Finding out the CPU clock frequency (per core, per processor)

Question

Programs like CPUz are very good at giving in depth information about the system (bus speed, memory timings, etc.)

However, is there a programmatic way of calculating the per core (and per processor, in multi processor systems with multiple cores per CPU) frequency without having to deal with CPU specific info.

I am trying to develop a anti cheating tool (for use with clock limited benchmark competitions) which will be able to record the CPU clock during the benchmark run for all the active cores in the system (across all processors.)

Answer 1

I'll expand on my comments here. This is too big and in-depth for me to fit in the comments.

What you're trying to do is very difficult - to the point of being impractical for the following reasons:

• There's no portable way to get the processor frequency. rdtsc does NOT always give the correct frequency due to effects such as SpeedStep and Turbo Boost.
• All known methods to measure frequency require an accurate measurement of time. However, a determined cheater can tamper with all the clocks and timers in the system.
• Accurately reading either the processor frequency as well as time in a tamper-proof way will require kernel-level access. This implies driver signing for Windows.

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There's no portable way to get the processor frequency:

The "easy" way to get the CPU frequency is to call rdtsc twice with a fixed time-duration in between. Then dividing out the difference will give you the frequency.

The problem is that rdtsc does not give the true frequency of the processor. Because real-time applications such as games rely on it, rdtsc needs to be consistent through CPU throttling and Turbo Boost. So once your system boots, rdtsc will always run at the same rate (unless you start messing with the bus speeds with SetFSB or something).

For example, on my Core i7 2600K, rdtsc will always show the frequency at 3.4 GHz. But in reality, it idles at 1.6 GHz and clocks up to 4.6 GHz under load via the overclocked Turbo Boost multiplier at 46x.

But once you find a way to measure the true frequency, (or you're happy enough with rdtsc), you can easily get the frequency of each core using thread-affinities.

Getting the True Frequency:

To get the true frequency of the processor, you need to access either the MSRs (model-specific registers) or the hardware performance counters.

These are kernel-level instructions and therefore require the use of a driver. If you're attempting this in Windows for the purpose of distribution, you will therefore need to go through the proper driver signing protocol. Furthermore, the code will differ by processor make and model so you will need different detection code for each processor generation.

Once you get to this stage, there are a variety of ways to read the frequency.

On Intel processors, the hardware counters let you count raw CPU cycles. Combined with a method of precisely measuring real time (next section), you can compute the true frequency. The MSRs give you access to other information such as the CPU frequency multiplier.

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All known methods to measure frequency require an accurate measurement of time:

This is perhaps the bigger problem. You need a timer to be able to measure the frequency. A capable hacker will be able to tamper with all the clocks that you can use in C/C++. This includes all of the following:

• clock()
• gettimeofday()
• QueryPerformanceCounter()
• etc...

The list goes on and on. In other words, you cannot trust any of the timers as a capable hacker will be able to spoof all of them. For example clock() and gettimeofday() can be fooled by changing the system clock directly within the OS. Fooling QueryPerformanceCounter() is harder.

Getting a True Measurement of Time:

All the clocks listed above are vulnerable because they are often derived off of the same system base clock in some way or another. And that system base clock is often tied to the system base clock - which can be changed after the system has already booted up by means of overclocking utilities.

So the only way to get a reliable and tamper-proof measurement of time is to read external clocks such as the HPET or the ACPI. Unfortunately, these also seem to require kernel-level access.

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To Summarize:

Building any sort of tamper-proof benchmark will almost certainly require writing a kernel-mode driver which requires certificate signing for Windows. This is often too much of a burden for casual benchmark writers.

This has resulted in a shortage of tamper-proof benchmarks which has probably contributed to the overall decline of the competitive overclocking community in recent years.