How can I accurately determine the number of physical processors and cores, taking into account hyper-threading configurations?

Determining Physical Processor and Core Count with Hyper-Threading Considerations

Detecting the number of physical processors and cores is crucial for optimizing multi-threaded applications to run efficiently. To accurately determine these counts, consider potentially enabled hyper-threading dependencies.

Step-by-Step Process:

1. Identify CPU Vendor: Execute the CPUID instruction (function 0) to identify the CPU vendor, either 'GenuineIntel' or 'AuthenticAMD'.
2. Check Hyper-Threading Support (Intel): Examine the CPU's features register (EDX) from CPUID function 1. If bit 28 is set (EDX bit 28 = 1), hyper-threading support is active.
3. Determine Logical Core Count: Obtain the logical core count per physical core from EBX[23:16] of the CPUID function 1 result.

4. Distinguish Physical Core Count:

   • If the vendor is Intel, the physical core count is 1 plus EAX[31:26] from CPUID function 4.
   • If the vendor is AMD, the physical core count is 1 plus ECX[7:0] from CPUID function 0x80000008.

C Implementation:

The following C program exemplifies these steps:

<code class="c++">#include <iostream>
#include <string>

void cpuID(unsigned i, unsigned regs[4]);

int main() {
  unsigned regs[4];
  char vendor[12];

  // Get vendor
  cpuID(0, regs);
  ((unsigned *)vendor)[0] = regs[1];
  ((unsigned *)vendor)[1] = regs[3];
  ((unsigned *)vendor)[2] = regs[2];
  string cpuVendor = string(vendor, 12);

  // Get CPU features
  cpuID(1, regs);
  unsigned cpuFeatures = regs[3];

  // Logical core count per CPU
  cpuID(1, regs);
  unsigned logical = (regs[1] >> 16) & 0xff;
  cout << " logical cpus: " << logical << endl;
  unsigned cores = logical;

  if (cpuVendor == "GenuineIntel") {
    // Get DCP cache info
    cpuID(4, regs);
    cores = ((regs[0] >> 26) & 0x3f) + 1;
  } else if (cpuVendor == "AuthenticAMD") {
    // Get NC: Number of CPU cores - 1
    cpuID(0x80000008, regs);
    cores = ((unsigned)(regs[2] & 0xff)) + 1;
  }

  cout << "    cpu cores: " << cores << endl;

  // Detect hyper-threads
  bool hyperThreads = cpuFeatures & (1 << 28) && cores < logical;
  cout << "hyper-threads: " << (hyperThreads ? "true" : "false") << endl;

  return 0;
}</code>

By adhering to these steps and utilizing the provided C program, developers can accurately determine the number of physical processors and cores, considering the nuances of hyper-threading configurations across different platforms.

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