$ProcessorCount isn't equal to the real number of CPUs

Questions
1

I’m running Mathematica13.2 on a CentOS7 machine with two AMD Epyc 9754 but $ProcessorCount=12, which is much less.

The shell command “lscpu” and C function sysconf(_SC_NPROCESSORS_ONLN) in <unistd.h> both give that the number of cpus is 256, and I’m sure the program is on the computing node.

I made some checks, such as SetSystemOptions[“ParallelOptions” → “ParallelThreadNumber” → 256], and $MaxLicenseProcesses gives Infinity. But they don’t work.

This problem is complicated, maybe the key is how does Mathematica know the number of cpus. I guess it may be associated with the MKL lib, but I don’t know how to test.

Thanks for help and discussion.

2

Yes, the automatic detection is done through MKL and a very common reason for an unexpected result is having an environment variable override (such as OMP_NUM_THREADS or MKL_NUM_THREADS).

If that does not resolve the issue, you may define $ProcessorCount in any desired way, for example

Unprotect[$ProcessorCount];
$ProcessorCount = 256;
Protect[$ProcessorCount];

in initialization code like init.m. The value does not have any impact on parallel computation except for deciding how many parallel kernels to launch by default via LaunchKernels[].

Other important parameters would be "ParallelThreadNumber" and "MKLThreadNumber" in SystemOptions["ParallelOptions"], which again can be set to any desired values.

3

Thank you very much! The two environment variables exactly as you mentioned are set to 12 (maybe done by the root), and it works fine after changing them to 256.