() might give the CPU time of the process of the 
program, while GetTickCount() measures "real-time".