Replying to myself sorry... > Quite right. This was related to the QueryPerformanceCounter() using the > 8254-compatible real-time clock which could take several thousand cycles. > The HAL of Pentium's and above should use Intel's RDTSC (Read Time Stamp > Counter) and not suffer this problem.
Apart from problems with RDTSC mentioned elsewhere (PCI problems, CPU speed changes, problems with >2.1GHz) ... I'm not sure when RDTSC is used as there seems to be a bit of information indicating uniprocessor HAL's using the real-time clock instead of RDTSC, including boost http://lists.boost.org/MailArchives/boost/msg31392.php Couldn't find anything definitive at MS. Others might know. The last bit of http://msdn.microsoft.com/msdnmag/issues/0500/hood/default.aspx seemed the most (in)definitive google turned up. But it seems only the multiprocessor HAL (which can run on a uniprocessor but is not the default) supports RDTSC. If this is the case then timing granularity will vary quite a bit per platform. Could code RDTSC directly (note quite portable C++ ;-)).... P J Naughter and J M McGuiness wrote the following code that is publicly available but copyrighted: http://www.thecodeproject.com/datetime/ccputicker.asp#xx1843xx But this would need to be different for each supported compiler :-( and you still have to calculate the processor frequency, perhaps using QueryPerformanceCounter() on win32... :-( #pragma optimize("",off) void CCPUTicker::Measure() { if (m_bHasRDTSC) { volatile ULARGE_INTEGER ts; //on NT don't bother disabling interrupts as doing //so will generate a priviledge instruction exception if (!m_bRunningOnNT) { _asm { cli } } _asm { xor eax,eax push eax push ebx push ecx push edx _emit 0x0f ; cpuid - serialise the processor _emit 0xa2 pop edx pop ecx pop ebx pop eax _emit 0x0f ; rdtsc _emit 0x31 mov ts.HighPart,edx mov ts.LowPart,eax } if (!m_bRunningOnNT) { _asm { sti } } m_TickCount = ts.QuadPart; } else { m_TickCount=0; } } #pragma optimize("",on) m_TickCount is __int64 Regards, Matt. --wasting too much time _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
