This threshold is currently eight, so if count is less than or equal to 8, the vectored I/O operation occurs in a very memory-efficient manner off of the process’ kernel stack.
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I'm going to be presumptuous here and assume that you are not aware of the use of this in the programming sense. (If you are, I don't mean to be condescending.)
A stack in any programming sense is a FILO (First In, Last Out) structure. Think of this as a stack of plates at the buffet table: you can't take a plate in the middle, you must take the plate on top.
Stacks only have 3 operations:
My description of operation #2 has "off of" in it; that's what the original "off of"in your example means. To paraphrase: the I/O operations [pops] operations off in a memory-efficient manner.
I checked the context, which says that the segments (when small enough) are allocated on the Linux kernel stack, rather than on the specific process' kernel stack. This seems to indicate that small operations are allocated and executed not in the process' kernel stack, but in the Linux kernel stack. So off of in this case means out of, or not in.
"off of" means "from" or "on". It's actually a quite literal phrase.
The sentence is rather confusing. It could be be written as:
"If the count is less than or equal to 8, the vectored I/O operation occurs on the kernel stack of the process, which is very memory-efficient."