How to Detect the Number of Physical Processors / Cores on Windows, Mac and Linux How to Detect the Number of Physical Processors / Cores on Windows, Mac and Linux windows windows

How to Detect the Number of Physical Processors / Cores on Windows, Mac and Linux


EDIT: This is no longer 100% correct due to Intel's ongoing befuddlement.

The way I understand the question is that you are asking how to detect the number of CPU cores vs. CPU threads which is different from detecting the number of logical and physical cores in a system. CPU cores are often not considered physical cores by the OS unless they have their own package or die. So an OS will report that a Core 2 Duo, for example, has 1 physical and 2 logical CPUs and an Intel P4 with hyper-threads will be reported exactly the same way even though 2 hyper-threads vs. 2 CPU cores is a very different thing performance wise.

I struggled with this until I pieced together the solution below, which I believe works for both AMD and Intel processors. As far as I know, and I could be wrong, AMD does not yet have CPU threads but they have provided a way to detect them that I assume will work on future AMD processors which may have CPU threads.

In short here are the steps using the CPUID instruction:

  1. Detect CPU vendor using CPUID function 0
  2. Check for HTT bit 28 in CPU features EDX from CPUID function 1
  3. Get the logical core count from EBX[23:16] from CPUID function 1
  4. Get actual non-threaded CPU core count
    1. If vendor == 'GenuineIntel' this is 1 plus EAX[31:26] from CPUID function 4
    2. If vendor == 'AuthenticAMD' this is 1 plus ECX[7:0] from CPUID function 0x80000008

Sounds difficult but here is a, hopefully, platform independent C++ program that does the trick:

#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));  // ECX is set to zero for CPUID function 4#endif}int main(int argc, char *argv[]) {  unsigned regs[4];  // Get vendor  char vendor[12];  cpuID(0, regs);  ((unsigned *)vendor)[0] = regs[1]; // EBX  ((unsigned *)vendor)[1] = regs[3]; // EDX  ((unsigned *)vendor)[2] = regs[2]; // ECX  string cpuVendor = string(vendor, 12);  // Get CPU features  cpuID(1, regs);  unsigned cpuFeatures = regs[3]; // EDX  // Logical core count per CPU  cpuID(1, regs);  unsigned logical = (regs[1] >> 16) & 0xff; // EBX[23:16]  cout << " logical cpus: " << logical << endl;  unsigned cores = logical;  if (cpuVendor == "GenuineIntel") {    // Get DCP cache info    cpuID(4, regs);    cores = ((regs[0] >> 26) & 0x3f) + 1; // EAX[31:26] + 1  } else if (cpuVendor == "AuthenticAMD") {    // Get NC: Number of CPU cores - 1    cpuID(0x80000008, regs);    cores = ((unsigned)(regs[2] & 0xff)) + 1; // ECX[7:0] + 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;}

I haven't actually tested this on Windows or OSX yet but it should work as the CPUID instruction is valid on i686 machines. Obviously, this wont work for PowerPC but then they don't have hyper-threads either.

Here is the output on a few different Intel machines:

Intel(R) Core(TM)2 Duo CPU T7500 @ 2.20GHz:

 logical cpus: 2    cpu cores: 2hyper-threads: false

Intel(R) Core(TM)2 Quad CPU Q8400 @ 2.66GHz:

 logical cpus: 4    cpu cores: 4hyper-threads: false

Intel(R) Xeon(R) CPU E5520 @ 2.27GHz (w/ x2 physical CPU packages):

 logical cpus: 16    cpu cores: 8hyper-threads: true

Intel(R) Pentium(R) 4 CPU 3.00GHz:

 logical cpus: 2    cpu cores: 1hyper-threads: true


Note this, does not give the number of physically cores as intended, but logical cores.

If you can use C++11 (thanks to alfC's comment beneath):

#include <iostream>#include <thread>int main() {    std::cout << std::thread::hardware_concurrency() << std::endl;    return 0;}

Otherwise maybe the Boost library is an option for you. Same code but different include as above. Include <boost/thread.hpp> instead of <thread>.


Windows only solution desribed here:

GetLogicalProcessorInformation

for linux, /proc/cpuinfo file. I am not running linuxnow so can't give you more detail. You can countphysical/logical processor instances. If logical countis twice as physical, then you have HT enabled(true only for x86).