Software Tuning, Performance Optimization & Platform Monitoring
Discussion around monitoring and software tuning methodologies, Performance Monitoring Unit (PMU) of Intel microprocessors, and platform monitoring
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About crystal frequency slow down in Xeon (non skylake)


I saw a kernel path:  Fix erroneous TSC rate on Skylake Xeon  

In this patch, it mentioned that crystal frequency was a bit slower than expect on Skylake Xeon.

I wonder is this issue only would happen on Skylake Xeon or not?

Because on Cascade Lake Xeon, I haven't find the slow down.

I'm using TSC as the source of timestamp, so it's real important for me to figure it out.




Additional Information:


My question actually comes from <Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3> 18.7.3 Determining the Processor Base Frequency, there is a Table 18-85. Nominal Core Crystal Clock Frequency


Processor Families/Processor Number Series



Intel® Xeon® Processor Scalable Family with CPUID signature 06_55H.

25 MHz

6th and 7th generation Intel® Core processors and Intel® Xeon® WProcessorFamily.

24 MHz

Next Generation Intel® Atom processors based on GoldmontMicroarchitecturewith CPUID signature 06_5CH (does notinclude IntelXeon processors).

19.2 MHz


Xeon Scalable has CPUID signature 06_55H, but they may have different microarchitecture, e.g. Skylake, Cascade Lake. 


For Skylake Xeon, there is comment in kernel patch:  

All SKX subject the crystal to an EMI reduction circuit that
reduces its actual frequency by (approximately) -0.25%


I wonder is there the same issue in further microarchitecture? If there is, which means the  Nominal Core Crystal Clock Frequency of Xeon Scalable in manual is not that accurate. So I can't use it as timestamp source.

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