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I don't know if this is very far fetched to make a dent into 1 PH/s with (limited) CPUs.
Let's say some top-end CPU gets you 20 MH/s (wild-assed guess as this isn't being updated with modern CPUs anymore).
20 MH/s is 0.02 GH/s, which is 0.00002 TH/s, and 0.00000002 PH/s.
An obsolete, five year old Antminer S9 that costs $200 today can generate 14 TH/s, or 700,000X more hashrate than this top-end desktop CPU machine can.
To get 1 PH/s, you would need 50 million top-end desktop CPU machines hashing away.