Please realize that Intel's "Hyperthreading" is not the same as "Multi-threading" as implemented by other vendors. Hyperthreading will often give you worse performance than simply using one core (ie, turning hyperthreading off).
Multi-threading, as in real SMT, will actually give you a performance improvement, but just not a linear one like adding an extra full core would. For example, extra hardware threads on the Niagara give about the same improvement as about 50% of a CPU.
I've done tests to verify this on our servers, and as a result I disable Hyperthreading on all of our compute nodes. Don't take my word for it though, run tests yourself.
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