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Question:

/proc/cpuinfo shows 8 CPUs, but MOSIX claims that there are only 4

Answer:

There are in fact only 4 real CPUs (or cores) per node - the extra CPUs shown in /proc/cpuinfo reflect Hyper-Threading. You can tell that Hyper-Threading is enabled by the "ht" flag in the "flags" field of /proc/cpuinfo. Hyper-Threading may help a few threaded applications, such as browsers, but is useless and sometimes even detrimental for HPC applications. It is a good idea to disable the Hyper-Threading feature in the BIOS (some vendors name it "virtual processor").