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I'm working on a number-crunching application and I'm trying to squeeze all possible performance out of it that I can. I'm designing it to work for both Windows and *nix and even for multi-CPU machines.

The way I have it currently set up, it asks the OS how many cores there are, sets affinity on each core to a function that runs a CPUID ASM command (yes, it'll get run multiple times on the same CPU; no biggie, it's just initialization code) and checks for HyperThreading in the Features request of CPUID. From the responses to the CPUID command it calculates how many threads it should run. Of course, if a core/CPU supports HyperThreading it will spawn two on a single core.

However, I ran into a branch case with my own machine. I run an HP laptop with a Core 2 Duo. I replaced the factory processor a while back with a better Core 2 Duo that supports HyperThreading. However, the BIOS does not support it as the factory processor didn't. So, even though the CPU reports that it has HyperThreading it's not capable of utilizing it.

I'm aware that in Windows you can detect HyperThreading by simply counting the logical cores (as each physical HyperThreading-enabled core is split into two logical cores). However, I'm not sure if such a thing is available in *nix (particularly Linux; my test bed).

If HyperTreading is enabled on a dual-core processor, wil the Linux function sysconf(_SC_NPROCESSORS_CONF) show that there are four processors or just two?

If I can get a reliable count on both systems then I can simply skip the CPUID-based HyperThreading checking (after all, it's a possibility that it is disabled/not available in BIOS) and use what the OS reports, but unfortunately because of my branch case I'm not able to determine this.

P.S.: In my Windows section of the code I am parsing the return of GetLogicalProcessorInformation()

Bonus points: Anybody know how to mod a BIOS so I can actually HyperThread my CPU ;)? Motherboard is an HP 578129-001 with the AMD M96 chipset (yuck).

Chuck R
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  • A couple observations: 1. I've never heard of a system on which there are heterogeneous processors (ie, you'll get different results from CPUID from different cores) which your test seems to suggest may happen; and 2. you might *not* want to spawn two threads on an HT core. HT shares many of the core's computing resources, so you may take a big hit from running two number-crunching threads on a single core. HT was designed more for threads that often block on I/O, if I'm not mistaken. – Kevin Sep 21 '13 at 11:58
  • There are different values that can be reported; on MacOS X I can get the number of packages (physical chips), the number of cores, and the number of threads using sysconf. The numbers reported are the ones that the OS uses; you can for example as a developer disable cores at boot time to test how your software works with fewer cores. CPUID might return different results obviously. It would return "hyper threading" if the CPU can do it, even if disabled in the OS. – gnasher729 Apr 15 '14 at 10:44

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