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Windows 8, Windows 8.1, Windows Server 2012, and Windows Server 2012 R2 use TSCs as the basis for the performance counter. Windows 8, Windows 8.1, Windows Server 2012, and Windows Server 2012 R2 On systems where the TSC is not suitable for timekeeping, Windows automatically selects a platform counter (either the HPET timer or the ACPI PM timer) as the basis for QPC. Furthermore, there is no added overhead for concurrent calls and user-mode queries often bypass system calls, which further reduces overhead. On such systems, the cost of reading the performance counter is significantly lower compared to systems that use a platform counter. Windows 7 and Windows Server 2008 R2 use TSCs as the basis of QPC on single-clock domain systems where the operating system (or the hypervisor) is able to tightly synchronize the individual TSCs across all processors during system initialization. TSCs are high-resolution per-processor hardware counters that can be accessed with very low latency and overhead (in the order of 10s or 100s of machine cycles, depending on the processor type). The majority of Windows 7 and Windows Server 2008 R2 computers have processors with constant-rate TSCs and use these counters as the basis for QPC. This limits scalability of QPC if it is called concurrently from multiple processors. Such platform timers have higher access latency than the TSC and are shared between multiple processors. Windows Vista and Windows Server 2008Īll computers that shipped with Windows Vista and Windows Server 2008 used a platform counter (High Precision Event Timer (HPET)) or the ACPI Power Management Timer (PM timer) as the basis for QPC. Systems with flawed firmware that run these versions of Windows might not provide the same QPC reading on different cores if they used the TSC as the basis for QPC. However, some hardware systems' BIOS didn't indicate the hardware CPU characteristics correctly (a non-invariant TSC), and some multi-core or multi-processor systems used processors with TSCs that couldn't be synchronized across cores. QPC is available on Windows XP and Windows 2000 and works well on most systems. ![]() FREE PASCAL CONVERT TIMESTAMP TO DATE SOFTWAREHere we describe the characteristics of QPC on different Windows versions to help you maintain software that runs on those Windows versions. QPC was introduced in Windows 2000 and Windows XP and has evolved to take advantage of improvements in the hardware platform and processors.
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