How to Determine if Hyper-Threading is Enabled on Windows, Mac, and Linux?

Detecting the Existence of Enabled Hyper-Threading

In multi-threaded applications, utilizing one thread per physical processor core ensures optimal performance. For precise thread count determination, it's essential to distinguish between physical and hyper-threading cores. Here's how to detect hyper-threading support and its activation status on Windows, Mac, and Linux:

CPUID Instruction

Utilizing the CPUID instruction, we can gather information about the processor's capabilities and configuration. A step-by-step process is outlined below:

1. Vendor Detection: Execute CPUID Function 0 to identify the CPU vendor (e.g., Intel or AMD).
2. Hyper-Threading Bit Check: Execute CPUID Function 1 and check bit 28 in the EDX register. This bit indicates hyper-threading support.
3. Logical Core Count: Retrieve the number of logical cores per CPU from bits [23:16] of EBX in CPUID Function 1.

4. Physical Core Count:

   • Intel CPUs: If vendor is 'GenuineIntel,' use CPUID Function 4 to obtain the number of physical cores plus one by extracting bits [31:26] of the EAX register.
   • AMD CPUs: If vendor is 'AuthenticAMD,' execute CPUID Function 0x80000008 and derive the number of physical cores plus one by taking bits [7:0] of the ECX register.

Implementation

Here's a C program that implements this method:

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

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

int main() {
  unsigned regs[4];

  // Get CPUID information
  cpuID(0x00, regs);
  cpuID(0x01, regs);

  // Determine vendor
  char vendor[12];
  ((unsigned *)vendor)[0] = regs[1];
  ((unsigned *)vendor)[1] = regs[3];
  ((unsigned *)vendor)[2] = regs[2];
  std::string cpuVendor = std::string(vendor, 12);

  // Variables
  unsigned logicalCores = (regs[1] >> 16) & 0xff;
  unsigned cores = logicalCores;
  bool hyperThreads = false;

  // Detect hyper-threading
  if (cpuVendor == "GenuineIntel") {
    cpuID(0x04, regs);
    cores = ((regs[0] >> 26) & 0x3f) + 1;
  } else if (cpuVendor == "AuthenticAMD") {
    cpuID(0x80000008, regs);
    cores = ((unsigned)(regs[2] & 0xff)) + 1;
  }

  if (regs[3] & (1 << 28) && cores < logicalCores) {
    hyperThreads = true;
  }

  // Print results
  std::cout << "Logical cores: " << logicalCores << std::endl;
  std::cout << "Cores: " << cores << std::endl;
  std::cout << "Hyper-threading: " << (hyperThreads ? "true" : "false") << std::endl;

  return 0;
}</code>

Output Examples:

Intel Core 2 Duo E8400 (no hyper-threading):

Logical cores: 2
Cores: 2
Hyper-threading: false

Intel Core i7-7700K (with hyper-threading):

 Logical cores: 8
    Cores: 4
hyper-threads: true

AMD Ryzen 5 2600X (with SMT):

 Logical cores: 12
    Cores: 6
hyper-threads: true

The above is the detailed content of How to Determine if Hyper-Threading is Enabled on Windows, Mac, and Linux?. For more information, please follow other related articles on the PHP Chinese website!