How can I accurately determine the number of physical cores in my system, considering the presence of hyper-threading?

Detecting the Number of Physical Processors/Cores with Hyper-Threading Support

In multi-threaded applications that aim for maximum efficiency, knowing the number of physical processors or cores is crucial. Creating excessive threads can hinder performance, especially in scenarios where hyper-threading is supported.

Hyper-Threading Detection

To accurately determine the number of physical processors, you need to detect if hyper-threading is supported and enabled. Here's how you can do it:

1. Identify CPU Vendor: Execute the CPUID instruction with function 0 to retrieve the CPU vendor (e.g., "GenuineIntel" or "AuthenticAMD").
2. Check for Hyper-Threading (Intel): For Intel processors, check bit 28 in EDX from CPUID function 1. If it's set, hyper-threading is supported.
3. Check for Hyper-Threading (AMD): For AMD processors, execute CPUID function 0x80000008 to obtain the number of cores in ECX[7:0]. If this number is greater than zero, hyper-threading is supported.

Determining Physical Core Count

Once hyper-threading support is detected, follow these steps to determine the number of physical cores:

• For Intel processors, execute CPUID function 4 and get the count from EAX[31:26] 1.
• For AMD processors, use the previously obtained ECX[7:0] value from CPUID function 0x80000008 and add 1.

Example Implementation

The following C program demonstrates the detection of hyper-threading and the number of physical cores:

<code class="cpp">#include <iostream>
#include <string>

using namespace std;

void cpuID(unsigned i, unsigned regs[4]) {
#ifdef _WIN32
  __cpuid((int *)regs, (int)i);
#else
  asm volatile
    ("cpuid" : "=a" (regs[0]), "=b" (regs[1]), "=c" (regs[2]), "=d" (regs[3])
     : "a" (i), "c" (0));
#endif
}

int main(int argc, char *argv[]) {
  unsigned regs[4];

  // ... (Code for vendor detection, feature check, and logical core count)
  
  // Hyper-Threading detection
  bool hyperThreads = cpuFeatures & (1 << 28) && cores < logical;

  // ... (Code for physical core count based on vendor)

  cout << "hyper-threads: " << (hyperThreads ? "true" : "false") << endl;

  return 0;
}</code>

Conclusion

By following these steps, you can accurately detect the number of physical processors/cores while accounting for hyper-threading support. This information is invaluable for optimizing the performance of your multi-threaded applications.

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