这个问题是在Windows XP下发现的，当我想写一个像GetTicketCount64这样的平台上不存在的函数。 这是我的testing代码：
uint64_t GetTickCountEx() { #if _WIN32_WINNT > _WIN32_WINNT_WINXP return GetTickCount64(); #else // http://msdn.microsoft.com/en-us/library/windows/desktop/dn553408.aspx LARGE_INTEGER Frequency = {}; LARGE_INTEGER Counter = {}; BOOST_VERIFY(QueryPerformanceFrequency(&Frequency)); BOOST_VERIFY(QueryPerformanceCounter(&Counter)); return 1000 * Counter.QuadPart / Frequency.QuadPart; #endif } for (int i = 0; ++i < 1000; Sleep(30000)) { const auto utc = time(nullptr); // System time const auto xp = GetTickCount(); // API of Windows XP SP3 const auto ex = GetTickCountEx(); // Performance counter const auto diff = ex - xp; printf("%lld %I32u %I64u %I64u \n", utc, xp, ex, diff); } 我无法理解下面的结果。 从这篇文章 ，Angstrom的回复似乎不正确。 最后一栏显示，随着时间的推移，GTC和GPC的差距越来越近！ …几个小时之后，它会达到零？
所以，我的问题是：我的GetTickCount64的实现是否正确，为什么？
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coreinfo.exe提供的 CPU核心信息：Coreinfo v3.21 – 关于系统CPU和内存拓扑的转储信息Copyright（C）2008-2013 Mark Russinovich Sysinternals – www.sysinternals.com
Intel(R) Core(TM) i3 CPU M 380 @ 2.53GHz x86 Family 6 Model 37 Stepping 5, GenuineIntel HTT * Hyperthreading enabled HYPERVISOR - Hypervisor is present VMX * Supports Intel hardware-assisted virtualization SVM - Supports AMD hardware-assisted virtualization EM64T * Supports 64-bit mode SMX - Supports Intel trusted execution SKINIT - Supports AMD SKINIT NX * Supports no-execute page protection SMEP - Supports Supervisor Mode Execution Prevention SMAP - Supports Supervisor Mode Access Prevention PAGE1GB - Supports 1 GB large pages PAE * Supports > 32-bit physical addresses PAT * Supports Page Attribute Table PSE * Supports 4 MB pages PSE36 * Supports > 32-bit address 4 MB pages PGE * Supports global bit in page tables SS * Supports bus snooping for cache operations VME * Supports Virtual-8086 mode RDWRFSGSBASE - Supports direct GS/FS base access FPU * Implements i387 floating point instructions MMX * Supports MMX instruction set MMXEXT - Implements AMD MMX extensions 3DNOW - Supports 3DNow! instructions 3DNOWEXT - Supports 3DNow! extension instructions SSE * Supports Streaming SIMD Extensions SSE2 * Supports Streaming SIMD Extensions 2 SSE3 * Supports Streaming SIMD Extensions 3 SSSE3 * Supports Supplemental SIMD Extensions 3 SSE4a - Supports Sreaming SIMDR Extensions 4a SSE4.1 * Supports Streaming SIMD Extensions 4.1 SSE4.2 * Supports Streaming SIMD Extensions 4.2 AES - Supports AES extensions AVX - Supports AVX intruction extensions FMA - Supports FMA extensions using YMM state MSR * Implements RDMSR/WRMSR instructions MTRR * Supports Memory Type Range Registers XSAVE - Supports XSAVE/XRSTOR instructions OSXSAVE - Supports XSETBV/XGETBV instructions RDRAND - Supports RDRAND instruction RDSEED - Supports RDSEED instruction CMOV * Supports CMOVcc instruction CLFSH * Supports CLFLUSH instruction CX8 * Supports compare and exchange 8-byte instructions CX16 * Supports CMPXCHG16B instruction BMI1 - Supports bit manipulation extensions 1 BMI2 - Supports bit manipulation extensions 2 ADX - Supports ADCX/ADOX instructions DCA - Supports prefetch from memory-mapped device F16C - Supports half-precision instruction FXSR * Supports FXSAVE/FXSTOR instructions FFXSR - Supports optimized FXSAVE/FSRSTOR instruction MONITOR * Supports MONITOR and MWAIT instructions MOVBE - Supports MOVBE instruction ERMSB - Supports Enhanced REP MOVSB/STOSB PCLULDQ - Supports PCLMULDQ instruction POPCNT * Supports POPCNT instruction LZCNT - Supports LZCNT instruction SEP * Supports fast system call instructions LAHF-SAHF * Supports LAHF/SAHF instructions in 64-bit mode HLE - Supports Hardware Lock Elision instructions RTM - Supports Restricted Transactional Memory instructions DE * Supports I/O breakpoints including CR4.DE DTES64 * Can write history of 64-bit branch addresses DS * Implements memory-resident debug buffer DS-CPL * Supports Debug Store feature with CPL PCID * Supports PCIDs and settable CR4.PCIDE INVPCID - Supports INVPCID instruction PDCM * Supports Performance Capabilities MSR RDTSCP * Supports RDTSCP instruction TSC * Supports RDTSC instruction TSC-DEADLINE - Local APIC supports one-shot deadline timer TSC-INVARIANT * TSC runs at constant rate xTPR * Supports disabling task priority messages EIST * Supports Enhanced Intel Speedstep ACPI * Implements MSR for power management TM * Implements thermal monitor circuitry TM2 * Implements Thermal Monitor 2 control APIC * Implements software-accessible local APIC x2APIC - Supports x2APIC CNXT-ID - L1 data cache mode adaptive or BIOS MCE * Supports Machine Check, INT18 and CR4.MCE MCA * Implements Machine Check Architecture PBE * Supports use of FERR#/PBE# pin PSN - Implements 96-bit processor serial number PREFETCHW * Supports PREFETCHW instruction Maximum implemented CPUID leaves: 0000000B (Basic), 80000008 (Extended). Logical to Physical Processor Map: *-*- Physical Processor 0 (Hyperthreaded) -*-* Physical Processor 1 (Hyperthreaded) Logical Processor to Socket Map: **** Socket 0 Logical Processor to NUMA Node Map: **** NUMA Node 0 Logical Processor to Cache Map: *-*- Data Cache 0, Level 1, 32 KB, Assoc 8, LineSize 64 *-*- Instruction Cache 0, Level 1, 32 KB, Assoc 4, LineSize 64 *-*- Unified Cache 0, Level 2, 256 KB, Assoc 8, LineSize 64 -*-* Data Cache 1, Level 1, 32 KB, Assoc 8, LineSize 64 -*-* Instruction Cache 1, Level 1, 32 KB, Assoc 4, LineSize 64 -*-* Unified Cache 1, Level 2, 256 KB, Assoc 8, LineSize 64 **** Unified Cache 2, Level 3, 3 MB, Assoc 12, LineSize 64
你不能比较两个定时源，它们在PC上的实现有很大的不同。
GetTickCount()来自时钟滴答中断，即实时时钟产生的一个信号。 传统上，专用芯片，最初是摩托罗拉MC146818，现在集成在南桥。 它有用于手表的振荡器，晶体稳定，通常运行在32768赫兹。 当机器电源关闭时，该振荡器继续运行，运行锂电池或超级电容器。
所以分辨率相当差，但是通过周期性地将时钟与时间服务器提供的时间重新同步，使其具有非常好的长期稳定性，并且非常准确，大多数Windows机器使用time.windows.com。 有关详细信息，请查阅GetSystemTimeAdjustment（）。
QueryPerformanceCounter()使用芯片组中可用的频率源。 传统的8053计数器运行在1193182赫兹。 现在的HPET定时器，HAL（硬件抽象层）允许系统集成商选择他可用的任何频率源。 使用CPU时钟在便宜的设计中并不罕见。
所以分辨率非常高，但是不准确，没有任何机制来校准这个定时器。 从报道的QPF中偏离800ppm并不罕见。 这个计时器只能用于短时间间隔测量，例如一个分析器将使用的类型。
所以不，使用QueryPerformanceCounter（）作为GetTickCount64（）的替代方案不是一个好主意，除非你能忍受这个不准确。 从技术上讲，只要您跟踪GetTickCount（）溢出的值，就可以合成自己的64位计数器。 例如，如果前一个值为负数，新值为正数，则可以增加课程计数，表示溢出。 唯一的要求是，你经常对GetTickCount（）进行抽样，至少在24天内看到一次转换。
在和你联系的那个话题的同一个话题中， 陈百强的回复明确地说，你既不应该考虑任何事情。 只考虑时差（间隔）是相关数量。 因此，你应该测试的是间隔，例如在循环开始值之前，循环开始之后的每一次循环。