Currently std.parallelism.totalCPUs on Windows is implemented using the system call GetSystemInfo() which can return a maximum of 64 cores. On a Xeon Phi 7250 system with 272 cores it is impossible to exploit all cores. TBB (Threading building blocks) uses the Processor Groups to correctly identify all cores, like this; https://github.com/01org/tbb/blob/314792356bf75f4a190277536aea543b9b6b310b/src/tbb/tbb_misc_ex.cpp#L241 A Xeon Phi 7250 which has 68 cores and 4 threads per core is reported as 5 Processor Groups; 4 x 64 cores + 1 x 16 cores.
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